#4 for 100 amp sub panel ?

Got a question
225 amp main panel
Sub Panel 100 amp breaker is a number 4 copper ok to use for a feeder?
seems to a lot of confusion over this one i marked for being under sized and refereed it to a electrician, builder refused and sent it was Ok under
Article 310-15-B6 NEC #4 awg
Buyer want it changed on his report

No good. #4 THHN is only rated for 85 amps at 75° C. This would be for MC cable or conductors in a raceway. SE cable in thermal insulation or NM cable would be rated at 60° C which has an ampacity of 70 amps.

What the InterNachi says is #3 AWG for a remote distribution panel.
19th edition of Canadian Electrical says #3 RX 90XLPE copper

I still stayed with my statement and didn’t change the report. Seems to be a lot of confusion on this topic 600.000 Plus home 5 feet of wire Who knows

Mike Holt Knows and agrees with the #3 and #4 is only good for 70Amps, so Robert is right also.

Thanks guys, I am glad i stuck to my guns lolol

Depending on the connected load of the subpanel you might be able to change the OCPD to a smaller size without changing the conductors.

Thanks Robert, I told them that but i left it up to the Electrician .

There are two charts for wire sizing, one was for when the entire load of the dwelling was served and the other one that was for the other sizings. Many did not distinguish the difference.

The breaker still has to protect the conductor.

Seems like the builder has no idea how to read the codebook. What is the voltage system that this is being used on? That table is very specific in that it is only used for certain dwellings and 1Ø, 120/240 volt systems. Considering you mentioned a 225 amp main panel I would guess that neither of those criteria apply.

Remember…That smaller table [310.15(B)(7)] is only good if you are dealing with 120/240 volt, single phase dwelling service conductors, service- lateral conductors and main power feeders and in your example you have none of those. The client would have to use the traditional ampacity ratings in accordance with Section 310.15(B)(16) to be compliant.

Good lord we are still going around on this . I referred to a electrician, Builder is saying i am wrong and wants it changed in my report. According to this Paul #4 is Under sized Am I right?

Let’s start with the wiring method, is it conductors in a raceway, SE cable, MC cable, NM cable? The differences were listed in post #2.

Here is the panel in question
Main 225
Home built in 2007 ( never lived in)
sub 100 amp
one breaker 60 amp
rest 20 amp
Sub panel is right beside main wire is run through conduit
I am trying to up load a picture but nothing happens
I try later

OK since the conductors are in conduit then it’s likely that they can be used at the 75° C ampacity in Table 310.15(B)(16). #4 AWG is good for 85 amps @ 75° C. Since 85 amps is not a standard size in 240.6 then you can go up to the next standard size which is 90 amps. In order for the “other” table {310.15(B)(7)} to be used those conductors would need to be the main power feeder and they are not.

I agree witt robert…just have them replace the breaker supplying the remote distribution panel with a 90A OCPD and they are fine. (and we will assume the load does not exceed 85 amps and call it a day)

refer to NEC and applicable local codes…

In nutshell, NEC states the environment and also condition for use of #4 gage wire. Under the notes section * it basically states that 120/240 single phase application(home), the 3 wire in conduit could be used for 100amps. See also for deratings guidelines which would make it unacceptable. Sometimes people get too conservative and dont understand that at 4 gage, there are already derating factor for the wire. Keep in mind that the main panel to the subpanel should be protected by fuse or cb and it should blow/trip prior to insulation burning. In case of an industrical application, number 3 gage will have to be used.
Industrial invironments subject to heavy motor loads, so the application is different and its where raceways are used?

Sorry I couldnt paste the 2002 NEC chart, but please read it thoroughly…If I am missing something, please feel free to respond.