125 amp sub panel

[kg18]

The wire is going from house out to sub panel in pipe. 1/0 for the ungrounded conductors #1 for the neutral. Thats what I am seeing in the code book. Thanks

 

[kg18]

Re: 125 amp sub panel

WOW. Let me re phrase that. What size wires do I need for a 125 amp sub panel. Sorry about the other post. Thanks

 

[iwire]

Re: 125 amp sub panel

Most likely I would choose 2 AWG copper.

2 AWG CU is rated 115 amp use 240.4(B) move up to next standard size OCP.

Or if you want AL 1/0.

1/0 AL is rated 120 amp use 240.4(B) move up to next standard size OCP

I can not say what you need for a neutral as I do not know the load.

 

[George Stolz]

Re: 125 amp sub panel

Originally posted by iwire:
...use 240.4(B) move up to next standard size OCP...

I am ready to learn why this is reasonable and not weird. I agree, it's legal to code, but it doesn't make a whole lot of sense.

If I am installing a 125 amp panel, the customer is going to believe they have 125 amps available. If you were to try to explain that they have a 125 rated panel with 115 rated conductors, they will be scared to death that their installation is going to burn down over ten amps.

Try to explain it's legal by the NEC, and they would hear, "I have a loophole in the NEC protecting me. Bring your own marshmellows."

 

[iwire]

Re: 125 amp sub panel

Originally posted by georgestolz:
I am ready to learn why this is reasonable and not weird. I agree, it's legal to code, but it doesn't make a whole lot of sense.

George IMO you have a perfect handle on it.

We can round up the OCP but not the rating of the conductor.

On a fixed load this makes a whole lot more sense than it does on a panel.

IMO You are right using the 2 AWG CU the panel has only a 115 amp capicity. I would not sweat it.

 

[jwelectric]

Re: 125 amp sub panel

Would not the conductors need to be large enough for the calculated load?
In other words the calculated load for the sub panel would need to be 115 amps or less.

 

[kiloamp7]

Re: 125 amp sub panel

I agree with jw that the calculated load would have to be 115A or less.
I agree that the 115A rated #2 cu conductors would be properly protected by a 125A OCPD.

 

[james wuebker]

Re: 125 amp sub panel

Sorry guys but I believe he should be using #1 copper proving all lugs are rated 75c. Yes # 2 copper will get you 115 amps. Yes the load might not get that high but you never know. You wouldn't put # 2 copper in a 200 amp panel even if the load was going to be low. Maybe I'm missing something in this thread. Let me know if I am.
Thanks!
Jim

 

[sceepe]

Re: 125 amp sub panel

For commercial, I would go with the #1's assuming the panel has 125 Amp main. For residential the minimum required by code (#2's) may be more acceptable. Perhaps you could give the owner the choice and see if he is willing to pay for the extra "future capacity".

 

[pierre]

Re: 125 amp sub panel

2 AWG copper conductors are sufficient, exactly as Bob has mentioned. If the calculated load comes to more than 115 amps, but less than 125 amps, than a larger conductor will be necessary for this 125 amp rated panel.
(this would be the bare minimum)

You could always run 500s and be prepared for the future as well )

 

[iwire]

Re: 125 amp sub panel

Originally posted by james wuebker:
Sorry guys but I believe he should be using #1 copper proving all lugs are rated 75c. Yes # 2 copper will get you 115 amps. Yes the load might not get that high but you never know. You wouldn't put # 2 copper in a 200 amp panel even if the load was going to be low. Maybe I'm missing something in this thread. Let me know if I am.
Thanks!
Jim

All you seem to be missing is 240.4(B)

2002 NEC

240.4(B) Devices Rated 800 Amperes or Less. The next higher standard overcurrent device rating (above the ampacity of the conductors being protected) shall be permitted to be used, provided all of the following conditions are met:

(1)The conductors being protected are not part of a multioutlet branch circuit supplying receptacles for cord-and-plug-connected portable loads.

(2)The ampacity of the conductors does not correspond with the standard ampere rating of a fuse or a circuit breaker without overload trip adjustments above its rating (but that shall be permitted to have other trip or rating adjustments).

(3)The next higher standard rating selected does not exceed 800 amperes.

If you personally do not want to use this allowance thats fine, but it is code compliant.

It is very common to find 400 amp services fed with 500 Kcmil copper with a rating of 380 amps.

 

[james wuebker]

Re: 125 amp sub panel

iwire
That's a new area for me. So it's telling me that it's OK to run 2/0 copper for a 200 amp sub panel. To me I would be running 3/0. Now if it's a service then we can use 310.15(B). 240.4(B) it's new meaning to me. Yes, 500 mcm I've seen it run for a 400 amp service. We'll thanks for the insight.
Jim

 

[jeff43222]

Re: 125 amp sub panel

Originally posted by james wuebker:
iwire
That's a new area for me. So it's telling me that it's OK to run 2/0 copper for a 200 amp sub panel. To me I would be running 3/0. Now if it's a service then we can use 310.15(B). 240.4(B) it's new meaning to me. Yes, 500 mcm I've seen it run for a 400 amp service. We'll thanks for the insight.
Jim

Actually, 310.15(B)(6) covers services and feeders, so running 2/0 Cu to 200A subpanel would be fine.

 

[iwire]

Re: 125 amp sub panel

Originally posted by jeff43222:
Actually, 310.15(B)(6) covers services and feeders, so running 2/0 Cu to 200A subpanel would be fine.

True for some installations but that table applies only to dwelling units that are fed from a 240/120 system.

 

[sceepe]

Re: 125 amp sub panel

so running 2/0 Cu to 200A subpanel would be fine.

I want to clarify something. For any service other than residential, single phase 120/240 Delta:

2/0 THNW, 75 deg wire is rated 175 amps and 3/0 THHW, 75 deg wire is rated 200 amps. Since the ampacity of the 3/0 conductors corresponds to the std rating of the 200 amp breaker you can't protect 2/0 with a 200 amp breaker.

For residential, 120/240 Delta only, see 310.15(b)(6) which will let you protect 2/0 with a 200 Amp OCPD.

Agreed?

 

[charlie b]

Re: 125 amp sub panel

Originally posted by jwelectric:
In other words the calculated load for the sub panel would need to be 115 amps or less.

I agree with JW. This is the key question, and it is a question that "kg18" has not yet addressed. You have to start with a load calculation, per 220. Then you size the conductors on the basis of the results of that calculation.

On the other hand, if all you say is that there is a 125 amp panel, if there has been no load calculation, then you have to provide a feeder that is good for 125 amps.We can't leave the customer with, "well, if you add load and go beyond 115 amps, we'll get out the marshmellows."

 

[infinity]

Re: 125 amp sub panel

For residential, 120/240 Delta only, see 310.15(b)(6) which will let you protect 2/0 with a 200 Amp OCPD.

I wouldn't call a residential 120/240 volt system a Delta system. Delta implies that it's a portion of a three phase system. It's simply a center tapped, 3 wire, single phase system.

 

[iwire]

Re: 125 amp sub panel

Originally posted by charlie b:
We can't leave the customer with, "well, if you add load and go beyond 115 amps, we'll get out the marshmellows."

Actually Charlie I believe as an electrician working under the NEC I can do just that.

It will be the responsibility of the electrician adding load to the panel to ensure it has the capacity.

I do not find this any different than the rules for service conductors protected by multiple OCP devices.

230.90(A)Exception No. 3: Two to six circuit breakers or sets of fuses shall be permitted as the overcurrent device to provide the overload protection. The sum of the ratings of the circuit breakers or fuses shall be permitted to exceed the ampacity of the service conductors, provided the calculated load does not exceed the ampacity of the service conductors

Whoever adds load better check what has been installed.

 

[charlie b]

Re: 125 amp sub panel

Originally posted by iwire: It will be the responsibility of the electrician adding load to the panel to ensure it has the capacity.

I agree with this statement, and my earlier statement does not contradict it. What I said is that when the panel is first installed, if we do not know the load (i.e., no load calc), then a 125 amp panel needs a 125 amp feeder (or service) and an overcurrent device (or devices) that will protect the 125 amp conductors.

 

[iwire]

Re: 125 amp sub panel

Originally posted by charlie b:
What I said is that when the panel is first installed, if we do not know the load (i.e., no load calc), then a 125 amp panel needs a 125 amp feeder (or service) and an overcurrent device (or devices) that will protect the 125 amp conductors.

How did I choose a 125 amp panel without having an idea what the load was going to be?

No big deal, we just come at this from different perspectives.