Generator Wire Sizing Calculator + Charts: What Wire Size?

Most generators need big wires. You need a wire that can handle all the amps (+ accounting for 80% NEC rule) the generator produces. This is quite a well-known fact. However, exactly what size wire you need for a generator is quite difficult to find.

Using a wire that is too small will fry the wire or the circuit. That’s why it’s not a good idea just to pick the nearest wire to connect a generator.

how big should a wire for generator be
If you pick a wire that’s too big, it’s just overkill. If you pick one that is too small, the wires can overheat and catch on fire.

To help figure out what size wire you need, we have created generator wire sizing calculators and charts for any generator (home, standby, portable, etc). If you know how many amp or watt generator you have, you can simply consult the generator wire sizing chart and you will know if you need an 8 AWG wire, 10 AWG wire, 12 AWG wire, and so on.

Example For Generator Wattage: Let’s say that you have an 8,000W generator running on a 220V circuit. What size wire do you need for an 8,000-watt generator? First, you need to calculate the amps like this: 8,000W/220V = 36.4A. Then you have to account for the 80% NEC rule by multiplying the amps by a 1.25 factor like this: 36.4A×1.25 = 45.5A. This means you need a wire with at least 45.5A ampacity. 8 gauge AWG copper wire has an ampacity of 50A; this is the wire size you need for an 8,000-watt generator.

Example For Generator Amps: This is easier. Let’s say you have a 50 amp generator. What size wire do you need for a 50 amp generator? You have to multiply the amps by a 1.25 factor (80% NEC rule) like this: 50A×1.25 = 62.5A. That means you need a wire with at least 62.5A ampacity. 6 gauge AWG copper wire has an ampacity of 65A; this is the wire size you need for a 50 amp generator.

These are just two examples. To adequately illustrate how generator wire sizing works, we are going to explain the whole calculation first.

Below the explanation, you will find very useful resources that will help you adequately determine generator wire size:

  1. Generator Wire Sizing Calculator. You basically input the wattage or amps of your generator, and the calculator (there are two; one for watts and the second for amps) will tell you what wire size you need for a generator.
  2. Generator Wire Wattage Chart. In the first chart, you will find the wire sizes you need for all generators; from a 500W generator to a 50,000W generator.
  3. Generator Wire Amps Chart. In the second chart, you will find the wire sizes you need for all generators; from a 10 amp generator to a 200 amps generator.
    generator wire size calculator and chart
    You can find all these resources further on to correctly size a wire for any generator.

Note: If you don’t find your answer, you can use the comment section below, give us the wattage or amps of your generator, and we’ll try to help you out.

Let’s start by explaining how the generator wire size is calculated:

How To Calculate Generator Wire Sizes?

The whole point of choosing the correctly sized generator wire is to determine a big enough wire that can handle all the amps. Every AWG wire has a specified ampacity; this is the current (amps) it can safely carry.

If you can figure out the ampacity, you can consult the wire ampacity chart here, and check what size wire you need for your generator.

Figuring out the ampacity start with calculating the generator amp draw. This is already done if you know you have a 50 amp, 100 amp generator, and so on. If you only know the generator’s wattage, you can calculate the amps.

Here is the full step-by-step calculation of how to determine the wire size for a generator:

  1. Divide the generator’s wattage by the voltage to calculate the amp draw. Amp Draw = Wattage / Voltage. Example: A 10,000-watt generator on a 220V voltage draws 10,000W/220V = 45.5 amps.
  2. Account for the 80% NEC rule by multiplying the amp draw by 1.25. According to the National Electricity Code (NEC), the maximum load of any circuit breaker is 80% of the rated ampacity. Example: If you have a 50 amp generator, these 50 amps represent 80% (or lower) of the total wire ampacity. That means you need a wire that can handle 25% more amps (1/0.8 = 1.25). This is a safety measure that prevents you from overloading the circuit (overloaded wires can catch on fire).
  3. When you get the minimum ampacity needed, consult the wire ampacity chart and pick a wire that can handle that ampacity or higher. Example: A 50 amp generator needs a wire with at least 62.5A ampacity. You can’t use an 8 AWG wire because it only has 50A ampacity. You will need to use a 6 AWG wire because it has 65A ampacity; more than 62.5A.

Don’t worry if all this sounds a bit complex. You can use two calculators below that do all this calculating automatically. Further on, you will also find the calculated charts for various generators and the wire sizes needed:

Wire Size Wattage Calculator (Insert Wattage, Get Ampacity)

Here you just insert the size of your generator in watts (Example: 12,000W generator) and the calculator will determine the minimum ampacity a generator wire should have:

 

Let’s say that you have a 12,000W generator. The calculator will tell you that you need a wire with at least 68.18A ampacity. You just consult the wire ampacity chart and check which wire has more ampacity than that. In this case, the best wire for the 12,000-watt generator is a 4 AWG wire with 85A ampacity.

Note: You can also use multi-strand wires. For example, 10 AWG wire has an ampacity of 35A. Two 10 AWG wires (denoted as 10/2 wire) have a combined ampacity of 70A; this is more than the minimum needed 68.18A for a 12,000W generator). That’s why we can use a 10/2 wire here as well.

The next calculator is for guys who determine the size of their generators in amps (it’s basically the same thing but with amps):

Wire Size Amps Calculator (Insert Amps, Get Ampacity)

Here you just insert the size of your generator in amps (Example: 100 amp generator) and the calculator will determine the minimum ampacity a generator wire should have:

As we can see, if you have a 100 amp generator, you will need a wire with at least 125A ampacity (this accounts for 80% NEC rule). Consulting the wire ampacity chart, you see that 1 AWG wire has a 130A ampacity; this is the correct wire size for a 100 amp generator.

You can play around with these calculators to see how the minimal wire ampacity for a generator changes. We already did all the calculations for you, complete with the generator wire sizing. You can check both of the following charts to check what size generator wire you need:

Wire Size Wattage Chart (For 500W – 20,000W Generators)

Generator Wattage: Minimum Ampacity: Wire Size:
500 Watt 2.84 Amps 24 AWG
1,000 Watt 5.68 Amps 22 AWG
1,500 Watt 8.52 Amps 20 AWG
2,000 Watt 11.36 Amps 18 AWG
2,500 Watt 14.20 Amps 16 AWG
3,000 Watt 17.05 Amps 14 AWG
3,500 Watt 19.89 Amps 14 AWG
4,000 Watt 22.73 Amps 12 AWG
4,500 Watt 25.57 Amps 10 AWG
5,000 Watt 28.41 Amps 10 AWG
6,000 Watt 34.09 Amps 10 AWG
7,000 Watt 39.77 Amps 8 AWG
8,000 Watt 45.45 Amps 8 AWG
9,000 Watt 51.14 Amps 6 AWG
10,000 Watt 56.82 Amps 6 AWG
11,000 Watt 62.50 Amps 6 AWG
12,000 Watt 68.18 Amps 4 AWG
13,000 Watt 73.86 Amps 4 AWG
14,000 Watt 79.55 Amps 4 AWG
15,000 Watt 85.23 Amps 3 AWG
16,000 Watt 90.91 Amps 3 AWG
18,000 Watt 102.27 Amps 2 AWG
20,000 Watt 113.64 Amps 2 AWG
25,000 Watt 142.05 Amps 0 (1/0) AWG
30,000 Watt 170.45 Amps 000 (3/0) AWG
35,000 Watt 198.86 Amps 000 (3/0) AWG
40,000 Watt 227.27 Amps 0000 (4/0) AWG
45,000 Watt 255.68 Amps 250 kcmil
50,000 Watt 284.09 Amps 300 kcmil

As you can see, there is quite a range of wires that can be used to wire a generator. Most wires are AWG wires. With the massive 40,000W+ generators, you will have to use even bigger kcmil wires.

Here are the same calculations for generator size in amps:

Wire Size Amps Chart (For 5A – 200A Generators)

Generator Amps: Minimum Ampacity: Wire Size:
5 Amp Generator 6.25 Amps 20 AWG
10 Amp Generator 12.50 Amps 18 AWG
15 Amp Generator 18.75 Amps 14 AWG
20 Amp Generator 25.00 Amps 12 AWG
25 Amp Generator 31.25 Amps 10 AWG
30 Amp Generator 37.50 Amps 8 AWG
35 Amp Generator 43.75 Amps 8 AWG
40 Amp Generator 50.00 Amps 8 AWG
45 Amp Generator 56.25 Amps 6 AWG
50 Amp Generator 62.50 Amps  6 AWG
60 Amp Generator 75.00 Amps 4 AWG
70 Amp Generator 87.50 Amps 3 AWG
80 Amp Generator 100.00 Amps 3 AWG
90 Amp Generator 112.50 Amps 2 AWG
100 Amp Generator 125.00 Amps 1 AWG
125 Amp Generator 156.25 Amps 00 (2/0) AWG
150 Amp Generator 187.50 Amps 000 (3/0) AWG
175 Amp Generator 218.75 Amps 0000 (4/0) AWG
200 Amp Generator 250.00 Amps 250 kcmil

With this amp table, you can figure out what size wire you need for a generator.

If you need some additional help with generator sizing, you can check what size generator you need here. You can also check the list of the quietest generators currently on the market here; with home generators, people are mostly concerned with high noise outputs (which is understandable).

If you have a specific generator in mind, you can use to comments below and we’ll try to help you out with generator wiring.

Thank you.

4 thoughts on “Generator Wire Sizing Calculator + Charts: What Wire Size?”

  1. Thank you for the Calculator.
    I going with a 20,000 Watt Gen (Kohler, Generac or B&S) with a 200 amp switch and service.
    Question is, considering NG + Gen and Elec service will be on opposite side of house (100 Ft run end to end), how does that effect the calculation?
    Thanks, John S.

    Reply
    • Hello John, a great question. Yes, that 100 ft run will affect the wire size you need. The simplest way is to presume that you get a 20% voltage drop every 100 ft. That’s why you would need a wire with 20% higher ampacity. If you have a 200A switch, you would need 250A ampacity just due to NEC 80% safety rule. You will have to multiply that number by 20% (so 250A×1.2 = 300A) to get the wire size you need for a 200A generator 100 feet away. For 300A rated ampacity, you will need one of these biggest kcmil wires. If you check this chart, the 350 kcmil wire has a median ampacity of 310 amps. That’s the wire you should get for 200 amp service generator with 100 ft run.

      Reply
    • Hi Tom, that’s a massive load. It depends on the voltage; because amps are watts divided by volts. If you have a 240V, you need a wire that can handle 1042 amps. That’s, again, a massive current. This biggest individual wire size is 2000 kcmil wire with 665A median ampacity. In this case, you would need two of these massive 2000 kcmil wires. You can opt for three 500 kcmil wires as well. You can get more info on these big kcmil wire sizes and amps here, hope it helps.

      Reply

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