# Help understanding 310.12



## oldsparky52 (Feb 25, 2020)

310.12 stops when the conductors no longer are carrying 100% of the loads. So in your case, no that table would not apply to the conductors from the feed through lugs to the interior panel. 

So, 2/0 allowed from meter to outside MB panel but 3/0 from the feed through lugs to ... 

Crazy, right?


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## mburtis (Sep 1, 2018)

Ok that's how I was interpreting it based on 310.12(b) but what's the point of 310.12(c) then.


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## oldsparky52 (Feb 25, 2020)

mburtis said:


> Ok that's how I was interpreting it based on 310.12(b) but what's the point of 310.12(c) then.


Excellent question, I await someone capable of answer that (along with you).


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## mburtis (Sep 1, 2018)

It really seems that if you have a 200 amp service with 2/0 wire feeding a 200 amp panel which then feeds another 200 amp panel the feeder to the second panel should be allowed to be 2/0 as well. Just seems stupid otherwise. Now if the sub panel was a 100 amp panel then the feeder would have to be sized for the whole 100 amps not 83 percent of that. I would be a lot more confident if there wasn't the picture in the handbook pointing to a sub panel saying it didn't apply.


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## Dennis Alwon (May 9, 2009)

Ok this is the 2020 nec for those looking for 310.12... it doesn't exist in prior codes.

IMO, 310.12(C) is saying that the ampacity of the conductor does not have to be greater than the service conductor. So, if you have 2/0 for a dwelling and you want to feed an interior panel at 200 amps but you cannot use the residential rule, then IMO you would use 3/0 because the ampacity of 3/0 is 200 amps and it is not greater than the ampacity of the 2/0 exception.

Many will disagree but that is how I see it.


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## oldsparky52 (Feb 25, 2020)

Dennis Alwon said:


> Ok this is the 2020 nec for those looking for 310.12... it doesn't exist in prior codes.


Took me a while to figure that out.


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## HertzHound (Jan 22, 2019)

mburtis said:


> It really seems that if you have a 200 amp service with 2/0 wire feeding a 200 amp panel which then feeds another 200 amp panel the feeder to the second panel should be allowed to be 2/0 as well. Just seems stupid otherwise. Now if the sub panel was a 100 amp panel then the feeder would have to be sized for the whole 100 amps not 83 percent of that. I would be a lot more confident if there wasn't the picture in the handbook pointing to a sub panel saying it didn't apply.


In your example the first panel would see the total load of the whole dwelling with all the diversity, but what loads are in the second panel? It's not the whole load with all the diversity. Maybe the first panel has all the smaller loads and the second panel has all the air conditioning, well pump and kitchen circuits etc.. Probably in reality the second panel would never see close to 200A, so it wouldn't matter. But the code has no idea what will be in the panel, so they limit the use of the 83% rule.

As for part (C), I have no idea what it means. Part (C) was in previous codes, but I haven't seen an explanation. I might have to dig in the basement for an old handbook.


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## mburtis (Sep 1, 2018)

But if the service entrance conductors are 2/0 into a 200 amp main breaker what difference does it make if the wires downstream are 2/0 as well?


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## Dennis Alwon (May 9, 2009)

mburtis said:


> But if the service entrance conductors are 2/0 into a 200 amp main breaker what difference does it make if the wires downstream are 2/0 as well?



That is some peoples reasoning but what isn't considered is the term ampacity. Just because a conductor is larger does not make it have a higher ampacity. On the surface it makes total sense to say 2/0 is fine fr the sub panel but that changes the diversity of the load and that is why we are able to use a smaller conductor for 200 amps.

Personally I see no problem with it especially in a dwelling but that is not how I read the section


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## mburtis (Sep 1, 2018)

If it was just sections a and b it would make sense and I could go on with my life. I still would think it's silly but I would understand it. For the life of me I still cant figure out what the point of section c is or what's it's trying to tell me.


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## Dennis Alwon (May 9, 2009)

mburtis said:


> If it was just sections a and b it would make sense and I could go on with my life. I still would think it's silly but I would understand it. For the life of me I still cant figure out what the point of section c is or what's it's trying to tell me.


I already told you but obviously that answer is not good enough for you ...hahaha


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## oldsparky52 (Feb 25, 2020)

Dennis Alwon said:


> IMO, 310.12(C) is saying that the ampacity of the conductor does not have to be greater than the service conductor. So, if you have 2/0 for a dwelling and you want to feed an interior panel at 200 amps but you cannot use the residential rule, then IMO you would use 3/0 because the ampacity of 3/0 is 200 amps and it is not greater than the ampacity of the 2/0 exception.
> 
> Many will disagree but that is how I see it.


Count me in the camp that disagrees. It reads to me that you don't have to exceed the service entrance conductor size.


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## mburtis (Sep 1, 2018)

I guess we are just reading it different, I'm sure your probably right but Im to stupid to see it. The way i read it section c says that no feeder conductors in a single dwelling need to be bigger than the ampacities from sections a and b.... which is 83 percent of the service rating.


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## wiz1997 (Mar 30, 2021)

The panel a feed through type correct?
First panel has a 200 amp main, correct?
I've done a few and they all past inspection.
If the service conductors are terminated in the breaker attached to one end of the buss and the out going are lugged to the other end of the buss, I did not change wire size.

I look at it like running the service conductors, after the service disconnect, into a gutter, then dropping into two panels under the gutter.
Let's not bother with tap rules.
You would run the same size conductor to each panel, correct?

I did not even consider derating the conductor size to the second panel because you don't know which panel will have the larger load.


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## Dennis Alwon (May 9, 2009)

oldsparky52 said:


> Count me in the camp that disagrees. It reads to me that you don't have to exceed the service entrance conductor size.


You certainly aren't the only one who disagrees. Did you read the 2020 wording. So tell me does it say anything about the feeder size or does it mention ampacity? You could have other wires in the conduit with those feeders which would cause de-rating. You could be going thru an ambient temp that is very high-- all those factors would change the ampacity of the conductor so you would need a larger conductor... It is the same idea...



> (C) Feeder Ampacities.
> 
> 
> In no case shall a feeder for an individual dwelling unit be required to have an ampacity greater than that specified in 310.12(A) or (B).


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## oldsparky52 (Feb 25, 2020)

How do you interpret the 1st three words of (C)? "In no case"

Also, if derating is a factor, then Table 310.12 is not allowed, right?


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## Dennis Alwon (May 9, 2009)

oldsparky52 said:


> How do you interpret the 1st three words of (C)? "In no case"
> 
> Also, if derating is a factor, then Table 310.12 is not allowed, right?


You are missing the point.. In no case shall the feeder have a greater ampacity....., which is different from In no case a feeder shall not be larger...


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## oldsparky52 (Feb 25, 2020)

Dennis Alwon said:


> You are missing the point.. In no case shall the feeder have a greater ampacity....., which is different from In no case a feeder shall not be larger...


Okay, then "In no case" shall the feeder be required to have an ampacity of greater than 83% of the the service rating.


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## Dennis Alwon (May 9, 2009)

That is an interesting way to look at it however the 2/0 is allowed for a 200 amp load. You have a good point and as I have said others see it differently... They have re-written this code section many times. I see it as the 2/0 ampacity is allowed for 200 amps not 166 amps. We are not limited to 166 amps on the service with 2/0 so I will stay with my thinking. 

The reason they did away with the table and went to 83% was because of this exact issue of de-rating. Installers would run a cable thru insulation and did not consider the de-rating. The table never considered de-rating

Read (B)



> (B) Feeders.
> 
> 
> For a feeder rated 100 amperes through 400 amperes, the feeder conductors supplying the entire load associated with a one-family dwelling, or the feeder conductors supplying the entire load associated with an individual dwelling unit in a two-family or multifamily dwelling, shall be permitted to have an ampacity not less than 83 percent of the feeder rating. *If no adjustment or correction factors are required, Table 310.12 shall be permitted to be applied.*


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## mburtis (Sep 1, 2018)

oldsparky52 said:


> Okay, then "In no case" shall the feeder be required to have an ampacity of greater than 83% of the the service rating.


This is exactly how I read it meaning that if there are no adjustment circumstances 2/0 is fine to a second panel.


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## oldsparky52 (Feb 25, 2020)

Dennis Alwon said:


> That is an interesting way to look at it however the 2/0 is allowed for a 200 amp load. You have a good point and as I have said others see it differently... They have re-written this code section many times. I see it as the 2/0 ampacity is allowed for 200 amps not 166 amps. We are not limited to 166 amps on the service with 2/0 so I will stay with my thinking.
> 
> The reason they did away with the table and went to 83% was because of this exact issue of de-rating. Installers would run a cable thru insulation and did not consider the de-rating. The table never considered de-rating
> 
> Read (B)


I'm not really arguing against your point, it's just that I have the same question as this


mburtis said:


> If it was just sections a and b it would make sense and I could go on with my life. I still would think it's silly but I would understand it. For the life of me I still cant figure out what the point of section c is or what's it's trying to tell me.


Why did the authors of the NEC use the verbiage "in no case" in (C)?


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## oldsparky52 (Feb 25, 2020)

Dennis Alwon said:


> That is an interesting way to look at it however the 2/0 is allowed for a 200 amp load. You have a good point and as I have said others see it differently... They have re-written this code section many times. I see it as the 2/0 ampacity is allowed for 200 amps not 166 amps. We are not limited to 166 amps on the service with 2/0 so I will stay with my thinking.


Why did they allow the smaller conductors for services? Is it because we are over calculating the demand load to a rather large %? What would you believe the demand load on a 200-amp service that is sufficient per NEC calculations? I bet it doesn't hit 166 amps an if it ever did, it would not be for long. 

I still want to know why (C) starts with "in no case"?


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## mburtis (Sep 1, 2018)

So originally I thought this section was based on diversity of the load, your never going to use all the calculated loads at once in a residence. Then last night I ran across a post somewhere saying it was originally based on the fact that there are only 2 current carrying wires in a single phase service vs 3 as in table 310.16 in addition to the load diversity. At the end of the day if your first panel is fed with 2/0 I dont see any reason not to feed a second panel with 2/0 as long as adjustment factors dont come into play.


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## Dennis Alwon (May 9, 2009)

oldsparky52 said:


> Why did they allow the smaller conductors for services? Is it because we are over calculating the demand load to a rather large %? What would you believe the demand load on a 200-amp service that is sufficient per NEC calculations? I bet it doesn't hit 166 amps an if it ever did, it would not be for long.
> 
> I still want to know why (C) starts with "in no case"?






mburtis said:


> So originally I thought this section was based on diversity of the load, your never going to use all the calculated loads at once in a residence. Then last night I ran across a post somewhere saying it was originally based on the fact that there are only 2 current carrying wires in a single phase service vs 3 as in table 310.16 in addition to the load diversity. At the end of the day if your first panel is fed with 2/0 I dont see any reason not to feed a second panel with 2/0 as long as adjustment factors dont come into play.



They allowed smaller conductors because of the diversity in the load. It has nothing to do with the number of current carrying conductor 's . The nec determined because it's a dwelling it is extremely unlikely that all loads would come on at the same time and that with the calculated load of 200 amps there would never be a chance that the load on the house would be 200 amps... In fact it would be very much less than that... Why they decided to make exception for dwellings might be because copper was expensive or not in abundance and they thought here was one way we could save without hurting anything, IDK.

I agree there probably would never be a situation where 2/0 would be an issue as a feeder. But, the nec does state it must carry the entire load of the dwelling to use this rule so why is that there if you feel like you can ignore it any time you have a 200 amp feeder that does not have the total load of the service?

Say you have a house that is 400 amps and you have 2- 200 amp panels. Now per your example we could run 2/0 to each panel. Now say the electrician thought it would be a good idea to put all the heating load and high energy loads in one panel. Now you have a 200 amp panel with a main breaker protected by a 175 amp conductor. You could potentially have over 175 amps on the one panel and basically nothing on the other. I get it-- it would be very rare and probably never an issue but I think since the nec cannot control this arrangement they had to do something to control diversity.

The only true way to have a diversified load is for all loads to be on the same conductor.

I don't know how to respond to your question about In no case.... I don't see the issue with it.

I think the problem is the wording. It seems to be causing trouble as they have changed the word "ampacity" to "larger than" back to "ampacity. I think most inspector would allow the 2/0. This is where 4/0 aluminum actually outperforms the copper because 4/0 alum is rated 180 amps so the next higher size breaker is allowed as long as the calculated load isn't over 180 amps. 2/0 copper is rated 175 amps and there is a 175 amp breaker so you cannot go to the next size breaker. So 4/0 would work regardless of the rule in 310.15 but 2/0 would depend on interpretation


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## oldsparky52 (Feb 25, 2020)

Dennis Alwon said:


> I don't know how to respond to your question about In no case.... I don't see the issue with it.


I guess it's more of a curiosity. They specifically put that in, and I bet for a reason. I am curious as to why? What "case" are they concerned could be made to require a conductor with a higher ampacity than 83% of the service rating?


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## mburtis (Sep 1, 2018)

I truly think that part c means that there is no need to run a conductor with higher capacity than that calculated in parts a or b downstream, to a sub panel for example as long as there is not derating involved. What sense does it make to run 3/0 downstream of 2/0 when all the power has to go through the 2/0 to get to the 3/0. Your parallel panels on a 400 amp service is different in my mind, in that case parts a and b would work out to 83% of 400 amps or 332 amps and the parallel feeders would have to be sized for the load they will see.


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## Dennis Alwon (May 9, 2009)

mburtis said:


> I truly think that part c means that there is no need to run a conductor with higher capacity than that calculated in parts a or b downstream, to a sub panel for example as long as there is not derating involved. What sense does it make to run 3/0 downstream of 2/0 when all the power has to go through the 2/0 to get to the 3/0.


 Agreed



> Your parallel panels on a 400 amp service is different in my mind, in that case parts a and b would work out to 83% of 400 amps or 332 amps and the parallel feeders would have to be sized for the load they will see.


Agreed but just to be clear those are not parallel conductors assuming you mean feeding 2- 200 amp panels..



> 310.10(H) Conductors in Parallel.
> (1) General. Aluminum, copper-clad aluminum, or copper
> conductors, for each phase, polarity, neutral, or grounded
> circuit shall be permitted to be connected in parallel (*electrically
> joined at both ends*).......


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## mburtis (Sep 1, 2018)

Not parallel conductors per say I guess but I just meant the panel loads where in parallel. Thanks for the help understanding this.


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## Matt Hermanson (Jul 18, 2009)

mburtis said:


> I dont have really any experience in residential but have been trying to learn lately. Recently became aware of 310.12 allowing 2/0 copper for 200 amp residential feeders. My question is if you have a 200 amp main breaker supplying an exterior panel (feeding garage, ac, etc) with feed through lugs, which then feeds a 200 amp interior panel, does 310.12 apply to the feeders between the feed through lugs and the interior panel?


Executive Summary:
NO feeder serving a dwelling unit (either then entire dwelling unit load or not) ever has to be sized larger than 83% of the service size. So if you have a 200 amp service, then no feeder has to have an ampacity larger than 166 amps.

In the old days we could size any dwelling unit feeder to the old table and, for example, use #4 copper for a 100 amp subpanel feed from a 200 amp main panel. It was the ability to down size the feeders to smaller subpanels that was lost.

But for a dwelling unit you most definitely could feed 2/0 copper from the utility drop to a 200-amp meter-main. And then follow that with 2/0 copper from the meter-main into the main house panel. All this even though you might have a 50-amp plug at the meter-main or even a second set of 2/0's feeding a 200-amp panel in a detached garage. Even after all this, your load calculation might still allow that 200-amp service.

Now from a logical standpoint, why on earth would I be allowed to install 2/0 copper upstream from a 200-amp main and only be protected by the utility transformer fuse (basically no protection at all) but be required to use 3/0 copper after the 200-amp main when those conductors would actually be protected by the 200-amp main?
#TwoAughtWiresMatter


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## Djea3 (Mar 8, 2019)

Matt Hermanson said:


> Executive Summary:
> why on earth would I be allowed to install 2/0 copper upstream from a 200-amp main and only be protected by the utility transformer fuse (basically no protection at all) but be required to use 3/0 copper after the 200-amp main when those conductors would actually be protected by the 200-amp main?
> #TwoAughtWiresMatter


The logical answer to your question would seem to be only if the wires to the sub panel need derating by distance and or heat loads etc. I often wonder why some installations are allowed here in FL, an east, south or west facing conduit or panel in direct sun should be derated. Hell I have seen copper lugs not tight after 5 years from heat expansion and contraction, I have seen panels that I could barely touch they were so hot from the sun!
The real question is why does the author use nebulous verbiage in code books? Better to include a chart that allows full understanding than allow interpretation that varies or is confusing at all. An inspector should really never have a different definition than a field electrician. IF they do then the code book is the issue.


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## Dennis Alwon (May 9, 2009)

Djea3 said:


> The logical answer to your question would seem to be only if the wires to the sub panel need derating by distance and or heat loads etc. I often wonder why some installations are allowed here in FL, an east, south or west facing conduit or panel in direct sun should be derated. Hell I have seen copper lugs not tight after 5 years from heat expansion and contraction, I have seen panels that I could barely touch they were so hot from the sun!
> The real question is why does the author use nebulous verbiage in code books? Better to include a chart that allows full understanding than allow interpretation that varies or is confusing at all. An inspector should really never have a different definition than a field electrician. IF they do then the code book is the issue.


Have you ever tried to write a code section? It is remarkably difficult to get the wording just right. I have written a few proposals and I could spend a long time on the wording and then someone would say-- what about this situation? Ugh . Remember the NEC is not written by one person. There are hundreds of people involved plus anyone who wants to write a proposals, of course, it would have to be accepted.


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## Marmathsen (Jun 27, 2013)

Dennis Alwon said:


> Say you have a house that is 400 amps and you have 2- 200 amp panels. Now per your example we could run 2/0 to each panel.





mburtis said:


> Your parallel panels on a 400 amp service is different in my mind, in that case parts a and b would work out to 83% of 400 amps or 332 amps and the parallel feeders would have to be sized for the load they will see.





Dennis Alwon said:


> Agreed but just to be clear those are not parallel conductors assuming you mean feeding 2- 200 amp panels..


Are we all in agreement that a 400 amp residential service where there are two sets of service conductors feeding from the meter (320) or CT to two 200 amp panels wouldn't even qualify for 310.12(A) since they aren't carrying 100% of the load in the dwelling? I have seen inspections fail for making that mistake.

However in a scenario where there is one set of conductors that runs say from a weatherhead to a CT or from a CT to a 400A disconnect, then in that case you can size them at 83% of the service.



Djea3 said:


> The real question is why does the author use nebulous verbiage in code books? Better to include a chart that allows full understanding than allow interpretation that varies or is confusing at all. An inspector should really never have a different definition than a field electrician. IF they do then the code book is the issue.





Dennis Alwon said:


> Have you ever tried to write a code section? It is remarkably difficult to get the wording just right. I have written a few proposals and I could spend a long time on the wording and then someone would say-- what about this situation? Ugh . Remember the NEC is not written by one person. There are hundreds of people involved plus anyone who wants to write a proposals, of course, it would have to be accepted.


Or more to the point, they are written by people and interpreted by people, as infallible as they are. I doubt they deliberately make it nebulous.

Very good thread. This has been rather eye opening for me although wasn't it already in past editions as 310.15(B)(7)(1)-310.15(B)(7)(3)?


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## taglicious (Feb 8, 2020)

I will make a point to come back to this with some pictures. It's been done, legally, with inspectors approval. I worked on a place a couple years ago. 2 homes, a garage, and a shop all off of the same service. I cant remember if it was a 400 but From the flashbacks, I dont see it.


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## Matt Hermanson (Jul 18, 2009)

oldsparky52 said:


> 310.12 stops when the conductors no longer are carrying 100% of the loads. So in your case, no that table would not apply to the conductors from the feed through lugs to the interior panel.
> 
> So, 2/0 allowed from meter to outside MB panel but 3/0 from the feed through lugs to ...
> 
> Crazy, right?


Nope.
Read it again.
No feeder is required to be larger than the 2/0.
Now the 100 amp sub-panel feed from a 200 amp service panel is a different story.


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## Matt Hermanson (Jul 18, 2009)

Dennis Alwon said:


> Ok this is the 2020 nec for those looking for 310.12... it doesn't exist in prior codes.
> 
> IMO, 310.12(C) is saying that the ampacity of the conductor does not have to be greater than the service conductor. So, if you have 2/0 for a dwelling and you want to feed an interior panel at 200 amps but you cannot use the residential rule, then IMO you would use 3/0 because the ampacity of 3/0 is 200 amps and it is not greater than the ampacity of the 2/0 exception.
> 
> Many will disagree but that is how I see it.


It was NOT new in the 2020 edition.
(c) states that no feeder has to be larger than your service conductor calculation at 83%.


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## Dennis Alwon (May 9, 2009)

Matt Hermanson said:


> It was NOT new in the 2020 edition.
> (c) states that no feeder has to be larger than your service conductor calculation at 83%.



I was saying that the section 310.12 did not exist. It was a different section in other editions. Section 310.12 is a new section where they moved an old code section to it.

IOW, the wording was there but it was in a different section. Members looking for 310.12 in a 2017 book will not find it.


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## taglicious (Feb 8, 2020)

240.21 i think if you treat it as taps, it will help on answering this. 408.16 There is a lot of useful info here. Also. What is the bussing rated for? Siemens, typically are rated for 225-residential 200A. Also whatever you choose, will still be protected by the 200A main as your OCPD.


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