Wiring Subpanel off Subpanel

brgmgb · 08-25-2011, 06:26 PM

I want to put a small subpanel in a pole barn. I have a few questions.

The house has 200 amp service. The main box has a double 100 amp breaker on 2 gauge wire to a subpanel in the garage. The distance from the panel in the basement to the panel in the garage is approximately 50 feet. The garage has a 100 amp box.

We built a small pole barn about 50 feet from the garage. I want to wire the pole barn with a few outlets (one for a garage door opener) and lights and a 30 amp outlet for a camper. I would like to put a small panel in the pole barn with two breakers - 30 amp for the single RV outlet and 20 amp for the lights and opener. The kids use the camper for sleep overs and the stone pad is between the garage and pole barn.

The distance from the garage subpanel to the proposed pole barn subpanel is about 110 feet, and the proposed pole barn subpanel is about 160 feet from the main box in the basement.

(1a) If I follow this setup, can I run 8/3 with ground from the garage box to the pole barn box? Or do I need 6/3?

(1b) Do I still use just a double 30 amp breaker in the garage box?

(1c) In the pole barn, after the breaker, do I get to use 10 gauge and 12 gauge wire for the two circuits? I understand and can calculate voltage drop, but am unclear on what happens when we start putting subpanels in the middle. The garage was wired by an electrician and he used 12 gauge wire on the 20 amp circuits and 10 gauge on the unused 30 amp.

(2) Just to confuse matters, if I install an RV post box (like they use at the campground) on the back side of the garage, I shorten the distance from the garage box to the RV box to about 50 feet. I should be able to just run 10/2 wire to the RV box with a 30 amp breaker in the garage, and run 10/2 to the pole barn to power the outlets and lights. Can I run a single 10/3 wire to the RV box, using the black for the RV box, and then continue to the pole barn with 10/2 connected to the 10/3's red, white, and ground? I don't think this is the best way, but I already have an unused 10/3 Romex from the garage box into the garage attic that was intended for an electric heater. We installed a propane heater instead.

Thanks for your patience and suggestions.

AllanJ · 08-25-2011, 09:31 PM

A subpanel in the middle does not contribute to voltage drop. Do your calculations based on the distance and wire size from the main panel to the first subpanel (where you are tapping off) and the distance and wire size from the first subpanel to the second subpanel.

You can go ahead and run the feed from the first subpanel to the RV outlet box and then to the second subpanel. But this is very limiting for future expansion. The RV would soak up some of the power that you might want to use in the pole barn in the future due to the shared black/white part of the circuit. You may not run a second red/white/black/ground or whatever feed cable from the garage to the pole barn for more power in the future.

brgmgb · 08-29-2011, 05:18 PM

Based on the calculations I have done, I plan to run 6/3 wire from the garage subpanel to the pole barn subpanel.

In the pole barn, I will have three circuits - a 30 amp for the RV outlet, a 15 amp for the ceiling lights and opener outlet, and 15 amp for a few wall outlets. I'll group the two 15 amp breakers on one lug and the 30 amp on the other.

In the garage subpanel, what size breaker should I use? My first thought is a double-pole 30, but should I make it larger?