I am running two 100 amp sub panels from a 200 amp service. My question is the wire size. I have seen the chart via The NEC provides wire sizing based on the type to be used and the application.

I am going 20 feet away from service. I have the calculator for voltage drop. What is the percentage I should use in allowable voltage drop? 1 or 3? I come up with 1 awg which seems rather small.

I'd rather not overkill with 1/0 or 2/0 for the sake of lack of knowledge.


3 Answers 3


The size of wire you use will depend on a few factors, including how it is being run.

If you plan to use conduit to run both sets of SE cables, you need to derate them, unless the conduit is able to enforce proper spacing (which it likely isn't). For two sets of 3-wire cables (four with ground), that is 4 conductors (neutrals aren't counted since they only carry imbalance current in a split-phase cable). In that case, your derating is to 80% of wire ampacity for its temperature rating.

Conduit Derating Table

In this case, 2GA copper wire is appropriate because it services 130A, which derates to 104A. If you have them running separately, then you can use up to 4GA. See this table.

Lastly, you were concerned with voltage drop. I'd say don't be. 4GA over 20 ft has a resistance of 5mΩ, which means at 100A, you would get .5V total drop. If you include the return trip (40ft total length), that's still only 1V... Check out this calculator.

All of this, however, assumes that each subpanel has a separate feeder breaker in the main breaker panel. If you plan to feed-through or do a T-splice from one cable to two panels, you will need to size for 200A since the cables can carry the full 200A load.

Also see this question.

  • 1
    thank you and i have all the room in the world for conduit there is No dry wall yet and its two separate 100 amp breakers in 200 amp service panel one for each sub panel. so 2 awg is Ok
    – dan d
    Feb 10, 2017 at 19:03
  • If they are run outside conduit, then you can use 2GA Al or 4GA Cu wires. Make sure you use SEU or SER. As a feeder wire, I believe you need to use Service Entry cable, not NM.
    – Hari
    Feb 10, 2017 at 19:09
  • 3
    @HariGanti oughta know, he just did it. Just the same run it through one of the voltage drop calcultors, I like Southwire's. Consider Al, as there's nothing wrong with it (really) and it's a money saver. I recently had to install 1000kcmil Cu, only because there's no way to do that in Al without changing out a whiole 1000A panel and conduit. Ouch. Feb 10, 2017 at 19:56
  • 1
    Standard SE cable does not have an insulated neutral and cannot be used for a sub-panel feeder. You would have to get 3 wire SE with ground for this application and make sure the neutral is not downsized. Additionally, SE cable doesn't need conduit. If you are using the abbreviation for the term Service Entrance that would be a misnomer. These are feeders not service entrance wires.
    – ArchonOSX
    Feb 11, 2017 at 7:30

20 feet is a short run and voltage drop is not a problem in that short of a distance.

Running 2 - 100 amp sub-panels you should run #3 THHN wire to each panel with a 100 amp breaker to feed each one.

The NEC recommends 3% voltage drop for either the feeder or the branch circuit and 5% total for both. This is only a recommendation and never appears in the text of the code so it not enforceable. However, it is a very good idea.

This would be a feeder and if you run the cables separately in their own conduits you don't have de-rate.

Good luck!


Use # 2SER AL or #4 SER Copper. : 4 conductor

  • 1
    Hello, and welcome to Stack Exchange. We prefer detailed answers that give the reasoning behind their conclusions. Feb 10, 2017 at 23:25
  • thanks for all your time the actual wire calculator tells me 1 awg but the 2 awg is whats on every house i have looked at
    – dan d
    Feb 10, 2017 at 23:41

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.