Distribution panel size

[Chuck Lambert]

Ok, how do you report the size of distribution panels? By the capacity of the breaker feeding it ? or By the max breaker capacity that can feed it?

Thanks,
Chuck

[Brandon Whitmore]

By the the lesser of the wire rating, main breaker size, or panel rating.

[Chuck Lambert]

By the the lesser of the wire rating, main breaker size, or panel rating.

Brandon,
So which is it a panel rated at 200 AMPS with a 150 AMP breaker feeding it with a 1/0 copper conducter.

So is it a 200, 150 or a 125 amp panel?

Chuck

[Jim Port]

Brandon,
So which is it a panel rated at 200 AMPS with a 150 AMP breaker feeding it with a 1/0 copper conducter.

So is it a 200, 150 or a 125 amp panel?

Chuck

1/0 CU is rated at 175 amps per table 310.15(B)(6) for a service.

[Jerry Peck]

Ok, how do you report the size of distribution panels? By the capacity of the breaker feeding it ? or By the max breaker capacity that can feed it?

I reported it as I found it:
- feeder size and rating, i.e., feeder conductors: 1/0 copper feeders/175 amp rating
- panel rating as shown on the panel, i.e., panel rating shown as: 150 amp rated mains
- breaker size feeding it, i.e., fed from: 150 amp breaker
- or
- breaker size feeding it, i.e., fed from: 175 amp breaker - BREAKER IS OVERSIZED FOR THE PANEL, MAXIMUM BREAKER SIZE PROTECTING THIS PANEL SHOULD BE 150 AMPS OR LESS
- NOT saying it was a "150 amp panel" and implying a rating of the INSTALLATION

[Chuck Lambert]

Thanks Jerry sounds good!!

Chuck

[Matt Fellman]

JP's way is much more informative but if you're looking for just one number it's the "chain is only as strong as the weakest link" idea.

Try to not think of it as some little guy at the power pole "allowing" a certain amount of electricity. In reality, you can pull 200 amps through a 150 amp panel and wire.... it's just not safe to do (and, hopefully, the breaker is sized right and will trip). The rating is the lowest of the three factors (panel rating, wire size, main breaker).

[Jerry Peck]

JP's way is much more informative but if you're looking for just one number it's the "chain is only as strong as the weakest link" idea.

Which is precisely where HIs have allowed themselves to become trapped.


Try to not think of it as some little guy at the power pole "allowing" a certain amount of electricity. In reality, you can pull 200 amps through a 150 amp panel and wire.... it's just not safe to do (and, hopefully, the breaker is sized right and will trip). The rating is the lowest of the three factors (panel rating, wire size, main breaker).


"The rating is the lowest of the three factors (panel rating, wire size, main breaker)."

"The rating" ...

WHICH "rating"?

The "allowable" rating for use?

The "allowable" rating before the overcurrent trips?

The rating at which it burns up?

Case in point - the example given:


I reported it as I found it:
- feeder size and rating, i.e., feeder conductors: 1/0 copper feeders/175 amp rating
- panel rating as shown on the panel, i.e., panel rating shown as: 150 amp rated mains
- (deleted this line)
- or
- breaker size feeding it, i.e., fed from: 175 amp breaker - BREAKER IS OVERSIZED FOR THE PANEL, MAXIMUM BREAKER SIZE PROTECTING THIS PANEL SHOULD BE 150 AMPS OR LESS
- NOT saying it was a "150 amp panel" and implying a rating of the INSTALLATION

175 amp breaker protecting 175 amp feeders to a 150 amp rated panel, which makes "the panel rating" the lowest of the ratings and simply calling that as a 150 amp rated panel is asking for trouble.

Instead, call it out for the breaker being oversized for the panel and forget about stating any rating for the panel, after all, you have already acknowledged that "the system" is "the rating", not the panel, so to say it is a "150 amp rated panel" is wrong when in fact "the panel" may actually be 225 amp rated. Trying to "rate" the installed system by then saying it is a "150 amp rated panel" is incorrect. The correct way would be to say "the breaker protecting the feeders is shown as xxx amps rating, the feeders are xx size/rated xxx amps, and the panel is shown as xxx amps rated" as that is "the system" you are reporting on.

Also, by stating "is shown as" you are reporting *what the manufacturer* has rated the breaker, not your, the feeders would be backed up by the code (oh-oh, that word many HIs refuse to get involved with), and the panel is (again) shown to be rated as that by the manufacturer, not you.

What if you had a 60 amp breaker protecting a 150 amp feeder to a 150 amp rated panel, and your client wants to upgrade to a 150 amp rated panel?

Yep, the panel ALREADY IS 150 amp rated, says so right on it, so the electrician points out that the HI SHOULD HAVE SEEN that, and that the feeders are ALREADY rated for 150 amp and the HI SHOULD HAVE KNOWN that too, and, gosh, all you have to do is replace the breaker 60 amp breakers with a 150 amp breaker and you have it all rated at 150 amps ... the electrician could very easily make you, the HI, look like you had no idea what you were doing. How would that affect what your client now thinks about everything else you looked at and reported? Like you did not know what you were doing.

HIs have a tendency to try to "dumb down" the a "single rating" as much as they can, when in reality doing so is not correct and can lead to errors.

Just the facts, ma'am, just the facts.

[Matt Fellman]

HIs have a tendency to try to "dumb down" the a "single rating" as much as they can, when in reality doing so is not correct and can lead to errors.


Every HI text book I've read and every other HI I've talked to accepts the 'rating' of a house's service to the lowest of the 3 items discussed. This really isn't that complicated.

[John Kogel]

Every HI text book I've read and every other HI I've talked to accepts the 'rating' of a house's service to the lowest of the 3 items discussed. This really isn't that complicated.Not this HI. :D
Why not just call it panel amperage? That's the size of the main breaker, if it has one.
Panel rating, that's the MAX ALLOWable for that panel, not what was installed.

Then if service is oversized, almost never the case, then that needs to be described as something else.
I had oversized service wires the other day, but it's lateral (undergound) wiring, so I don't know what the supply from the transformer looks like. I only saw big 4/0 or larger wires coming from the meter, and a 150 Amp breaker installed. Panel max rating is 225 amps. I told the client "it looks like upsizing to 200 amps would be relatively easy, but an electrician would need to check it out".

[James Duffin]

In my reports the service rating is the size of the single main breaker if the panel has one and if it does not have a single main breaker then I base the size of the service on the wire size feeding the main lugs.

[dana1028]

1/0 CU is rated at 175 amps per table 310.15(B)(6) for a service.

I'd like to clarify a point that is often misunderstood - the statement above is correct for a service.

1/0 is not permitted to serve just any feeder. The original poster indicated this 1/0 was feeding a distribution panel,he did not indicate this was the main power feeder.

There is a difference and the code over several cycles has been slightly changing the language in NEC 310.15(B)(6) the clarify that this table cannot be used for other than main power feeders.

Example
A 200A main service panel with multiple circuit breakers has a 150A, 2-pole breaker feeding another distribution panel....this 'other' distribution panel is not the 'main' distribution [the service is] and thus cannot use Table 310.15(B)(6) to size the feeder conductors.

A 200A main service panel that has just a main disconnect [not multiple breakers] that feeds the main distribution panel within the dwelling can use Table 310.15(B)(6) to size the feeder.

BTW - see section 110.14(C)(1) for the instructions on conductor ratings with respect to terminations.