20 Amp Wire Manual Guide: what size wire is best for 20 amp circuits

When examining your home's electrical system, outlets and extension cords ubiquitously use 20 amp cords. Choosing the right gauge of wire is an important consideration, and it's not just about avoiding potential hazards.

However, safety measures, wire length, type and material, and a number of other details must be considered when calculating the correct wire thickness.

Catalog

Maximum Theoretical Current-Carrying Capacity

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	-	-	-

According to the American Wire Gauge (AWG) chart, a 12-gauge solid copper wire boasts a diameter of 2.0525 millimeters and a cross-sectional area of 3.3088 square millimeters. This wire theoretically transmits a current of 20 amperes when exposed to conditions of 60°C/140°F. Yet, despite meeting the requisite current capacity, it proves unsuitable for residential use because its surface temperature cannot exceed 60°C.

While a 12-gauge solid copper wire can theoretically bear 20 amperes at 60°C, practical realities like insulation type, ambient temperature, and installation conditions determine its safe application. By understanding these nuances and integrating conservative safety measures grounded in real-world experiences, substantial improvements in the safety and efficiency of electrical installations can be achieved.

80% Rule: Ensuring Safety in Electrical Systems

Electrical systems are engineered with safety margins to avert overloading and potential hazards. Although a 20-ampere circuit is safeguarded by a 20-ampere circuit breaker, the associated wiring must comply with the 80% safety rule. This rule specifies that the maximum safe operating current should be 80% of the circuit's rated capacity. To illustrate:

\[ I (A) = \frac{20 A}{0.8} = 25 \text{ amperes} \]

Practically speaking, the wire selected must be capable of carrying 25 amperes to adhere to the safety rule. According to the AWG (American Wire Gauge) wire size chart, AWG 10 wire is rated for 30 amperes under operating conditions of 60°C (140°F). Therefore, using AWG 10 wire in residential applications is sensible to ensure the surface temperature of the wire doesn't surpass 60°C, thereby promoting both efficiency and safety.

Due compliance with the 80% rule aids in mitigating the risk of overheating, which can escalate into electrical fires. Devices and appliances in many residential settings may draw varying levels of current. Ensuring the wiring can support slightly higher amperages provides a buffer against unexpected surges, a crucial measure for safety.

Impact of Wire Length

When the length of a wire extends beyond several tens of feet, one must consider increasing the current-carrying capacity to mitigate energy loss. Specifically, for every additional 10 feet of wire, the required current capacity may need to be increased by approximately 50%. For instance, a 50-foot wire might require a capacity of 27.5 amperes, while a 100-foot wire might need around 30 amperes. This raises a pertinent question: How do surface temperature and wire length collectively influence current capacity? Adjusting the American Wire Gauge (AWG) value based on both the length of the wire and surface temperature is vital. This ensures the wire can efficiently conduct the necessary current without overheating, providing a more reliable and safer electrical system.

Adjusting AWG Values for Extended Wire Lengths

The following table shows the AWG values required for wire lengths:

Wire Length / Surface Temperature	@60°C/140°F	75°C/167°F	90°C/194°F
<50 feet (25 Amps)	AWG 10 (30A)	AWG 12 (25A)	AWG 12 (30A)
50 feet (27.5 Amps)	AWG 10 (30A)	AWG 10 (35A)	AWG 12 (30A)
100 feet (30 Amps)	AWG 10 (30A)	AWG 10 (35A)	AWG 12 (30A)
150 feet (32.5 Amps)	AWG 8 (40A)	AWG 10 (35A)	AWG 10 (40A)
200 feet (35 Amps)	AWG 8 (40A)	AWG 10 (35A)	AWG 10 (40A)

Proper selection of AWG values is critical when dealing with extended wire lengths. For residential circuits designed to carry 20 amperes, using AWG 12 wire may be too thin and insufficient. Instead, a thicker AWG 10 wire should be chosen. This ensures that the wire can comfortably carry the 20-ampere current while minimizing energy loss over longer distances. This leads to an interesting thought: Could misjudging wire thickness be the root of many electrical failures in households? In everyday practice, incorrect wire sizing can lead to significant issues, including voltage drops and potential overheating. These problems can compromise the safety and efficiency of electrical systems. It’s fascinating how such a small detail can have such a big impact, isn't it?

To summarize:

- For a residential 20-amp circuit, AWG 12 wire might prove insufficient due to its limited capacity over extended lengths.

- Selecting an AWG 10 wire helps ensure that the wire can carry the current efficiently, reducing overall energy loss and preventing overheating during prolonged use.

Should any uncertainties arise, consulting a qualified electrician is recommended to ensure proper compliance and safety standards are maintained. Isn't it reassuring to know that expert advice can safeguard your home’s electrical systems?