# Size feeder for Resi 200A panel. 4/0? - tricky



## bullheimer

i guess unless you know the code for allowing 4/0 to feed a 200A panel, you might just say 4/0 since that's what we 'always' use.

but the way i read that exception that allows you to derate the service 83%, the ENTIRE resi load has to come from the panel you are feeding.

What i would like to do is put a mobile home type meter/main on the outside of the house and then feed the main panel (making it a sub panel) from it with 4/0.

This WILL work if i run ALL the circuits from there....it fits the definition.

What i WANT to do is feed the A/c Heat Pump from the outside Main panel because it's a straight shot down the side of the house. However, if i do that, then the Entire load will not come from the Sub and i will be forced to use 250 mcm to feed my sub panel. the heat pump is outside the house, but i dont think that matters.

Am I reading this right or not? somebody give me some exception to the "entire house" rule if there is one. thanks.

ps the other way around it is just mount a meter base outside and the main on the inside wall as per usual. but this garage is huge with 10' ceilings, hence my desire to use a sub closer to all the circuits in the house. also WA state is on the 2017 code


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## Arrow3030

You're reading it right. It's a silly rule in many ways.

My helpful input is, do you really need the sub to be 200 amps since the AC isn't off It?

Also, feeders and branches never have to be larger than the service conductors. That may be your best loop hole


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## Dennis Alwon

It is ridiculous that if you take some load off the interior panel then you have to upsize the wire. However, depending on your code cycle 4/0 ser may be used at 75C which would be rated 180 amps. 

Article 240.4(B) allows me to upsize the breaker to 200 amps as long as the load doesn't exceed 180 amps. In most cases you may be able to use 4/0

2017



> 338.10(B)(4) Installation Methods for Branch Circuits and Feeders.
> (a) Interior Installations. In addition to the provisions of
> this article, Type SE service-entrance cable used for interior
> wiring shall comply with the installation requirements of Part II
> of Article 334, excluding 334.80.
> For Type SE cable with ungrounded conductor sizes 10 AWG
> and smaller, where installed in thermal insulation, the ampacity
> shall be in accordance with 60°C (140°F) conductor temperature
> rating. The maximum conductor temperature rating shall
> be permitted to be used for ampacity adjustment and correction
> purposes, if the final derated ampacity does not exceed
> that for a 60°C (140°F) rated conductor.


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## Helmut

Dennis Alwon said:


> It is ridiculous that if you take some load off the interior panel then you have to upsize the wire.


Explain this please?


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## Dennis Alwon

Helmut said:


> Explain this please?


The code states that you can use 83% of the service size to size your service conductors. So a 200 amp service would require a 166 amp conductor- 2/0 copper is rated at 175 amps at 75C so it can be used as the service conductors. This section also allows any feeders that carry the entire load of the dwelling to be sized the same way, therefore, a 2/0 copper can be run to a 200 amp subpanel as long as that panels carries all the load.

If one installs an a/c circuit from the main service panel then the wire to the sub panel can no longer use the 83% and must be sized to 3/0


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## HackWork

Helmut said:


> Explain this please?


The provision that allows us to use 4/0 for 200A does so if the entire dwelling unit is being fed by it, such as a house service, or feeder for an entire apartment subpanel.

But if you take one of the loads off of that feeder, such as the OP wants to do, you can no longer use that provision which allows you to use the smaller conductor.


There used to be a table which specified if the service or feeder was powering an entire dwelling unit, you could use 4/0 al or 2/0 cu for 200a and #2 al or #4 cu for 100A, but they convoluted that into some stupid 83% rule.l.

The code keeps getting worse and worse, they refuse to clear up known issues and change things that were clear into confusion.


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## Helmut

So, he could install a 150A meter main, leave the 4/0 Alum, and then tap off the MB with a fusible disconnect for the AC, no?


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## HackWork

Helmut said:


> So, he could install a 150A meter main, leave the 4/0 Alum, and then tap off the MB with a fusible disconnect for the AC, no?


I'm not sure about that. 

But as Dennis described in his first post, you can often still get away with using 4/0 al on a 200A breaker as long as the calculated load is less than 180A. 

Another very convoluted rule. The _calculated_ load is meaningless, it's just a guess. Either 4/0 can handle 200A or it can't. :vs_mad:


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## Dennis Alwon

Helmut said:


> So, he could install a 150A meter main, leave the 4/0 Alum, and then tap off the MB with a fusible disconnect for the AC, no?


Yeah but now you have a 150 amp service. Why would you do that?


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## Helmut

Dennis Alwon said:


> Yeah but now you have a 150 amp service. Why would you do that?


A/c load is off that panel, How much do you need?

He wouldn't need 250 mcm either.


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## bullheimer

Arrow3030 said:


> You're reading it right. It's a silly rule in many ways.
> 
> My helpful input is, do you really need the sub to be 200 amps since the AC isn't off It?
> 
> Also, feeders and branches never have to be larger than the service conductors. That may be your best loop hole



i like that option the best, would love to find that Article. as well as the info on the 200A breaker for a 180A load. Although i'm sure around here they would want a $20 Plaque saying the conductors were only rated at 180A

however i guess i no longer need to know, since the owner only wanted to pay me $500 for material to run the sub, so there is NO money in there for labor. my only incentive is shorter home runs, but i think the extra work installing a combo meter out front, S.E.R and a sub panel is more work than that. I dont look forward to those home runs across the garage primarily because it's an 11 foot ceiling. however the whole inside of the house is 10' ceilings, so i wont be enjoying any of it.

thanks for all these great answers man. i'll keep them stored in the middle of my brain!


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## Arrow3030

310.15(b)(7) is what to read.
nec 2014


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## sbrn33

Thing is that most guys want to use a Milbank 8/16 feedthru since it is the cheapest. Problem it is a 200 amp feed thru. It is just a really stupid code that "our" code committee didn't think thru and now they are to proud to admit they ****ed up. 
Sounds like our politicians.


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## Wiresmith

i believe you all are assuming some things incorrectly. this part of the code is perfectly logical.

you have to remember there are a lot more pages in the book than the one you are reading and they are all related.


-240.4 conductors shall be protected against over-current in accordance with their amapcities. 

-310.15(B)(7) is not raising the ampacity of a conductor, it allows you to not use the full capacity of the service(how much current the utility will provide to you)

- i believe you are confusing branch circuit rating and service rating, branch circuits are rated based on ocpd


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## Wiresmith

read blue text on page 262 of handbook

the ocpd is based on the wire ampacity (which is based on the load) not the other way around

the service size is not your main breaker in the panel


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## bullheimer

i absolutely will read that. not that i will be putting that into use on this job. he declined to accept my bid. i've been told he'll do it himself. have fun. but i will learn something when i read this i'm sure. right now i got to get some tax stuff together.


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