ECN Forum Forums Code Related Discussion NEC & other Code issues Sizing a Pull/Junction Box

You said you are using 4/0s


If you look at parts B, C and D of this you will see no mention of cubic inches.

I would like to see this inspector use his box in this situation. The 3" conduit would not fit inside the 3" dimension. The best part would be watching what he would do with the locknut. On second thought I think I have seen some of his work, and I am sure he was in some way responsible for the electrical test question that says, if the ID of the conduit is larger than the OD will the hole be on the outside.

I believe that the boxes you describe (12" wide, 10" high, 8" deep) with the wires that you describe (2x 4/0 + ground in, 2x 4/0 + ground out, 2x 10 + ground out) could reasonably be used for a safe installation. However I've got to chime with everyone else: it won't meet the NEC.

Consider: if this were a 'cabinet' with internal permanently mounted terminal lugs connecting for connecting the wires, this box would be large enough to provide all the necessary 'wire bending space' needed for 4/0 wires. (See article 312) But this is not a cabinet; it is a junction box.

Additionally, your 3" conduit is rather oversized for the conductors in question. 314.28(A)(3) permits conduit bodies or junction boxes to be smaller than that ordinarily required by a given conduit, provided that the junction boxes be 'listed for and permanently marked with the maximum number and maximum size of conductors permitted.' My guess is that the particular boxes that you've found are not so listed...but I bet that they are large enough that they could be listed if you wanted to pay the right testing lab My _hunch_ is based on the size in comparison to wire bending space in article 312, and size of an appropriate box for the _minimum_ conduit which would hold these conductors.

Question: Why use 3" conduit? By oversizing the conduit, you are forced to oversize all the other components. If we presume that you have to pull 3 4/0 conductors (two circuit conductors and an oversized ground conductor), then the minimum conduit size is 2". If you were to use 'compact' conductors, then you may even be able to use 1.5" conduit...though this depends upon which insulation you are using, and pulling the conductors could be a real pain. Depending upon circuit amp rating, the ground may be smaller than 4/0, freeing up some space.

But if you use smaller conduit, then you can use smaller conduit bodies and junction boxes, without any sort of interpretation or UL evaluation. If the oversize conduit is to provide future expansion capability, then the undersized boxes will prevent this. So why not properly size the conduit, and then properly size the smaller junction boxes.

-Jon

Just a reminder if this a 20 or 30 amp circuit the "over sized" ground will be NEC (2002) required to be 4/0 if the ungrounded conductors are 4/0s.

250.122(B)

I also do not think you want to try to work 4/0s in a 10" wide box.

Uh-oh.
Bob, I think that the EGC would only have to be increased proportionally to the increase of the circuit conductors, thus a #4.
Bomb me, buddy!
bobwk, I agree with the rest. Your inspector is wrong, unless he/she is willing to cite 90.4...S

From the original post.


If this is fed from a 15, 20 or 30 amp circuit the EGC will have to be a 4/0 also.

The EGC for these size circuits is a 1 to 1 ratio and remains 1 to 1 no mater how big you make the ungrounded conductors.

Bob

Read 314.28 (A)(2) carefully. The last paragraph says;

"When transposing cable size into raceway size in 314.28(A)(1) and (A)(2), the minimum metric designator (trade size) raceway required for the number and size of the conductors in the cable shall be used."

In other words you size the box off the minimum conduit size allowed for the wire, which is 2" and still run the oversized 3" conduit. Oversizing the conduit does NOT mean you have to oversize the pull box.

So the minimum box size is 2"x8=16"
and 7 1/2". Something like 16x12 or 16x16 is probably more reasonable.

GJ

[This message has been edited by golf junkie (edited 03-14-2004).]

GJ that is for sizing the box when using cables no raceways.

In that case you would find the smallest allowable raceway for the cable you are using and base the box size from that.

If you have 3" raceways entering the box you must size the box based on those.

I still will say 24" x 7.5" x 4" is the absolute smallest dimensions allowed by the NEC for this installation.

Bob

Oh NO, iwire
It looks you're right.
This is a goofed up part of the Code.
Agreed?

What's goofed up about it?

These 4/0s fed from a 15, 20, or 30 amp breaker need a 4/0 EGC run with them.

Change the breaker to a 100 amp and the EGC run with the 4/0s would only have to be a 1 AWG (actually within 10 cir mils of a 2 AWG)

Why would that make us scratch our head?

Almost worth while using a 100 amp breaker and placing in-line fuses at each pole base.

Bob