subpanel
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.
This is an example of a 100 amp Sub-panel coming off of a 200 amp Main-panel. Note the errors to current NEC code. Since there was an upgrade to the system, this one needed to be upgraded to current standards.
The ground had been connected to the neutral lug and the ground stake had also been connected to same. Also, note the horizontal bus connector bar.
(Believe it or not, the local electrical inspector missed this during remodeling with added circuits.)
This shows the corrected connections in the same panel.
This is the removed bar:
Your options are a minimum of #1 aluminum or #3 copper.
Since it's only 65 feet I wouldn't upsize it for voltage drop.
You don't need service entrance or direct burial type cable if you are running conduit underground you can use type THWN wire. Individual wires not cable. If you don't want to run conduit you can buy direct burial type wire like You are talking about.
Good luck and stay safe!
Best Answer
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.
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.