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What Size Ground for 100 Amp Service? (Quick Answers)

When installing a new service or upgrading an existing service, you will need to know the right size ground. You will need a ground fault circuit interrupter (GFCI) installed on a service that will run at 100 amps or more. 

Find out what the ground requirements are for a 100-amp service.

Ground size for 100 amp service

It is recommended to use at least #8 AWG copper conductors or #6 AWG aluminum conductors or copper-clad aluminum conductors for a 100-amp service. Copper conductors sized #8 AWG should be grounded at least 18 inches away from the service and tied into a ground electrode system (GES). 

Additionally, you must install a properly grounded outlet in addition to the ground size requirement. It is necessary to install a grounding electrode system (GES) when the circuit breaker has been reset and an arc or open flame is possible.

Bare/Earth ground:

Grounds connected to metal water pipes or distribution mains outside the building must be bare/earth ground. You need either #8 AWG copper wire or 6# AWG aluminum bare/earth ground for 100-amp service. 

You need either #8 AWG copper wire or 6# AWG aluminum wire because thinner wires can spark and break in enclosed areas, whereas higher gauge wires won’t. The rule of thumb is that the smaller the AWG, the more powerful the wire.

Aluminum ground:

A grounding electrode must be within 12 inches of an aluminum ground if you are using an aluminum ground. Aluminum grounding electrodes are metal strips embedded in concrete or masonry. The aluminum ground wire should be #6 AWG for 100-amp service. 

You can go up to #4 AWG, but not below #6. AWG #6 aluminum ground wire has a smaller diameter than AWG #8 copper wire and will not cause as much interference. A spark can ignite something else in the electrical system if there is any interference between the two wires.

Equipment ground:

Wire sizes range from 14 AWG through 1 AWG, with 100 amps or less rated for the circuit. Therefore, the 60C table determines the conductor’s ampacity. For the 100-amp service, the ground wire size should be #4 AWG. 

There are four main reasons #4 AWG is used for equipment ground: It is small enough not to interfere with other electrical wiring in the inspector’s facility. Installation and maintenance are easier with thin wire. Due to its low resistance, it can carry a sufficient amount of current.

Ufer ground:

The concrete should have at least 20′ of #4 Copper or 1/2″ Rebar when using a UFER ground for 100-amp service. UFER Grounds are conductors inserted into the earth between two structures, usually utility poles. 

UFER ground systems reduce galvanic corrosion between utility poles and electrical wiring in adjacent structures. It accomplishes this by providing an opposing current to electrolysis.

Copper ground:

Copper grounding is the most common and effective grounding method. For 100 amps service, the copper ground wire should be at least #8 AWG. In the inspector’s facility, the wire size is large enough to prevent interference with another electrical wiring. 

Because the wire is thick, it provides more insulation and protection from physical damage. Compared to #4 and #8 AWG wires, it has a high resistance, which reduces loss and improves efficiency.

Water ground:

In most cases, electrical wiring is not installed in water. It can, however, be effective in certain circumstances. Use at least #6 AWG copper conductor when using water ground for 100-amp service. 

The wire size is large enough to prevent interference with other electrical wiring in the inspector’s facility, as well as provide physical protection from damage. Thick wire provides more insulation and protection against physical damage. 

It has high resistance, which reduces loss and improves efficiency over #4 or #8 AWG wires.

AWG wire for 100 amps: NEC Code

NEC guides electrical wiring installation, maintenance, and use. Various types of wire are specified in the code, including those used in residential, commercial, and industrial settings.

The National Electric Code (NEC) requires “Distance from the subpanel due to voltage drop (NEC 310-16 Code),” and by using the factor you will find that a 100-amp service requires a maximum of #3 AWG ground wire. A wire with a diameter of 3 AWG can handle 100 amps.

The NEC code specifies the distance between a sub-panel and the wiring. It is recommended that the distance between a sub-panel and wiring is two times the voltage drop on that circuit (NEC 310-16, Table 16). 

The maximum separation recommended is 720 ft or nearly two football fields since 240 volts will drop 60 volts.

How do I choose a ground wire size?

You should consider the voltage drop on your circuit and the distance between the wiring and substation when choosing a grounding wire size. Maximum separation of two times the voltage drop on that circuit is recommended by the code (NEC 310-16 table 16). 

Here are some tips for choosing a ground wire.

Find the Amp:

The first thing you need to do is determine the amp rating of your circuit. It can be found on a label attached to the main electrical panel or in the service manual. If you don’t have an amp rating, use the circuit’s safe wattage.

Find the voltage:

The next step is to find out what voltage is present in your circuit. There will be a label attached to the main electrical panel or in the service manual which will provide you with this information.

Find out the voltage drop

The next step is to determine the voltage drop on your circuit. NEC code Table 16 contains this information. A qualified electrician can measure it for you if you cannot find this information.

Do the calculation:

You can now select the size of a ground wire based on the voltage drop and the distance between your wiring and the substation. Using the following equation, you can do this:

Volts x (2xDistance) = AWG

Or divide the amp rating by the voltage to find the AWG of the wire. As an example, 50 amps divided by 120 volts equals 6. To protect your equipment from electrical shock, you would need a #6 AWG grounding wire.

How to install ground wires for 100 amp service?

Installing ground wires for a 100-amp service can be done in two ways. The first method is to run the wire underground, and the second method is to run it through the wall. Choose the route you want to take, then follow the steps below.

Things you will need:

  • Wire strippers
  • Tape measure
  • Circular saw or a power drill with a bit that matches the amp rating of your circuit
  • Spade bits

Select the way:

Decide where you want to install the ground wire. In addition to running the wire underground, you can also run it through the walls. Mark out the area where you will install it using a tape measure, and then use a circular saw or power drill to cut out the hole in the wall.

Install the ground wire:

After you have installed your ground wire, it is time to install the connector. Wire strippers can be used to remove about 2 inches of insulation from each end of the wires. Each stripped wire should be soldered to a neutral screw on your circuit breaker panel (usually black or green). 

The other ends of these wires should be soldered to grounding screws near where you will run them along floors or walls after trimming any excess insulation.

Test and verify:

After installing your ground wire, it’s time to test it. When the circuit breaker is off, turn on one light switch at a time until you see sparks or hear a loud noise coming from the fixture. 

Turn off all of the lights and test them again if everything looks good. Ensure that all the lights are working by turning on the circuit breaker.

Content wire to the fixtures:

You can now install the wire to your fixtures if everything is working as it should. Make sure the wires are buried deep enough so that they are out of sight under floors and around walls. 

Insulating wires may also prevent someone from getting electrocuted if they can touch them without getting electrocuted.

Final Thoughts

Overall, it is recommended to use #6 AWG copper or #8 AWG aluminum for 100-amp service. NEC Code 4-09.3 requires a grounding conductor for installations over 100 volts and less than 3,000 watts. To find the AWG of wire, use the following formula: AWG = volts x (2xDistance).