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01-26-2010, 08:15 AM   #1
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## Box-Fill Calculations

Are my box-fill calculations correct?

(1) 4-gang switch box = 63.5 cu. in. (stamped on inside)
(3) #12/3 BX = 13.5 (6 x 2.25)
(2) #12/3 NM = 9.0 (4 x 2.25)
(4) Dimmer Switches = 9.0 (4 x 2.25)
Largest GND Wire is #12 = 2.25

Total Volume Required = 33.75 (29.75 spare)

Notes:
1. The third wire in each cable group is the ground. From what I've read you are only required to count one (1) ground and that needs to be the largest ground wire in the box.

2. There are no cable clamps in the box.

01-26-2010, 08:20 AM   #2
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Quote:
 (1) 4-gang switch box = 63.5 cu. in. (stamped on inside) (3) #12/3 BX = 13.5 (6 x 2.25) (2) #12/3 NM = 9.0 (4 x 2.25) (4) Dimmer Switches = 9.0 (4 x 2.25) Largest GND Wire is #12 = 2.25
What wire are you using ?
12-3 has red/black/white & ground

20.25 (3) #12/3 BX = 13.5 (9 x 2.25) 3 current carrying per wire
13.5 (2) #12/3 NM = 9.0 (6 x 2.25) 3 current carrying per wire
18 (4) Dimmer Switches = 9.0 (8 x 2.25) count as (2) each
2.25
***********
54 total from what I can see

01-26-2010, 08:26 AM   #3
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Quote:
 What wire are you using ? 12-3 has red/black/white & ground
The 12/3 includes the ground. The BX is BLK//WHT/GRN and the NM is BLK/RED/GND. Yes, I forgot to count each switch as two (2).

UPDATED:

(1) 4-gang switch box = 63.5 cu. in. (stamped on inside)
(3) #12/3 BX = 13.5 (6 x 2.25)
(2) #12/3 NM = 9.0 (4 x 2.25)
(4) Dimmer Switches = 18.0 (8 x 2.25)
Largest GND Wire is #12 = 2.25

Total Volume Required = 42.75 (20.75 spare)

Last edited by MartinD; 01-26-2010 at 08:31 AM.

 01-26-2010, 08:34 AM #4 CodeEnforcer     Join Date: Dec 2009 Location: Massachusetts Posts: 137 Rewards Points: 99 If this is a 4 gand plastic box you cannot use for BX
01-26-2010, 08:36 AM   #5

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Quote:
 Originally Posted by MartinD The 12/3 includes the ground. The BX is BLK//WHT/GRN and the NM is BLK/RED/GND. Yes, I forgot to count each switch as two (2).
Two #12 conductors plus ground is called 12/2 not 12/3.

Quote:
 the NM is BLK/RED/GND
Are sure this isn't BLK/WHT/BARE?

The 42.75ci should be correct.
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01-26-2010, 08:45 AM   #6
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Quote:
 Originally Posted by HIVOLT If this is a 4 gand plastic box you cannot use for BX
Yes, this is a metal box.

01-26-2010, 08:49 AM   #7
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Quote:
 Originally Posted by HouseHelper Two #12 conductors plus ground is called 12/2 not 12/3. Are sure this isn't BLK/WHT/BARE? The 42.75ci should be correct.
Two #12 conductors plus ground is called 12/2 not 12/3.
Thank you for the correction.

Are sure this isn't BLK/WHT/BARE?
Yes, you are correct, it is a bare copper wire. Thank you for helping with the terminology.

Thanks again for all your help.

 01-26-2010, 11:12 AM #8 Electrical Contractor     Join Date: Jun 2004 Location: Newnan GA Posts: 7,027 Rewards Points: 650 On a side note, what is the load going to be on the dimmers? 4 600 watt dimmers in a single box must be derated, and I'm guessing to 300 watts each or less. __________________ "The problem isn't that Hillary Clinton lies. We all know she lies. The problem is that her supporters don't seem to care"
01-26-2010, 02:10 PM   #9
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Quote:
 Originally Posted by jbfan On a side note, what is the load going to be on the dimmers? 4 600 watt dimmers in a single box must be derated, and I'm guessing to 300 watts each or less.
Derated? Could you elaborate?

Dimmer #1: 600W (300W = 4 x 75W)
Dimmer #2: 600W (150W = 2 x 75W)
Dimmer #3: 1000W (600W = 8 x 75W)
3-Speed Fan Switch: 3A (2.5A = 2 x 1.25A)

Could please reference the NEC spec that you are referring to?

 01-26-2010, 03:08 PM #10 CodeEnforcer     Join Date: Dec 2009 Location: Massachusetts Posts: 137 Rewards Points: 99 Actually it is not in the code. It is the manufacturers tech specs. www.lutron.com
01-26-2010, 06:58 PM   #11

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When you place multiple dimmers in a ganged box, you have to break off cooling fins on one or both sides. When you do that, it reduces the wattage rating of the dimmer.

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