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Standby generators deliver substantial amounts of electricity, meaning they must have wires large enough to carry the energy.
But what size wire should you use for your standby generator? Read on to find the answer for your generator size…
What Size Wire Is Needed for a Standby Generator?
Wire sizes from 18 AWG to 1 AWG are appropriate for safe household standby generator installations ranging from 1 kW to 24 kW.
In addition, when considering generator wire sizing, we also need to size the breaker and transfer switch correctly. They all work together and cannot be sized in isolation from each other. Sizing the wire properly for a generator installation takes these parameters into consideration.
Wire Sizes for Different Standby Generator Outputs
There are two approaches to estimating the copper wire size required for a particular size of standby generator: the wattage approach, and the amperage approach.
These figures are for a typical home in the United States that runs on 120/240 Volt, 60 Hz, single-phase electricity. Most standby generators are connected with a 240 Volt cable to the house.
Copper Wire Sizes by Generator Watts
| Wattage | / Voltage | = Amperage | / 80% NEC | = Adjusted Amps | Wire Gauge AWG |
| 1,000 | 240 | 4.2 | 0.8 | 5.2 | 18 |
| 2,000 | 240 | 8.3 | 0.8 | 10.4 | 16 |
| 4,000 | 240 | 16.7 | 0.8 | 20.8 | 12 |
| 6,000 | 240 | 25 | 0.8 | 31.3 | 10 |
| 8,000 | 240 | 33.3 | 0.8 | 41.7 | 8 |
| 10,000 | 240 | 41.7 | 0.8 | 52.1 | 6 |
| 12,000 | 240 | 50 | 0.8 | 62.5 | 6 |
| 17,000 | 240 | 70.8 | 0.8 | 88.5 | 3 |
| 20,000 | 240 | 83.3 | 0.8 | 104.2 | 2 |
| 24,000 | 240 | 100 | 0.8 | 125 | 1 |
NOTE: If your generator is rated in Kilowatts (kW) then simply multiply the kW rating by 1,000 to get Wattage.
To calculate wire size by generator Wattage you start by dividing Wattage / Voltage to get Amperage. If you have an 8,000-watt generator running on 220V, then 8,000 / 220 = 36.4 amps.
Next, you apply the 80% National Electricity Code (NEC) rule to calculate an adjusted (higher) Amperage. The 80% NEC rule creates a 25% cushion to prevent overloading the wire and starting a fire. This is done by dividing the Amperage by 0.8. In this case we get 45.5 amps (NEC adjusted).
Finally, look for the copper wire size that can handle the adjusted NEC Amperage or higher. Consult an ampacity table like this one (opens PDF in a separate tab) to find the right gauge wire for your standby generator.
Copper Wire Sizes by Generator Amps
| Amperage | / 80% NEC | = Adjusted Amps | Wire Gauge AWG |
| 5 | 0.8 | 6.3 | 18 |
| 10 | 0.8 | 12.5 | 16 |
| 15 | 0.8 | 18.8 | 14 |
| 20 | 0.8 | 25 | 12 or 10 |
| 25 | 0.8 | 31.3 | 10 |
| 30 | 0.8 | 37.5 | 8 |
| 50 | 0.8 | 62.5 | 6 |
| 75 | 0.8 | 93.8 | 3 |
| 100 | 0.8 | 125 | 2 |
| 125 | 0.8 | 156.3 | 2/0 |
Circuit Breaker Sizes for Different Standby Generator Sizes
The breaker size must be matched to the correct generator wire size you obtained from the tables above. All wire sizes below are for copper wires.
The maximum generator wattage is calculated by multiplying Breaker Size * Volts * 80% NEC factor.
| Wire Gauge (AWG) | Breaker Size | Volts | Maximum Generator Watts |
| 18 | 10A | 240 | 1,920 |
| 16 | 13A | 240 | 2,496 |
| 14 | 15A | 240 | 4,500 |
| 12 | 20A | 240 | 6,000 |
| 10 | 30A | 240 | 9,000 |
| 8 | 40A | 240 | 12,000 |
| 6 | 60A | 240 | 18,000 |
| 4 | 70A | 240 | 21,000 |
| 3 | 90A | 240 | 27,000 |
| 2 | 100A | 240 | 30,000 |
| 1 | 110A | 240 | 33,000 |
| 1/0 | 125A | 240 | 37,500 |
| 2/0 | 150A | 240 | 45,000 |
Here are some great circuit breakers to choose from for standby generator installations 12 kW and up:
- 2-pole 50 amp circuit breaker
- 3-pole 50 amp circuit breaker
- 2-pole 70 amp circuit breaker
- 3-pole 70 amp circuit breaker
- 2-pole 100 amp circuit breaker
- 3-pole 100 amp circuit breaker
Transfer Switch Sizes by Generator Output
The transfer switch is used to switch your house (or other building) between grid power and continuous standby generator power. Transfer switches can be manual or automatic transfer switches (ATS). A transfer switch must be able to handle the maximum output from the generator, as well as the size and types of loads expected on the generator circuit.
In simple terms, a standby generator transfer switch size should be equal to or larger than the largest output in Amps from the generator. This table shows how you can calculate the transfer switch size from the maximum output Wattage of your generator. Please note that the 80% NEC rule is applied to transfer switch sizing, as well as to sizing the wire and breaker (see above).
The transfer switch Amp sizing should be higher than the expected maximum Amp from the generator to prevent overloading the switch under peak loads. The smallest standby generator installations use at least a 30 Amp transfer switch, which can handle about 5 kW of power.
| Wattage | / Voltage | = Amperage | / 80% NEC | = Transfer Switch Size (rounded up to nearest standard switch size) |
| 1,000 | 240 | 4.2 | 0.8 | 15A |
| 2,000 | 240 | 8.3 | 0.8 | 15A |
| 4,000 | 240 | 16.7 | 0.8 | 20A |
| 6,000 | 240 | 25 | 0.8 | 30A or 50A |
| 8,000 | 240 | 33.3 | 0.8 | 50A |
| 10,000 | 240 | 41.7 | 0.8 | 50A or 70A |
| 12,000 | 240 | 50 | 0.8 | 70A |
| 17,000 | 240 | 70.8 | 0.8 | 90A or 100A |
| 20,000 | 240 | 83.3 | 0.8 | 125A |
| 24,000 | 240 | 100 | 0.8 | 125A or 150A |
Here are some highly rated transfer switches to choose from:
Pro/Tran2 30-Amp 10-Circuit 2 Manual Transfer Switch
Pro/Tran2 50-Amp 10-Circuit 2 Manual Transfer Switch
Generac 100 Amp 120/240V Single Phase NEMA 3R Automatic Transfer Switch
Generator Wire Size Questions & Answers
This video should help understand further how to correctly size the wire, breaker, and transfer switch for a standby generator setup:
Can 10-gauge copper wire carry 50 amps?
No. A 50-amp load can be carried by a 10-gauge bare copper wire, but the resistance in the cable will cause it to overheat significantly.
How far can you run 12 gauge wire to a standby generator on a 20 amp circuit?
You can run a 12 gauge wire on a 20 amp circuit a maximum of 50 feet before you exceed the 3% voltage drop limit.
What wire size is required for a generator to deliver 100 amps across 100 feet?
The wires and conductors on the 100-amp circuit running 100 feet will need to be 3 AWG copper or 1 AWG aluminum.
Can 6 gauge wire handle 60 amps?
Yes. Using 6 AWG you can draw a maximum of 65 amps. If you expect current peaks up to 75 amps then go with 4 AWG wire.
How many amps is #10 copper wire good for?
It is good for 30 Amps.
Which is thicker 8 gauge or 10-gauge wire?
In general, the lower the gauge number is, the thicker the wire. In 1857, the United States created a standard technique for measuring the thickness of a cable. This method is still used. So, an 8-gauge wire is thicker in diameter than a 10-gauge wire.
How far will 10-gauge wire carry 30 amps from a generator?
30 Amps can be carried by a 10-gauge (10 AWG) wire up to 150 feet. 8 AWG or 6 AWG wire should be used for distances greater than 150 feet.
What size wire do I need to run 300 feet at 100 Amps generator output?
For a 100 amp service at a distance of 300 feet, you need direct burial 1/0 gauge copper. You can run it in conduit, but the wire must still be underground-rated required by NEC code.
Will 8-gauge wire carry 50 amps?
Yes, the 8-gauge wire will carry 50 amps. Only if the current surpasses 55 amps will it be a concern.
Will 14-gauge wire carry 20 amps?
No. The maximum amps 14 AWG wire can carry is 15 amps. To run 20 amps of current through 14 gauge wire will overheat it and potentially cause a fire.
What size wire is good for 20 amps?
12 gauge wire and up works OK for 20 amps.
What size wire do I need to run 100 amp service from a generator at a distance of 150 feet?
Copper wire 2/0 AWG can run 100 amps for a distance of 150 feet.
What wire size do I need to run a 60 amp service from a generator for 100 feet?
Copper wire 4 AWG can fulfill this role.
What size wire do I need to run 30 amps for 200 feet?
A 200-foot run of 30 amps can be done with 4 AWG wire. 3 AWG wire is an option if you are concerned with current loss.
How many amps will #6 aluminum wire carry?
40 Amps can be carried by #6 aluminum wire.
What is #4 aluminum wire rated for?
For use in both wet and dry environments, #4 aluminum wire is rated at 600 volts and 90 degrees Celsius.
Generator Breaker Size FAQs
What size breaker do I need for a 20 kW generator?
If you’re using a generator with a capacity of 20,000 Watts on 240 Volts, a 70 Amp breaker should be used.
What is the maximum breaker size for 12 gauge wire?
A maximum of 20 amp breaker for the 12-gauge wire running 240 Volts should be used. A maximum breaker size of 25 or 30 amps should be used for 10-gauge wire.
Generator Transfer Switch Size FAQs
Do I need a 30 amp or 50 amp transfer switch?
30 amp transfer switches are typically used for low-power generators (5 kW or less) and 110V interior circuits. They can quickly overload if your generator outputs more than 5 kW, or if you run heavy loads on the circuit, such as a washer-dryer, A/C, and refrigerator all at once.
50 amp transfer switches are used for most mid-power 5 kW to 10 kW standby generators and typically run on 240V exterior circuits. A 50 amp transfer switch is appropriately sized for any generator putting out 5 kW to 10 kW continuously.
Above 10 kilowatts of home standby generator output, you will need wiring, breakers, and a transfer switch rated for between 70 amps and 150 amps.
How do I choose a generator transfer switch?
The simplest approach to figure out what size transfer switch you need is to see how much power your generator can handle. Your generator’s biggest outlet should be matched to this. A 30-amp transfer switch should be purchased for a generator with a 30-amp outlet in order to obtain the full power load.
What size transfer switch do I need for my generator?
Manual transfer switches should be sized to match the generator’s biggest outlet. A 30-amp transfer switch, for example, is required if the biggest outlet on your generator is only rated for 30 amps.
Can you plug a 50 amp generator into a 30 amp transfer switch?
Technically you can plug a 50 amp generator output into a 30 amp transfer switch. However, this is dangerous, as the transfer switch is not rated for the maximum load of the generator outlet. This could lead to overheating of the transfer switch and connectors, potentially causing a fire.
Conclusion
The size of the generator, breaker, and transfer switch are factors to consider when looking for the right wire to install in your standby generator. Make sure to check with the manufacturer’s handbook for specific details related to wiring before proceeding.
The “what size wire for a generator to house?” question is one that gets asked often in standby generator installations. The answer is that the best size wire to use depends on how big your generator will be and what type of power it will produce.
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