# ATS / Generator question



## JohnR (Apr 12, 2010)

The wire only has to be 115% of the generator rating per 445.13 
" 2008 NEC
445.13 Ampacity of Conductors. The ampacity of the
conductors from the generator terminals to the first distri-
bution device(s) containing overcurrent protection shall not
be less than 115 percent of the nameplate current rating of
the generator. It shall be permitted to size the neutral con-
ductors in accordance with 220.61. Conductors that must
carry ground-fault currents shall not be smaller than re-
quired by 250.30(A). Neutral conductors of dc generators
that must carry ground-fault currents shall not be smaller
than the minimum required size of the largest conductor."

This tells me that if your generator is only 125A then multiply 125X1.15=143.75A 
Then check the 75deg scale T310.16 =1/0 cu.
that would be my install. Can't derate the Neut. either.

Edit: Any wiring that is on your main service to the ats and out to breaker panel will still need to be protected at rating of breaker ahead of them.


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## wildleg (Apr 12, 2009)

I may be mistaken, but I believe that the code article you quoted only applies from the generator motor to the main breaker on the generator.

the conductors from the ats have to be rated for the max current they will carry, and protected accordingly. (might be helpful to sketch your installation and post.)

If your service is 300A, then the conductors from the ATS to the main panel would need to be sized for the 300A service size. Since you are feeding the genset side with 150A I'm guessing you have load shedding ? Am I understanding that the ATS has built in overcurrent protection at 400A ? Hopefully this isn't your main, otherwise these are the wrong size.


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## JohnR (Apr 12, 2010)

After re-reading the article, Yes Wildleg you are correct.:icon_redface:
I left the post up for clarity though.

Well that changes my answer. I would rate the conductors from the 125A breaker to the ATS @ 125A then. 
A sketch would help.


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## yucan2 (Jun 9, 2009)

It sounds like your ATS is also your service disconnect. That being the case, as Wildleg stated, that scenario won't fly without load shed capability and implementation to keep maximum load at or below generator capability.

As for the wiring from the meter pan being rated for 300 amps, that's fine.


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## Shorty Circuit (Jun 26, 2010)

yucan2 said:


> It sounds like your ATS is also your service disconnect. That being the case, as Wildleg stated, that scenario won't fly without load shed capability and implementation to keep maximum load at or below generator capability.
> 
> As for the wiring from the meter pan being rated for 300 amps, that's fine.


Yes Yucan2 that's right. There was a change from the 2005 code to the 2008 code. The connected load without automatic load shedding must be no larger than the generator capacity which in this case should be no more than 100 amps per phase (because the main breaker is 80% rated.) This is true even for standby generators that are not legally required. You can't depend on diversity factor anymore. Evidently too many gen-sets tripped out on overload when the stars lined up all wrong. 

With a manual transfer switch you can do what you want. The code assumes that sufficient load will be shed before the switch is thrown by someone who knows what they are doing :laughing:


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## jsb (Apr 5, 2009)

the original questions was how to size the wire from the meter to the ATS

i understand you to be using the ats as the service disconnect. if so, the wire feeding the meter base and the wire from the meter base to the ats all have to be sized off of the 400A ATS service disconnect.


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## Shorty Circuit (Jun 26, 2010)

"the original questions was how to size the wire from the meter to the ATS"

I've never seen an ATS used as a service disconnect. Is it rated for that service? Whatever, the feeders into it from commercial power and out of it to the load would have to be sized to the upstream protection even if that were calculated from the primary side protection of an upstream transformer as the commercial source. In this case I presume it's 300 amps as that is what was implied as the upstream fuse or breaker.

As I understand it, the service is rated at 300 amps, the meter pan is rated at 400 amps, and the electrically operated breakers inside the ATS are rated at 400 amps. 

More typical would be a 400 amp MDP with a 300 amp MCB and a 125amp subfeed to the ATS. Non critical loads would be fed from the MDP or other subpanels.


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## DavidRoberts (Jun 20, 2010)

jsb said:


> the original questions was how to size the wire from the meter to the ATS
> 
> i understand you to be using the ats as the service disconnect. if so, the wire feeding the meter base and the wire from the meter base to the ats all have to be sized off of the 400A ATS service disconnect.


I am in *110%* agreement with this! I love it when someone agrees with my point of view, makes me feel smart.:thumbup:


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## Shorty Circuit (Jun 26, 2010)

DavidRoberts said:


> I am in *110%* agreement with this! I love it when someone agrees with my point of view, makes me feel smart.:thumbup:


 
If the main service is only 300 amps, why would I need to provide 400 amp feeders downstream of that? Suppose the meter pan were rated at 600 or 800 amps, would I have to provide even larger feeders?


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## yucan2 (Jun 9, 2009)

My original reply was accurate, however a 300 amp disconnect before the ATS would be required. Should have read OP's request more closely. 

He has a 300 amp service. He wants to incorporate a 400 ATS that will become the new service disconnect. 

Increasing the wire size from the meter pan to the ATS to have 400 amp capacity is not the answer. Reason being, the service riser will still have only 300 amp capacity.


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## Mr. Sparkle (Jan 27, 2009)

Now you guys see why I am so confused?:001_huh:


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## yucan2 (Jun 9, 2009)

Mr. Sparkle said:


> Now you guys see why I am so confused?:001_huh:


Always ask questions on Monday's. Check back Tuesday for replies. Again Wednesday for rebuttals. And then again Thursday for detailed modified responses. Schedule all work for Fridays. :laughing:


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## Mr. Sparkle (Jan 27, 2009)

Oh the job is done, finished, complete, inspected, A ok'ed etc....I am just wondering if it is done properly because if it isn't nobody caught it.....:whistling2:


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## Shorty Circuit (Jun 26, 2010)

Mr. Sparkle said:


> Now you guys see why I am so confused?:001_huh:


Oh ho! So that's the reason! And here I thought it was just the general state of affairs :no:


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## jsb (Apr 5, 2009)

wow-im way confused

if you have a wire coming down the side of the building into a meter, then into a transfer switch (service rated) with a 400a breaker-everything is sized off of that 400a breaker and it is a 400a service with the ATS serving as the service disconnect.

if your wire comes down the building into a meter, then into a 300a disc, then into a transfer switch, the transfer switch doesnt even need to contain an OCPD. its a 300a service with wire sized accordingly and the 300a disc is your service disconnect.


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## Shorty Circuit (Jun 26, 2010)

jsb;

"wow-im way confused" 

Mr. Sparkle;

"v=Ok, if I have a Generator with a 125A Breaker going into an ATS that is only available with 400A Breakers for each side and it is feeding twin 150A MB panels does the wire between the ATS and the Meter Pan have to be rated for the 400A ATS Mains or can it be rated for the existing 300A service?

I'm confused."

First of all, what does a 300 amp service mean? It means somewhere upstream there's an OCPD rated at 300 amps. Therefore a 400 amp metering pan is the correct size. This means you don't have to use more than 300 MCM downstream of the OCPD. Mr. Sparkle didn't say if this is a residence or more likely a leased space that is subdivided. That would be my guess. The 300 amp would be the feeder breaker from a large DP. But even if it is the utility transformer it must be from the size of the fuses. (It's not the transformer itself, every utility I've ever dealt with will tell you they don't worry until their transformers reach at least 1,000,000% of their nameplate rating  )

The problem is that if the ATS is the only service disconnect and the MCB on the generator is 125 amps, unless there is automatic load shedding you can't connect more than 100 amps of load (80% of 125) which is how the breaker has to be sized for the generator's continuous rating. The 100 amp connected load comes from 700.5(A) and 702.5(B)(2)...(I'm especially dangerous when I'm in my office and have access to my code book. :cowboy: )

One of the two panels should be connected upstream of the ATS say by a splice. This would allow a total of up to 230 amps load continuous, 250 amps noncontinuous between the two panels. Personally I prefer a single service entrance disconnect such as in this case an L frame unit breaker with a 300 amp trip or a 400 amp fused switch with 300 amp fuses would provide but even this is not necessary if the ATS and the non critical panel are close to each other because of the 6 throw rule. You could also downsize the feeders to the line and load sides of the ATS and non critical panel using the tap rule if it applies. 

BTW, I'm taking both of you off my bidders list. :2guns: In the unlikely event the owner asks why he had to pay for 500 MCM you could try to tell him you were concerned with voltage drop  he probably won't know better anyway, in fact he probably won't know what you are talking about. Or you could just tell him that's the code on Tuesdays.


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