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National Electric Codes - Wire in Electrical Box NEC-Table370-16a

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Summary: The National Electrical Code explains the Maximum Number of Wires that can be installed into a box, otherwise known as Box Fill. This code is based upon the type of box, wires, wire sizes, wire clamps and conduit fittings. Adjustments are made for the ground wire as you will see in the table below.

Calculating The Minimum Size Junction Box for Electrical Wiring

Wire in Electrical Box - Box Fill Calculations

Part (1) of Section 370-16(a) describes in detail the method of counting wires, as well as clamps, fittings, or devices (i.e., switches, receptacles, combination devices) - by establishing an equivalent conductor-value for each.

These values are added together to get a total number of conductors.

The minimum size box is the smallest one in the Box Fill Table (shown) that can accommodate that number of conductors.

BOX FILL TABLE

DEDUCTIONS

Volume Required per Conductor Size per Deductions	#18 (1.50)	9.00	10.50	12.00	13.50	15.00	16.50	18.00	19.50	21.00
	#16 (1.75)	10.50	12.25	14.00	15.75	17.50	19.25	21.00	22.75	24.50
	#14 (2.00)	12.00	14.00	16.00	18.00	20.00	22.00	24.00	26.00	28.00
	#12 (2.25)	13.50	15.75	18.00	20.25	22.50	24.75	27.00	29.25	31.50
	#10 (2.50)	15.00	17.50	20.00	22.50	25.00	27.50	30.00	32.50	35.00
	#8 (3.00)	18.00	21.00	24.00	27.00	30.00	33.00	36.00	39.00	42.00
	#6 (5.00)	30.00	35.00	40.00	45.00	50.00	55.00	60.00	65.00	70.00

Points to Remember:

No matter how many ground wires come into a box, they only count as one conductor within the box.
Any wire running unbroken through the box counts as one wire.
Each wire coming into a splice device (crimp or twist-on type) is counted as one wire.
Each wire coming into the box and connecting to a device counts as one wire of that size.
Where devices are mounted in the box, the total conductor count must be increased by two for each mounting strap because the strap takes up space.

NEC Table 370-16(a). Metal Boxes
Number of Conductors in Outlet, Device, and Junction Boxes
Box Dimension in Inches Trade Size or Type	Minimum Capacity (in.3)	Maximum Number of Conductors
Box Dimension in Inches Trade Size or Type	Minimum Capacity (in.3)	No. 18	No. 16	No. 14	No. 12	No. 10	No. 8	No. 6
4 X 1-1/4 round or octagonal 4 X 1-1/2 round or octagonal 4 X 2-1/8 round or octagonal	12.5 15.5 21.5	8 10 14	7 8 12	6 7 10	5 6 9	5 6 8	4 5 7	2 3 4
4 X 1-1/4 square 4 X 1-1/2 square 4 X 2-1/8 square	18.0 21.0 30.3	12 14 20	10 12 17	9 10 15	8 9 13	7 8 12	6 7 10	3 4 6
4-11/16 X 1-1/4 square 4-11/16 X 1-1/2 square 4-11/16 X 2-1/8 square	25.5 29.5 42.0	17 19 28	14 16 24	12 14 21	11 13 18	10 11 16	8 9 14	5 5 8
3 X 2 X 1-1/2 device 3 X 2 X 2 device 3 X 2 X 2-1/4 device 3 X 2 X 2-1/2 device 3 X 2 X 2-3/4 device 3 X 2 X 3-1/2 device	7.5 10.0 10.5 12.5 14.0 18.0	5 6 7 8 9 12	4 5 6 7 8 10	3 5 5 6 7 9	3 4 4 5 6 8	3 4 4 5 5 7	2 3 3 4 4 6	1 2 2 2 2 3
4 X 2-1/8 X 1-1/2 device 4 X 2-1/8 X 1-7/8 device 4 X 2-1/8 X 2-1/8 device	10.3 13.0 14.5	6 8 9	5 7 8	5 6 7	4 5 6	4 5 5	3 4 4	2 2 2
3-3/4 X 2 X 2-1/2 masonry box / gang 3-3/4 X 2 X 3-1/2 masonry box / gang	14.0 21.0	9 14	8 12	7 10	6 9	5 8	4 7	2 4