Wire Gauge & Ampacity Basics: Guide to Select The Right Wire Size

While setting up the wiring of a residence or commercial entity, one of the primary concerns is to select the safest cable. But for anyone who’s new to the industry, deciphering all the ratings, numbers and conventional metrics can be pretty tough. 

In this 3-chapter guide, we’ve taken a handy approach to simplify the process of how you can calculate the diameter and cross-sectional area of an electric wire. Later on, you’ll find a 2-step process to find the right type and size of wire for any given circuit load. 

Seems interesting? Put on your handyman hat and let’s proceed!

Chapter 1: Definitions

What is Wire Gauge?

Simply put, a wire gauge is a metric to signify the thickness and area of round and solid electric wires. It’s usually represented by a number, and there are no particular units of it. Smaller wire gauge numbers mean thinner wires, and higher gauge numbers mean thicker wires.

Across the globe, the most accepted wire gauge metric is AWG or American Wire Gauge. You’ll see the following AWG gauges in the market:

• Thick Wire: 0000 (4/0) AWG, 000 (3/0) AWG, 00 (2/0) AWG, and 0 (1/0)AWG.
• Medium-thick Wire: 1, 2, 3, 4, 5, 6, 7, 8, 9 ,10, 11, 12, 13, 14, and 15 AWG. 
• Thin Wire: 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, and 40 AWG. 

Credit: CarsonDunlop.com 

Example- A 5 AWG wire is 4.62mm in diameter and 16.8mm² in area. Where a smaller 2 AWG wire is 6.54mm in diameter and 33.6mm² in terms of area. 

Here, AWG is one of the convenient standards to measure and express wire gauge. We’ll talk about this standard later in this guide. But for now, let’s have a look at all of the convenient wire gauge metrics across the globe:

• AWG(American Wire Gauge)
• MCM(Multi-Chip Module) or kcmil(1 thousand circular mils).
• Millimeter(mm) and millimeter-square(mm²)
• Inch(in) and Inches square(in²)

As per convenience, MCM or kcmil are pretty backdated metrics of wire gauge. Throughout the guide, we’d rather stick to mostly AWG, and show you how to convert from AWG to mm/mm² and in/in².

What is Ampacity/Load Bearing Capacity?

Ampacity is the short form of ‘Ampere Capacity’ and had been sourced from the word ‘Ampere or Amp’. It simply defines the maximum ability to carry electricity of a wire under normal conditions. In other words, you can also call it a wire’s load-bearing capability. The unit of ampacity is ‘Ampere’.

Ampacity varies on what gauge of wire, the metal of the wire you’re talking about, what temperature the wire is at, and a few other environmental factors. When it comes to determining the load-bearing capacity or the ampacity of a wire, a change in any of these factors would change the ampacity of the wire. 

Let’s clarify this with a couple of examples:

A copper wire of 8 AWG will have a 30 amp load-bearing capacity at 60⁰C, where another wire of 2 AWG will have 95 amp capacity under the same temperature. 

If you change the temperature from 60⁰C to 90⁰C of the first wire(8 AWG copper), the ampacity would increase to 40 amp. Hence, the change of temperature is one of the contributing factors in ampacity.  

Chapter 2: Calculating Wire Size (Diameter & Area) from Wire Gauge

Before you decide which wire to install on your residential or commercial electric circuits/system, it’s very important to learn the parameters of the wire. 

In this chapter, we’ll learn how you can measure the exact diameter and cross-section area from a given wire gauge. 

Steps of Calculating Wire Diameter from Wire Gauge

We’ll learn the steps of converting Wire Gauge(AWG) to wire diameter in units of inches and mm in this section. 

Here go the steps:

• Take the wire gauge in the AWG unit. Call it ‘n’
• Assume that the wire diameter is called ‘d_n’
• Use the following formula to determine ‘d_n’ in ‘inches’ from any given value of ‘n’

Dn (in) = 0.005 in × 92^(36-n)/39 

• Use the following formula to determine ‘d_n’ in ‘mm’ from any given value of ‘n’

d_n (mm) = 0.127 mm × 92^(36-n)/39

Steps of Calculating Wire Cross Section Area from Wire Gauge

We’ll learn the steps of converting Wire Gauge(AWG) to wire cross-section area in units of inches², mm², and kilo-circular mils (kcmil) in this section. 

Here go the steps:

• Take the wire gauge in the AWG unit. Call it ‘n’
• Assume that the wire diameter is called ‘d_n’
• Assume that the wire cross-section area is called ‘A_n’
• Use the following formula to determine ‘A_n’ in ‘inches²’ from any given value of ‘n’

A_{n (in}²₎ = (π/4)×d_n² = 0.000019635 in² × 92^(36-n)/19.5 

• Use the following formula to determine ‘A_n’ in ‘mm²’ from any given value of ‘n’

A_{n (mm}²₎ = (π/4)×d_n² = 0.012668 mm² × 92^(36-n)/19.5

• Use the following formula to determine ‘A_n’ in ‘kilo-circular mils (kcmil)’ from any given value of ‘n’

A_{n (kcmil)} = 1000×d_n² = 0.025 in² × 92^(36-n)/19.5 

Based on both of formulas, below is a worked-out chart that shows up the diameter and area of wires of different AWG ratings:

AWG gauge	WireDiameter (In)	WireDiameter (mm)	WireCross Section Area (kcmil)	WireCross Section Area (mm2)
0000 (4/0)	0.4600	11.6840	211.6000	107.2193
000 (3/0)	0.4096	10.4049	167.8064	85.0288
00 (2/0)	0.3648	9.2658	133.0765	67.4309
0 (1/0)	0.3249	8.2515	105.5345	53.4751
1	0.2893	7.3481	83.6927	42.4077
2	0.2576	6.5437	66.3713	33.6308
3	0.2294	5.8273	52.6348	26.6705
4	0.2043	5.1894	41.7413	21.1506
5	0.1819	4.6213	33.1024	16.7732
6	0.1620	4.1154	26.2514	13.3018
7	0.1443	3.6649	20.8183	10.5488
8	0.1285	3.2636	16.5097	8.3656
9	0.1144	2.9064	13.0927	6.6342
10	0.1019	2.5882	10.3830	5.2612
11	0.0907	2.3048	8.2341	4.1723
12	0.0808	2.0525	6.5299	3.3088
13	0.0720	1.8278	5.1785	2.6240
14	0.0641	1.6277	4.1067	2.0809
15	0.0571	1.4495	3.2568	1.6502
16	0.0508	1.2908	2.5827	1.3087
17	0.0453	1.1495	2.0482	1.0378
18	0.0403	1.0237	1.6243	0.8230
19	0.0359	0.9116	1.2881	0.6527
20	0.0320	0.8118	1.0215	0.5176
21	0.0285	0.7229	0.8101	0.4105
22	0.0253	0.6438	0.6424	0.3255
23	0.0226	0.5733	0.5095	0.2582
24	0.0201	0.5106	0.4040	0.2047
25	0.0179	0.4547	0.3204	0.1624
26	0.0159	0.4049	0.2541	0.1288
27	0.0142	0.3606	0.2015	0.1021
28	0.0126	0.3211	0.1598	0.0810
29	0.0113	0.2859	0.1267	0.0642
30	0.0100	0.2546	0.1005	0.0509
31	0.0089	0.2268	0.0797	0.0404
32	0.0080	0.2019	0.0632	0.0320
33	0.0071	0.1798	0.0501	0.0254
34	0.0063	0.1601	0.0398	0.0201
35	0.0056	0.1426	0.0315	0.0160
36	0.0050	0.1270	0.0250	0.0127
37	0.0045	0.1131	0.0198	0.0100
38	0.0040	0.1007	0.0157	0.0080
39	0.0035	0.0897	0.0125	0.0063
40	0.0031	0.0799	0.0099	0.0050

Chapter 3: Calculating Wire Load Bearing Capacity

For buying electricity-carrying wires, the maximum allowable ampacity or load-bearing capacity plays a significant role. But unlike the previous processes, it can not be directly determined from the AWG rating of a wire. There are 4 factors in total that have to be taken into account for the process- 

• Wire Gauge in AWG or kcmil
• Wire type (copper or aluminum)
• Insulation or temperature – 60⁰C, 75⁰C, and 90⁰C
• The placement of the wire – raceway, cable, buried or open air

For wire size for residential appliances and typical industrial purposes, the fourth parameter is usually fixed to the ‘Cable’ type. For the sake of simplification, we’ll be omitting the placement type consideration from next on. 

Below is the two-step process of calculating the right wire that has the maximum load-bearing capacity/ampacity for your purpose.

Part 1: Steps of Calculating for The Load Requirement of Your Circuit

Given that you’re working with either a residential or commercial circuit(or a number of circuits), we’ll first learn how to calculate the total load requirement of your system. Here go the steps:

• Determine how many devices will be plugged in/powered from the circuit, and find the wattage of each. 
• Add up the watts found in all of the devices combined. 
• Determine the voltage your system is running at. For North America, it’s either 120V or 220V. 
• Now divide the tall wattage by the total voltage to obtain the total load requirement(ampacity) of the system”

Load Requirement(amp) = Total Watt(W) / Total Voltage(V)

For convenience, here is the worked-out load requirements(approximate) under 110-120V of voltage:

EQUIPMENT	AMPERAGE DRAWS
RV Converter (charging)	1-8 Amps
Lights (per bulb)	0.5 -1.5 Amps
Water Heater (6-gallon, heating)	8-13 Amps
RV Air Conditioner (Start-up)	16-18 Amps
RV Air Conditioner (Running)	13-16 Amps
Window Air Conditioner	5-10 Amps
RV Roof Fan	3-5 Amps
Ceiling Fan (low to high setting)	0.2-6 Amps
Oscillating Fan	0.5-1 Amps
Furnace Fan	7-9 Amps
Space Heater (1600 watts)	7-13 Amps
Space Heater (800 watts)	4-10 Amps
Refrigerator	5-8 Amps
Portable Ice Maker	19.2 Amps
Blender	5-6 Amps
Microwave Oven (Standard)	7-13 Amps
Microwave Oven (Convection)	7-15 Amps
Portable Pizza Oven	12.2 Amps
Toaster	8-10 Amps
Coffee Maker	5-8 Amps
Electric Kettle	6-12 Amps
Frying Pan (Cooking – High)	7-12 Amps
Crock-Pot (Cooking – High)	3-5 Amps
Crock-Pot (Warming)	1-2 Amps
Food Processor	5-8 Amps
Hair Dryer (High)	7-13 Amps
Curling Iron	0.5-0.7 Amps
Iron (High)	8-10 Amps
Washer/Dryer	140-16 Amps
Vacuum (Hand-Held)	2-6 Amps
DVD, Disc Player, Radio	0.5-2 Amps
Television HD, Digital	1.5-4 Amps
Satellite Receiver/Game Console	0.5-0.8 Amps
Computer (Laptop)	2-3 Amps
Chargers (small electronics)	0.5 to 1.5 Amps
Drill	2-6 Amps
Golf Cart Charger	13-21 Amps

Part 2: Steps of Calculating for Wire Gauge from Load Requirement 

Once you have figured out the load requirement of your system, the next step will help you to find the right copper/aluminum wire gauge. This is calculated considering the maximum safe ampacity in mind. So the actual load might be less than what you’ve found from the previous part. 

Source: futurehousestore.co.uk 

The following chart contains wire gauge in units of AWG and kcmil. From the table, find the closest ampacity to your calculated ampacity, and that will lead you to the required wire gauge in the preferred unit:

Material	Copper	Copper	Copper	Aluminum	Aluminum
Temperature	60°C (140°F)	75°C (167°F)	90°C (194°F)	75°C (167°F)	90°C (194°F)
Wire Gauge Size	Wire Type: Wire, NM-B, UF-B	Wire Type: THW, THWN, SE, USE, XHHW	Wire Type: THWN-2, THHN, XHHW-2, USE-2	Wire Type: THW, THWN, SE, USE, XHHW	Wire Type: XHHW-2, THHN, THWN-2
14 AWG	15	20	25	—	—
12 AWG	20	25	30	20	25
10 AWG	30	35	40	30	35
8 AWG	40	50	55	40	45
6 AWG	55	65	75	50	55
4 AWG	70	85	95	65	75
3 AWG	85	100	115	75	85
2 AWG	95	115	130	90	100
1 AWG	—	130	145	100	115
1/0 AWG	—	150	170	120	135
2/0 AWG	—	175	195	135	150
3/0 AWG	—	200	225	155	175
4/0 AWG	—	230	260	180	205
250 kcmil	—	255	290	205	230
300 kcmil	—	285	320	230	260
350 kcmil	—	310	350	250	280
500 kcmil	—	380	430	310	350
600 kcmil	—	420	475	340	385
750 kcmil	—	475	535	385	435
1000 kcmil	—	545	615	445	500

Final Words

Thanks for being with the guide till this far! We hope this has been helpful for you to determine what size and type of wire you need for any circuit/electric system buildup.

The last tip is to keep in mind the VDI (Voltage Drop Indicator) if you’re going for a long run of wire instead of regular household runs.