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

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

Dn (in) = 0.005 in × 92(36-n)/39 

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

dn (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 ‘dn
  • Assume that the wire cross-section area is called ‘An
  • Use the following formula to determine ‘An’ in ‘inches2’ from any given value of ‘n’

An (in2) = (π/4)×dn2 = 0.000019635 in2 × 92(36-n)/19.5 

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

An (mm2) = (π/4)×dn2 = 0.012668 mm2 × 92(36-n)/19.5

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

An (kcmil) = 1000×dn2 = 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 gaugeWireDiameter (In)WireDiameter (mm)WireCross Section Area (kcmil)WireCross Section Area (mm2)
0000 (4/0)0.460011.6840211.6000107.2193
000 (3/0)0.409610.4049167.806485.0288
00 (2/0)0.36489.2658133.076567.4309
0 (1/0)0.32498.2515105.534553.4751

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:

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 Conditioner5-10 Amps
RV Roof Fan3-5 Amps
Ceiling Fan (low to high setting)0.2-6 Amps
Oscillating Fan0.5-1 Amps
Furnace Fan7-9 Amps
Space Heater (1600 watts)7-13 Amps
Space Heater (800 watts)4-10 Amps
Refrigerator5-8 Amps
Portable Ice Maker19.2 Amps
Blender5-6 Amps
Microwave Oven (Standard)7-13 Amps
Microwave Oven (Convection)7-15 Amps
Portable Pizza Oven12.2 Amps
Toaster8-10 Amps
Coffee Maker5-8 Amps
Electric Kettle6-12 Amps
Frying Pan (Cooking – High)7-12 Amps
Crock-Pot (Cooking – High)3-5 Amps
Crock-Pot (Warming)1-2 Amps
Food Processor5-8 Amps
Hair Dryer (High)7-13 Amps
Curling Iron0.5-0.7 Amps
Iron (High)8-10 Amps
Washer/Dryer140-16 Amps
Vacuum (Hand-Held)2-6 Amps
DVD, Disc Player, Radio0.5-2 Amps
Television HD, Digital1.5-4 Amps
Satellite Receiver/Game Console0.5-0.8 Amps
Computer (Laptop)2-3 Amps
Chargers (small electronics)0.5 to 1.5 Amps
Drill2-6 Amps
Golf Cart Charger13-21 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:

Temperature60°C (140°F)75°C (167°F)90°C (194°F)75°C (167°F)90°C (194°F)
Wire Gauge SizeWire Type: Wire, NM-B, UF-BWire Type: THW, THWN, SE, USE, XHHWWire Type: THWN-2, THHN, XHHW-2, USE-2Wire Type: THW, THWN, SE, USE, XHHWWire Type: XHHW-2, THHN, THWN-2
14 AWG152025
12 AWG2025302025
10 AWG3035403035
8 AWG4050554045
6 AWG5565755055
4 AWG7085956575
3 AWG851001157585
2 AWG9511513090100
1 AWG130145100115
1/0 AWG150170120135
2/0 AWG175195135150
3/0 AWG200225155175
4/0 AWG230260180205
250 kcmil255290205230
300 kcmil285320230260
350 kcmil310350250280
500 kcmil380430310350
600 kcmil420475340385
750 kcmil475535385435
1000 kcmil545615445500

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.

Scott Kelly