This will be all new construction. We are putting light pole fixtures on top of a bridge parapet wall. Below each pole fixture we are putting a Pull/Junction Box with cover plate in the parapet wall. The question is how big does the box need to be ? We want to minimize the size but it must comply with NEC. Each box will have 2 #4/0 + ground wire in a 3" horizontal conduit entering and the same exiting the box. Inside the box 2 # 10 + ground will be spliced with the # 4/0 + ground wires and the # 10 + ground will exit the top of the box in a 1.25" conduit to feed the light pole on top of the parapet wall. What is the minimum size ( length X width X depth) box we can use that will meet NEC ? Thank You

Box size is determined by straight or angle pulls and size and number of conduits entering the box. Check NEC article 314.28 (A)(B)&(C)as these provide the formula for box sizing.

Size of conduits entering and leaving the box, And the sides they enter and leave???

one 3" horizontal conduit will enter left side of box--one 3" horizontal conduit will exit right side of box--one 1.25" vertical conduit will exit top of box. That's all. Thanks for your help.

The minimum size would be 18" wide by 7.5" high.

You might get lucky and find a stocked box 18" x 12" but I doubt it.

A standard size will be 18" x 18"

The depth can be as shallow as want and still get the 3" raceways in.

If this really has to look good you may want to have some made 18"W x 10"H x 6"D

You may already know this but I would not cut the 4/0s I would tap them with some sort of connector.

If you can find a 4/0 to 10 awg hypress H tap you would really have a trouble free connection.

Also if this is a 30 or 20 amp circuit and you are under the 2002 code the grounding conductor will need to be 4/0 when run with the 4/0s and 10 awg when run with the 10 AWGs

If the circuit is higher than 30 the EGC will still need to be increased in proportion to the ungrounded conductors.

Yes the grounding conductor for a 20 or 30 amp circuit may be required to be larger than the grounding conductor for a 40 amp circuit.

Read 250.122(B) carefully for this info.

[This message has been edited by iwire (edited 03-11-2004).]

iwire
the way I read this it is a straight through pull which would require 8 times 3" or 24" width wouldn't it?

Doh! I figured as all angle pull (6x) but you are right about the straight through sizing.

So make that 24" x 7.5" minimum.

Perhaps you can use a piece of 24" x 10" x 10" trough.

Thanks for pointing that out so bobwk gets the right size box.

Bob

Hi,

I was wondering if we could have more details on this, eg. distances involved, VA per light pole, voltages, poles per circuit etc.

-Jon

First of all I want to thank everyone who took time to answer my question about sizing a box.

Probably I should have stated this question differently. We are trying to minimize the box since it is going to be installed in the bridge parapet wall and the bridge structural engineer needs to design his wall around the box. We found a box that is 12" horizontal X 10" vertical and 8" deep and the bridge structural engineer can live with that size so we are hoping that the 12" X 10" X8" box meet NEC.

Since I first posted this question I was able to talk to an Electrical Inspector. This is what he said. For the 3" conduits--3" X 8 = 24" For the 1.25" conduit--1.25" X 6 = 7.5" Minimum depth = size of largest conduit entering the box so depth = 3". Minimum size box = 24" X 7.5" X 3" = 540 cu. in. He said the bottom line is you need the cu. in. volume but you can change the size of length, width or depth as long as you maintain the minimum cu. in. volume of 540 cu. in. I told him we wanted to use the 12" X 10" X 8" box whcih equals 960 cxu.in. and he said the 12" X 10" X 8" box will meet NEC. I told him the wire sizes we are using and he said that we will not have problems with the 12" X 10" X 8" box.

Does anybody disagree ? Thanks

I disagree entirely with the inspector.


The minimum size is 24"W x 7.5"H x 4" deep.

You can not put a 3" raceway in a 3" box and you can not make the minimum dimensions smaller by increasing the depth to keep the cubic inches the same.

The only way to use a smaller box is to use smaller raceways.

It sounds to me like the inspector is not used to sizing cans with raceways.

All that aside if he is going to approve it, I guess the NEC does not matter.

I would also be surprised if he understands 250.122(B)

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

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

Was the wording changed from '99 to the '02 code? I'm sure that the AHJ here in Nebraska was allowing boxes sized on conductor size a while back.

GJ

The Nebraska inspector must be a cousin of the one that sizes pull boxes based on the cubic inch formula. There are inspectors out there that are human and don't know everything about the NEC. I have been in the trade almost 23 years and believe it or not I still learn something new about the code on an almost daily basis. Lately a good bit of that is thanks to you guys here at ECN.

"There are inspectors out there that are human"

Hey, let's not be starting that rumor!

GJ