These ground wire sizes are notoriously difficult to figure out. If you are looking for the ground wire size for 100 amp service, you may see some conflicting wire sizes; from as little as 10 AWG to as big as 00 AWG. To help everybody out, we are going to show exactly how you can determine which ground wire size to use for 100 amp service.
Namely, we can take the guesswork out of the equation. Why? Well, because can find clear guidance to 100 amp ground wire size in National Electric Code (NEC). Specifically, we need to look at the NEC Article 250, Table 250.66 titled ‘Grounding Electrode Conductor For Alternativing Current Systems’.
In this 250.66 table we get the data for:
- Which size of hot copper wire can be grounded with which size of ground copper wire. Example: 2 AWG hot copper wire is grounded with 8 AWG ground copper wire.
- Which size of aluminum copper wire can be grounded with which size of ground aluminum wire. Example: 1 AWG hot aluminum wire is grounded with 4 AWG ground aluminum wire.
As you can see, ampacities (amps) are not included in the NEC ground wire size table. That’s unfortunate but, with a little help, we can adequately figure out exactly what ground size wire we need for 100 amp service.
On top of that, we are going to look at 100 amp ground wire size for sub-panels that may be 50 feet, 100 feet, 150 feet, or 200 feet away. In these longer wires, we need to take into account voltage drop and potentially use thicker ground wires as we will show further on.
First, let’s look at how you can determine the size of ground wire needed for 100 amp service in 3 easy steps:
Determining 100 Amp Ground Wire Size (Based On NEC Table 250.66)
Here is the 3-step process we will use (with screenshots) to determine the size of a 100 amp ground wire:
- First you need to know if you are using copper or aluminum wire. 100 amp copper ground wire is not the same as 100 amp aluminum ground wire.
- Figure out which hot wire (copper or aluminum) can handle 100 amps. Here are the screenshots from the copper AWG wire amp chart and the aluminum AWG wire amp chart:
For 100 amp service, you will need to use 3 AWG hot copper wire because it has 100A ampacity. As we will see further on, if you need 100 amp service 50, 100, 150, 200 feet away, you need to use 2 AWG wire with 115A ampacity to negate the voltage drop across the wire. 
For 100 amp service, you will need to use 00 (2/0) AWG hot aluminum wire because it has above 100A ampacity (00 AWG aluminum wire can carry 108 amps). Even if you need a 100 amp service 50, 100, 150, 200 feet away, you can still use 00 AWG aluminum wire because it has above 100A ampacity (108A) that adequately negates the voltage drop across the aluminum wire. - Now we know that, for 100 amp service, we need to ground 2 AWG copper hot wire or 00 (2/0) AWG aluminum hot wire. We consult this ground wire size chart, based on the NEC Table 250.66, for both copper and aluminum conductors to get the 100 amp service ground wire size like this:
As we can see, 100A 3 AWG copper hot wire needs to be grounded with an 8 AWG copper ground wire. We use the same 8 AWG copper ground wire for grounding 115A 3 AWG copper wire to provide 100 amp service at a distance of 50, 100, 150, or 200 feet. 
As we can see, 108A 00 (2/0) AWG aluminum hot wire needs to be grounded with a 2 AWG aluminum ground wire. 2 AWG aluminum ground wire can also be used to provide 100 amp service to sub-panels that are 50, 100, 150, or 200 feet away from the source.
In summary, you need to use:
- 8 AWG ground copper wire for 100 amp service (grounding 3 AWG copper hot wire). Even if you have a sub-panel 50 feet, 100 feet, 150 feet, or 200 feet away, you can still use 8 AWG wire because it is used to ground a thicker 2 AWG copper hot wire with 115A ampacity.
- 2 AWG ground aluminum wire for 100 amp service (grounding 00 AWG aluminum hot wire). Even if you have a 100 amp service sub-panel some distance away, you can still use the 2 AWG ground aluminum wire.
This 3-step procedure is the only way how you can determine what size ground wire you need for 100 amp service. On top of that, you can use the same procedure to determine the ground wire size for any amps. Here is an example of using this procedure to figure out what size ground wire you need for 200 amp service.
If you have any questions regarding the 100 amp service ground wire size, you can use the comments below and we will try to help you out as best we can.