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#87466 - 03/11/04 05:01 AM Sizing a Pull/Junction Box
bobwk Offline
Junior Member

Registered: 05/16/03
Posts: 6
Loc: baltimore
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

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#87467 - 03/11/04 05:50 AM Re: Sizing a Pull/Junction Box
watthead Offline
Member

Registered: 10/13/01
Posts: 182
Loc: South Carolina
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.

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#87468 - 03/11/04 09:35 AM Re: Sizing a Pull/Junction Box
Yoopersup Offline
Member

Registered: 03/04/03
Posts: 826
Loc: Michigan
Size of conduits entering and leaving the box, And the sides they enter and leave???

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#87469 - 03/11/04 12:30 PM Re: Sizing a Pull/Junction Box
bobwk Offline
Junior Member

Registered: 05/16/03
Posts: 6
Loc: baltimore
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.

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#87470 - 03/11/04 03:01 PM Re: Sizing a Pull/Junction Box
iwire Offline
Moderator

Registered: 01/05/03
Posts: 4343
Loc: North Attleboro, MA USA
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).]
_________________________
Bob Badger
Construction & Maintenance Electrician
Massachusetts

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#87471 - 03/11/04 05:55 PM Re: Sizing a Pull/Junction Box
watthead Offline
Member

Registered: 10/13/01
Posts: 182
Loc: South Carolina
iwire
the way I read this it is a straight through pull which would require 8 times 3" or 24" width wouldn't it?

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#87472 - 03/11/04 09:47 PM Re: Sizing a Pull/Junction Box
iwire Offline
Moderator

Registered: 01/05/03
Posts: 4343
Loc: North Attleboro, MA USA
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
_________________________
Bob Badger
Construction & Maintenance Electrician
Massachusetts

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#87473 - 03/12/04 09:54 AM Re: Sizing a Pull/Junction Box
winnie Offline
Member

Registered: 09/15/03
Posts: 652
Loc: boston, ma
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

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#87474 - 03/12/04 01:11 PM Re: Sizing a Pull/Junction Box
bobwk Offline
Junior Member

Registered: 05/16/03
Posts: 6
Loc: baltimore
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

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#87475 - 03/12/04 01:31 PM Re: Sizing a Pull/Junction Box
iwire Offline
Moderator

Registered: 01/05/03
Posts: 4343
Loc: North Attleboro, MA USA
I disagree entirely with the inspector.

 Quote:
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.


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)
_________________________
Bob Badger
Construction & Maintenance Electrician
Massachusetts

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