View Full Version : How do I calculate the grounding conductor size????

March 6th, 2003, 02:25 PM
How do I calculate the grounding conductor size for a service panel.

I have a two hundred amp main panel with a new 125 amp service panel. This panel is fed with three TW-1 cables.



March 6th, 2003, 06:11 PM
YOu are a bit contradictory in terms in your post in my mind.

If you want to know what size grounding electrode conductor that must serve as that sole connection between your ground rod or metal water pipe being used as a grounding electrode then your 200 amp service per NEC Table 250.66 calls for a 4 awg copper wire unbroken from your metal water pipe if in direct contact with earth for 10 feet. A grounding electrode conductor from that 200 amp service panel to a ground rod would require a minimum conductor size of 6 awg copper wire again unbroken from end to end.

If you have a 125 amp sub panel, [this is where you throw me a bit, you said service panel but I suspect this 125 amp panel is a slave of a 200 amp main service rated panel] If this is the case then you would size the equipment grounding conductor by NEC Table 250.122 calling for a minimum equipment grounding conductor to be installed in the same cable or conduit as the feeders serving this 125 amp sub panel from that 200 amp main service panel grounding bar and sized no smaller than 6 awg copper equipment grounding conductor.

Hope this helps


March 7th, 2003, 10:55 PM
Thanks for the reply. You are correct. i want to replace an existing main service panel and a sub panel.

The county inspector didn't give me the permit because I couldn't answer that question and I didn't know how to calculate the answer. I also told him my service drop wa #1 cable and he said that was not rated for 200 amp service. There are no markings on that cable but I measured it when I got home and it is 1/2 inch on the outside and the copper is about 3/8". From checking cable specs I believe its 1-0 cable. If this is true theni was in arror again

The cable to the exisiting sub panel is a TW-1 which he reported is not rated for 125 amps. From checking I found its rated at 135 amps. I did not tell him it was TW insulaton. in the end he said do more home work and come back.

He (the county inspected) reported that I would need two ground rods six feet apart in addition to the cold water ground. what I am confused about is how to connect them. I assume I would run two conduits from the panel to the ground rods, use 4 AWG cable connected to seperate lugs on the ground bar in the panel, connect the cable to the ground rods with acorn clamps and, use those ground straps that connect the pipe to the ground rod too. I am to bury these connections after the inspections. I will also run a #6 grounding connector between the main service panel and the 125 amp sub panel.

Am I doing the grounds correctly this way and are my cable sizes right?

By the way. The inspector (Miami-dade County) said they rarely give out home owners a permit for this typ of work. He said, anybody can do lights and outlets but this stuff you better know what you are doing. I have done a lot of electrical work for my company but it was all drawn up and told how to do. On my own I see its a different ball game.

Thanks for your help,


March 8th, 2003, 12:41 AM

Go to the following link. There you will find detailed explainations with pictures that should get you a long way toward learning the proper wiring methods to install a service. YOu sub panel is 125 amp rated you will need to separate the neutral and grounding bars in that sub panel. [this is different in your 200 amp main service there they must be married together]. You will need two hot wires, one neutral and one equipment grounding conductor inside the conduit or cable feeder serving that sub panel. The minimum hot wire size for 125 amps would be 2 awg copper or 1/0 awg aluminum. Be careful about local rules such as the two ground rods mentioned by your inspector. Also be careful about local rules pertaining to your serving power company.

Service read the folllowing link that should get you well on your way to having the right answers your inspector needs to hear you to understand.


Also you will most likely be required to defend your service size by providing a dwelling demand load calculation. A format to do this calculation may be found at the following link.


If you want more reading on electrical go to the top left corner of this forum and click that link to our opening page of our homewiringandmore web site. Click on the 2002 top right of that opening page to see a complete book published there on the subject of wiring dwellings etc. Also found on that main page of our web site look for wiring schematics for switches etc. shown in drawings from that link also on our main page of our web site.

Let us know if these chapters linked above helps you.

Good Luck