250.66

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paul renshaw

paul renshaw

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Cordova, Maryland
In a building with a 400 amp meter can that feeds out into two 200 amp main breaker panels, do you have to add the size of each 3/0 copper together to determine proper GEC from 250.66 or do you use 3/0 as your reference size since they are not in parallel. Note 1 to 250.66 has arisen this question in a discussion at work. thanks for your inputs.
 
Re: 250.66

note 1 to table 250.66 is the proper reference to size the GEC, as exception 2 of 230.40 is the exception that allow us to install two service disconnects/panel from one service drop.

1. Where multiple sets of service-entrance conductors are used as permitted in 230.40, Exception No. 2, the equivalent size of the largest service-entrance conductor shall be determined by the largest sum of the areas of the corresponding conductors of each set.
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Re: 250.66

Originally posted by paul renshaw:
So you would total the area of the two 3/0 and that number would give you the reference for the table.
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This may help

GECtaps.JPG
 
Re: 250.66

Originally posted by wyatt:
Bob does 250.66(a) mean that you can use 6 awg to two ground rods?
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Yes that is exactly what it means, the NEC never requires a conductor only run to a ground rod to be larger than 6 CU regardless of the service size.
 
Re: 250.66

We did a service Chang on a metal building went to the same 400 to two 200 panels as above. they let us keep the old 6 awg ground. but what don't make since to me is had to run 3awg to the building frame and 3awg to 6" of metal pipe stubbed up that changed to plastic. I know it's in the code it just don't make since
 
Re: 250.66

Well it does make sense.

First the rod.

The ground rod simply does not 'connect' to the earth well enough to every require more than 6 AWG CU.

Have ground fault to a rod and you will be lucky to have 20 amps flowing through the rods GEC.

Now the water line.

Remember that they write the code for all situations and it is very possible the metal water line is continuous through out the area and is tied into many electrical services.

If that is true and you have a service fault to the water line that GEC will be subject to all the current the utility can supply and 'squeezes' through your service conductors. That may be thousands of amps for a short time.

The building steel

I am not so sure why that does not have an allowance like rods but it may be they are concerned with a service conductor somehow energizing the building steel, again in that case the building steel GEC would be subject to the full fault current the utility can supply.
 
Re: 250.66

Originally posted by wyatt:
Thanks. I like to learn the why behind the code.
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No problem I hope I helped :)

But also keep in mind as much as I like to think I have the answers I may be wrong. :eek:

Always question and verify for yourself the information you are told by anyone in the trade.

I learn a lot from these forums, when I see something new I look it it up myself to make sure I am getting good info.
 
Re: 250.66

Originally posted by wyatt:
Bob does 250.66(a) mean that you can use 6 awg to two ground rods?
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I am assuming that the two grounding electrodes are connected together. It would be a violation to not connect them together since you are only permitted to have a single grounding electrode system for each building. :D
 
Re: 250.66

Yes, it is nice to see Charlie back. Missed his insight to a lot of the POCO posts for a while now. Welcome back.

Trevor
 
Re: 250.66

iwire, in your example, the service entrance conductors are tapped from the 350's. The building I am referring to has a set of 3/0 cu from the meter to each panel,the panels and the meter are back to back. The existing Gec to building steel and water pipe is sized for 3/0 with a #4. Is that correct or are you supposed to add the 3/0's together and use that number as your reference size in the table? Note 1 is why I ask this, just trying to clarify. Thanks, Paul
 
Re: 250.66

Originally posted by paul renshaw:
iwire, in your example, the service entrance conductors are tapped from the 350's. The building I am referring to has a set of 3/0 cu from the meter to each panel,the panels and the meter are back to back. The existing Gec to building steel and water pipe is sized for 3/0 with a #4. Is that correct or are you supposed to add the 3/0's together and use that number as your reference size in the table? Note 1 is why I ask this, just trying to clarify. Thanks, Paul
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So you have parallel #3/0 feeding the meter?
 
Re: 250.66

The bottom of the meter has double barrel lugs, one set of 3/0 goes to one panel, one set to the other panel. The panels are side by side and the meter has two nipples out of the back containing a set of 3/0's in each going to the separate panels.
 
Re: 250.66

Originally posted by paul renshaw:
The bottom of the meter has double barrel lugs, one set of 3/0 goes to one panel, one set to the other panel. The panels are side by side and the meter has two nipples out of the back containing a set of 3/0's in each going to the separate panels.
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What size service entrance conductors do you have on the line side of the meter?
 
Re: 250.66

Paul don't know if this will help. we did the same set up as you and we had to add the 2 sets together.
 
Re: 250.66

The line side of the meter conductors are irrelevant in this case. They are part of the service lateral, the table says service entrance conductors, which are the ones from the bottom of the meter to the mains in the panels. The meter is the service point and the wires leaving it are the service entrance conductors.
 
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