What’s the largest breaker I can use in 100 amp panel to power a subpanel?

Question

What’s the highest breaker I can use in my main breaker box (100A service) to power my sub panel in the garage panel?

The sub panel is a 100A box doesn’t have a main breaker. If I have 50A in the main, how high can I go in both panels to get the power I need without creating issues?

The breaker that powers my garage right now is 50A with E18679 (ul) AWG 8 cu 3 CDR with AWG10 ground type Hm-B 600volts wire connecting the sub to the main.

Can I just change to a larger breaker in my main and put a big enough one in my sub panel to keep my welder and compressor running off one breaker, or do I need one breaker for each and what size should put in the main to handle those two things? What size breakers in the sub panel in the garage?

Answer 1

Given the facts you have provided, your maximum safe breaker size is 40 amps.

You are already overbreakered and you need to fix that. This is due to the 8 AWG cable connecting the two panels. Breakers protect wires and equipment. 8 AWG cable requires 40A protection. I gather downbreakering to 40A will be an inconvenience, but so will owing a mortgage on a burned out house because insurance denied the claim.

The ground wire being #10 instead of #8 is allowed.

If the cable were a larger size, you could run a larger breaker.

• 6 AWG copper / 4 AWG aluminum can be breakered as large as 60A
• 3 AWG copper / 1 AWG aluminum allows a full 100A.

Yes, you can put a 100A subpanel off a 100A main panel. And in that case, if your main panel has feed-through lugs on the bottom, you can tap off those. Otherwise they make "sub feed lug kits" which snap in like a breaker, but are just lugs (have no overcurrent capability). They're not much cheaper than actual breakers though, so if the cost is within 20% just use a proper breaker.

Answer 2

The #8 awg you have ran to your sub panel should be protected depending on its insulation rating. If you have #8 romex ran from your main to your sub panel then it should be protected with a 40a breaker. If the #8 feeder has THHN insulation it's code compliant to be protected with a 50amp breaker. If the feeder has a THHN-2 insulation rating then the feeder is actually good for 55 amps assuming everything is inside the house. But good luck finding a 55amp breaker!