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I’m adding a sub-panel to my house about 30 feet from the service panel. There will be two 240 volt circuits in the sub-panel. One will be a 240 v - 20 amp circuit and the other a 240v 30 amp circuit. What size breaker should I use in the service panel to power the sub-panel?


The line running from the main service panel will be a #8/3 w/ground stranded. At the subpanel one of the circuits will be powering an A/C unit with 240v which calls for a 20amp line. That 20 amp line runs about 10' from the subpanel to the A/C unit and is a #10/2 w/ground solid. The other circuit in the subpanel, will be running about 75' under the house to an outdoor sauna which requires a 240v 30amps. That line is a #10/3 w/a ground solid. I'm planning on splitting the line at the sauna to accommodate a 120volt for a light in the sauna. Here is a picture of the service panel;service panel

The red, black pair is currently powering the A/C. They will be removed and replaced with the aforementioned 8/2 line to the subpanel.

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  • What are the loads on the two circuits? Breakers and wires are sized on loads. can you include a picture of the main panel? – JACK Dec 23 '19 at 13:29
  • What size and type of wire will you use from main to sub? – Harper - Reinstate Monica Dec 23 '19 at 13:55
  • Can you post photos of your service panel? What are these 240V circuits powering? Is the path from the service panel to the new subpanel location through finished or unfinished space? – ThreePhaseEel Dec 23 '19 at 17:17
  • Can you get us a close-up shot of the breakers in the service panel? Also, am I correct that this main service panel is outdoors, and I take it the new subpanel will be outdoors as well? – ThreePhaseEel Dec 23 '19 at 19:05
  • Also, are you running this 8/3 cable by itself, or in a conduit? – ThreePhaseEel Dec 23 '19 at 21:41
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Nope. You can't run that cable through that conduit.

8/3 "underground Romex" as you call it, actually UF-B cable, is not permitted in that conduit because the cable is too large for the conduit. So are most cables. You are actually better off running the type that is already there, THWN-2 individual wires. You will need 1 white and 2 of black or a color. Your existing ground wire is fine and may be reused, if it is 10 AWG.

Romex in conduit (which will not fit) is good for 40A. THWN wires are allowed 50A.

Supply breaker ampacity is decided by the cable.

  • 10 AWG wire - 30A
  • 8 AWG NM or UF cable - 40A
  • 8 AWG THWN wire in conduit - 50A
  • 6 AWG NM or UF cable - 60A
  • 6 AWG THWN wire in conduit - 70A

If the subpanel is main-lug, the breaker also can't be larger than the busing on the subpanel, however that won't be a problem if you listen to my "think big" advice above...

Don't scrimp on subpanel size

Install the biggest subpanel you can possibly stand to get. Panel spaces are cheap. Running out of spaces in a panel later is an expensive, frustrating problem.

If you aim to scrimp, then a) skip the main breaker. Your subpanel does not need it if it's in the same building. And b) use aluminum wire, replacing #8Cu with #6Al... or #6Cu with #4Al. Aluminum wire is actually preferred at these large sizes. The subpanel input lugs will be aluminum!

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