40 Amp Wire Guide

To ensure electrical safety and reliability in 40 ampere current applications, it is critical to select and calculate the correct wire thickness. This article will explore in detail the principles and methods of selecting the right wire.

Catalog

AWG Wire Size Reference

Below are some of the most common AWG wire thicknesses as well as wire ampacity (wire current carrying capacity) and wire sizes:

AWG #	Diameter (mm/inches)	Area (mm2/in2)	Resistance (Copper) (mΩ/m;mΩ/ft)	Ampacity (A)
				@60°C/140°F	@75°C/167°F	@90°C/194°F
4/0 (0000)	11.6840 0.4600	107.2193 0.1662	0.1608 0.04901	195	230	260
3/0 (000)	10.4049 0.4096	85.0288 0.1318	0.2028 0.06180	165	200	225
2/0 (00)	9.2658 0.3648	67.4309 0.1045	0.2557 0.07793	145	175	195
AWG 0 (1/0)	8.2515 0.3249	53.4751 0.0829	0.3224 0.09827	125	150	170
1	7.3481 0.2893	42.4077 0.0657	0.4066 0.1239	110	130	145
2	6.5437 0.2576	33.6308 0.0521	0.5127 0.1563	95	115	130
3	5.8273 0.2294	26.6705 0.0413	0.6465 0.1970	85	100	115
AWG 4	5.1894 0.2043	21.1506 0.0328	0.8152 0.2485	70	85	95
5	4.6213 0.1819	16.7732 0.0260	1.028 0.3133	-	-	-
AWG 6	4.1154 0.1620	13.3018 0.0206	1.296 0.3951	55	65	75
7	3.6649 0.1443	10.5488 0.0164	1.634 0.4982	-	-	-
AWG 8	3.2636 0.1285	8.3656 0.0130	2.061 0.6282	40	50	55
9	2.9064 0.1144	6.6342 0.0103	2.599 0.7921	-	-	-
AWG 10	2.5882 0.1019	5.2612 0.0082	3.277 0.9989	30	35	40
11	2.3048 0.0907	4.1723 0.0065	4.132 1.260	-	-	-
AWG 12	2.0525 0.0808	3.3088 0.0051	5.211 1.588	20	25	30
13	1.8278 0.0720	2.6240 0.0041	6.571 2.003	-	-	-
AWG 14	1.6277 0.0641	2.0809 0.0032	8.286 2.525	15	20	25
15	1.4495 0.0571	1.6502 0.0026	10.45 3.184	-	-	-
16	1.2908 0.0508	1.3087 0.0020	13.17 4.016	-	-	18
17	1.1495 0.0453	1.0378 0.0016	16.61 5.064	-	-	-
AWG 18	1.0237 0.0403	0.8230 0.0013	20.95 6.385	10	14	16
19	0.9116 0.0359	0.6527 0.0010	26.42 8.051	-	-	-
20	0.8118 0.0320	0.5176 0.0008	33.31 10.15	5	11	-
21	0.7229 0.0285	0.4105 0.0006	42.00 12.80	-	-	-
22	0.6438 0.0253	0.3255 0.0005	52.96 16.14	3	7	-
23	0.5733 0.0226	0.2582 0.0004	66.79 20.36	-	-	-
24	0.5106 0.0201	0.2047 0.0003	84.22 25.67	2.1	3.5	-
25	0.4547 0.0179	0.1624 0.0003	106.2 32.37	-	-	-
26	0.4049 0.0159	0.1288 0.0002	133.9 40.81	1.3	2.2	-
27	0.3606 0.0142	0.1021 0.0002	168.9 51.47	-	-	-
28	0.3211 0.0126	0.0810 0.0001	212.9 64.90	0.83	1.4	-
29	0.2859 0.0113	0.0642 0.0001	268.5 81.84	-	-	-
30	0.2546 0.0100	0.0509 0.0001	338.6 103.2	0.52	0.86	-
31	0.2268 0.0089	0.0404 0.0001	426.9 130.1	-	-	-
32	0.2019 0.0080	0.0320 0.0000	538.3 164.1	0.32	0.53	-
33	0.1798 0.0071	0.0254 0.0000	678.8 206.9	-	-	-
34	0.1601 0.0063	0.0201 0.0000	856.0 260.9	0.18	0.3	-
35	0.1426 0.0056	0.0160 0.0000	1079 329.0	-	-	-
36	0.1270 0.0050	0.0127 0.0000	1361 414.8	-	-	-
37	0.1131 0.0045	0.0100 0.0000	1716 523.1	-	-	-
38	0.1007 0.0040	0.0080 0.0000	2164 659.6	-	-	-
39	0.0897 0.0035	0.0063 0.0000	2729 831.8	-	-	-
40	0.0799 0.0031	0.0050 0.0000	3441 1049	-	-	-

Please note that the current carrying capacity in the table is for enclosed wires at an ambient temperature of 30°C. Typically, the wire's surface temperature should not exceed 60°C in most installations. For example:

- At 60°C/140°F, an 8-gauge wire has a current-carrying capacity of 40A.

- At 75°C/167°F, an 8-gauge wire supports 50A.

- At 90°C/194°F, a 10-gauge wire manages 40A.

The 80% Rule

When targeting improved electrical safety, the 80% rule is crucial. It suggests leaving a safety margin when identifying the wire for carrying a specific current. The formula for this is:

\[ I_{required} = \frac{Current}{0.8} = \frac{40A}{0.8} = 50A \]

Thus, a wire with a 50A current carrying capacity is necessary. Here, specific examples include:

- At 60°C/140°F, a 6-gauge wire has a capacity of 55A.

- At 75°C/167°F, an 8-gauge wire supports 50A.

- At 90°C/194°F, an 8-gauge wire reaches 55A.

Interestingly, opting for a 6-gauge wire (AWG 6) generally enhances safety. This recommendation stems from extensive practical validation across multiple industries.

40 Amp cord length

Does wire length impact current carrying capacity? Absolutely. The wire length does affect current loss, necessitating an increase in capacity by 10% for every 50 feet of extension. Here’s what this looks like in practice:

- For 50 feet: \( capacity_{50ft} = 50A \times 1.1 = 55A \)

- For 100 feet: \( capacity_{100ft} = 50A \times 1.2 = 60A \)

- For 150 feet: \( capacity_{150ft} = 50A \times 1.3 = 65A \)

- For 200 feet: \( capacity_{200ft} = 50A \times 1.4 = 70A \)

Now we must check the AWG value required for a given wire length against the wire surface temperature - the relevant values are given in the table below:

Wire Length / Surface Temperature	@60°C/140°F	75°C/167°F	90°C/194°F
Default (40 Amps)	AWG 8 (40 Amps)	AWG 8 (50 Amps)	AWG 10 (40 Amps)
<50 feet (50 Amps)	AWG 6 (55 Amps)	AWG 8 (50 Amps)	AWG 8 (55 Amps)
50 feet (55 Amps)	AWG 6 (55 Amps)	AWG 6 (65 Amps)	AWG 8 (55 Amps)
100 feet (60 Amps)	AWG 4 (70 Amps)	AWG 6 (65 Amps)	AWG 6 (75 Amps)
150 feet (65 Amps)	AWG 4 (70 Amps)	AWG 6 (65 Amps)	AWG 6 (75 Amps)
200 feet (70 Amps)	AWG 4 (70 Amps)	AWG 4 (85 Amps)	AWG 6 (75 Amps)

40 Amp Wire Thickness - AWG 6 (6 gauge wire)

Under particular conditions involving a maximum temperature of 60°C and wire length no more than 50 feet, a 6-gauge wire stands out as an optimal choice. Its specifications are:

- Diameter: 4.1154 mm (0.1620 in)

- Cross-sectional area: 13.3016 mm² (0.0206 in²)

Default current carrying capacities:

- At 60°C/140°F: 55 amps

- At 75°C/167°F: 65 amps

- At 90°C/194°F: 75 amps

In conclusion, selecting the appropriate AWG 6 wire can ensure a safe and reliable electrical installation. This approach not only guarantees efficient system operation but also represents the collective wisdom gathered from repeated real-world applications.