# Large junction box calculation

kornbln Member
Join Date: Mar 2008
Location: Sacramento, CA
Posts: 27
Large junction box calculation

I've got 12 NM cables that I need to bring into a junction box and transition to conduit with THHN. Need some help with calculating what size box I need.

I have 2 6/2 aluminum, 1 10/3, 1 10/2, 1 12/3, 2 12/2, and 5 14/2.

The 6/2 aluminum will splice to #8 THHN. What is the box fill for #6 aluminum?

So far what I have is:

10/3 = 3 * 2.50 = 7.50
10/2 = 2 * 2.50 = 5.00

12/3 = 3 * 2.25 = 6.75
12/2 = 2 * 2.25 = 4.50
12/2 = 2 * 2.25 = 4.50

14/2 = 2 * 2.00 = 4.00
14/2 = 2 * 2.00 = 4.00
14/2 = 2 * 2.00 = 4.00
14/2 = 2 * 2.00 = 4.00
14/2 = 2 * 2.00 = 4.00
----------------------
Total: 43.75*2 = 87.50

4 #8 THHN = 4 * 3.00 = 12.00

87.50 + 12.00 = 99.50

Largest ground wire will be #10, so

99.50 + 1 * 2.50 = 102 CU

Couldn't I technically lower that number by discounting some of the neutrals in the MWBCs since they aren't consdiered current carrying?
I don't think I'd actually want to though considering I want extra room in the box to make things easier.

How well do these calculations work when sizing your box? It may all fit according to code, but will it be tough to work with in the real world? I was thinking of just using a 10x10x4 box just to make sure I have plenty of room. I'll have a few polaris connectors in there to splice the AL to CU.

Tolyn Ironhand Group Moderator
Join Date: Nov 2007
Location: United States
Posts: 14,960
I've got 12 NM cables that I need to bring into a junction box and transition to conduit with THHN. Need some help with calculating what size box I need.
You also have to take into account the conduits going to the box. You will need 6 times the size of your largest pipe plus the sum of the other pipes. This is calculated from each side of the box. However, it is exempt if you come into the back of the box.

The 6/2 aluminum will splice to #8 THHN. What is the box fill for #6 aluminum?
#6 is 5 cubic inches.

So far what I have is:

10/3 = 3 * 2.50 = 7.50
10/2 = 2 * 2.50 = 5.00

12/3 = 3 * 2.25 = 6.75
12/2 = 2 * 2.25 = 4.50
12/2 = 2 * 2.25 = 4.50

14/2 = 2 * 2.00 = 4.00
14/2 = 2 * 2.00 = 4.00
14/2 = 2 * 2.00 = 4.00
14/2 = 2 * 2.00 = 4.00
14/2 = 2 * 2.00 = 4.00
----------------------
Total: 43.75*2 = 87.50

4 #8 THHN = 4 * 3.00 = 12.00

87.50 + 12.00 = 99.50

Largest ground wire will be #10, so

99.50 + 1 * 2.50 = 102 CU
Looking at your math you look a little off and I don't understand why you doubled the total. From what I see it looks like you have a total of 48.25 +4 #8 = 60.25 +2 #6 = 70.25 + #10 ground = 72.75

Couldn't I technically lower that number by discounting some of the neutrals in the MWBCs since they aren't consdiered current carrying?
No, you may not. You must add it for box fill which you did above.

How well do these calculations work when sizing your box? It may all fit according to code, but will it be tough to work with in the real world? I was thinking of just using a 10x10x4 box just to make sure I have plenty of room. I'll have a few polaris connectors in there to splice the AL to CU.
In most cases a bigger box is always better. (within reason) Make sure when you splice your AL to Copper you use antioxidant paste and your connector that are rated for Aluminum.

kornbln Member
Join Date: Mar 2008
Location: Sacramento, CA
Posts: 27
You also have to take into account the conduits going to the box. You will need 6 times the size of your largest pipe plus the sum of the other pipes. This is calculated from each side of the box. However, it is exempt if you come into the back of the box.

#6 is 5 cubic inches.

Looking at your math you look a little off and I don't understand why you doubled the total. From what I see it looks like you have a total of 48.25 +4 #8 = 60.25 +2 #6 = 70.25 + #10 ground = 72.75

No, you may not. You must add it for box fill which you did above.

In most cases a bigger box is always better. (within reason) Make sure when you splice your AL to Copper you use antioxidant paste and your connector that are rated for Aluminum.
I doubled the total because I am counting the NM conductors going to the box plus the THHN in conduit. Do I only count one? Show Printable Version Email this Page