Help with wire size

How many amps?

OK I see what is going on.

You originally said 100a?? The inspector screwed up and quoted 310.15(B)(6) then caught his mistake and quoted 310.16 (which is correct for a garage feeder)

That gets you #3 copper or #1 aluminum in the 75c column

If you use a cable (not wire in conduit) you are usually in the 60c column. What is the cable type the counter man is quoting? There are

75c cables.

Assuming you're using THHN and copper, #4 is only good for 95 amps which would bump you up to #3. However, the rule of thumb is the size is good for every 100 feet of the run. So if your run was 100 feet or less the #3 would be okay, but since the run is up around 200 feet you go one size larger to #2. This upsizing is to offset voltage drop you'll get due to the resistance in the wire. Since this is a sub-panel and not a main service, you cannot under size the ground, it must be the same size as the hots and neutral wires. You have to make this run in conduit, underground cable is not allowed. From what I see, you will need to install 4 #3 THHN conductors in a

1-1/4" conduit run.

I am so confused. Why cant i use underground cabel and you said I can use #3 but earlier in your paragraph you said I cant because the run is over 100 feet?

I am not sure I will have to ask the counter person. The inspector said somethign about using the "60 degree mark for 100 amps.?"

Wire is rated by the temperature of it's insulation and the temperature rating of the breakers you hook it to. Generally breakers are 75c rated as is wire you pull in conduit but "cables" are mostly rated at 60c and the amount of current they can carry is less.

There is a direct burial wire called USE that would work at 75c.

The other side of this is how much you want to dig. Direct burial wire or cable needs to be down 2 feet. PVC Conduit only has to be down 18"

You can't use UF cable because NEC 340.12(1) prohibits UF cable from being used as service entrance cable. That's why you have to run conduit and pull

4 #2 THHN conductors into it. I apologize for confusing you because when I wrote, "install 4 #3 THHN conductors in a 1-1/4" conduit run" in my original message I wrote #3 by mistake. I should've wrote, " install 4 #2 THHN conductors in a 1-1/4" conduit run." That's 2 hots, 1 neutral, and 1 ground, all #2 THHN copper.

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Why does the ground need to be #2?

The inspector told me #3 with copper and a #6 ground. He said the ground could be bare or jacketed but must be run as a bundle with the oter wires.

He said something about the 60 C rating but I am not sure what that meant.

I can get #2 copper for .87/foot. FOr three wires that adds up over

210 feet.

This is not a service entrance, it is a feeder and you could certainly use Underground Feeder (UF) cable for it. You can also run a smaller neutral (based on the load calc) and a #8 gauge copper, 6ga al, equipment ground (based on 100a)

Why does the ground need to be #2?

The inspector told me #3 with copper and a #6 ground. He said the ground could be bare or jacketed but must be run as a bundle with the oter wires.

He said something about the 60 C rating but I am not sure what that meant.

I can get #2 copper for .87/foot. FOr three wires that adds up over

210 feet.

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If the inspector told you he would accept #3 for the hots and neutral with a #6 for the ground, then that's what you use.

When you say 210 feet is that the distance between the two panels or do you mean 210 feet of wire as in 70 ft. X 3 runs?

250.122 says you can use #8 copper or #6 aluminum for 100a. The neutral size is either the ungrounded conductor size or smaller as calculated based on the load. Typically you see 1-2 sizes less than the ungrounded conductor..

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210 feet is the distance between the two panels

Then the inspector would be correct except for the upsizing of the wire one size to compensate for voltage drop.

The inspector has no business ordering an upsize of the conductor for voltage drop. The NEC does not prescribe voltage drop limits. Fine Print Notes are just comments on good practice. They are not part of the code and cannot be enforced by the authority having jurisdiction.

-- Tom Horne

The inspector has no business ordering an upsize of the conductor for voltage drop. The NEC does not prescribe voltage drop limits. Fine Print Notes are just comments on good practice. They are not part of the code and cannot be enforced by the authority having jurisdiction.

-- Tom Horne

Yes he can and yes the NEC does. The NEC allows for a maximum voltage drop at the furthest point of power usage to be 5% (3% on the branch circuit, 2% on the feeders), article 210.19(A)(1) FPN #4. Fine Print Notes are more than just comments for good practice, they are there to clarify the intent of the article or provide addition guidance.

The important thing you do have to remember is if you upsize the phase conductors, you also have top upsize the ground accordingly.

FPN meaning voltage drop comments are in a "Fine Print Note".

NEC 90.5-C Explanatory Material. "Fine print notes are informational only and are not enforceable as requirements this Code."

Seems kinda like what Tom wrote.

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I'm sorry to seem so quarrelsome but you are just plain wrong. Section 90.5 paragraph C is very clear on this point vis.

90.5 Mandatory Rules, Permissive Rules, and Explanatory Material. (C) Explanatory Material. Explanatory material, such as references to other standards, references to related sections of this Code, or information related to a Code rule, is included in this Code in the form of fine print notes (FPNs). Fine print notes are informational only and are not enforceable as requirements of this Code.

-- Tom Horne

To further explain why you can't mandate a voltage drop number on this installation, we don't have a clue what the load will actually be and without a definite load, you can't compute the voltage drop. Most guys with 100a. panels in their garage, never use more than about

30 of it at any given time. The NFPA calls that load diversity.