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Posted by David Sherman [24.32.202.83] on Tuesday, December 15, 2009 at 17:48:35 :
In Reply to: Re: OT - 60air or 80 gallon tank? posted by Desoto61 [138.163.0.44] on Tuesday, December 15, 2009 at 16:40:37 :
8 gauge is legal and would be safe. It could even squeak by with a 40 amp breaker (80% load is the requirement and 32/40 is 80% exactly). However, if the panel is any distance at all from the compressor, I would go with #6 wire because a compressor draws a pretty heavy start-up current and you don't want it stalling out due to voltage drop. Plus, even when it's running smoothly, squirrel cage motors do not do well at all with under-voltage conditions. They lose power and efficiency pretty fast as the voltage drops and the slip angle increases. If there's any doubt, measure the running voltage at both ends of the wire. If the compressor comes with a cheesy power cord, best upgrade it at the same time.
Regardless of the wire gauge, it'd still be your call as to whether to use a 40 or 50 amp breaker. 40 is legal with #8, 50 requires #6. Nobody says you can't use heavier wire than the minimum for a given breaker rating. The codes are based on avoiding fires, not on voltage drops. The minimums are generally fine in a normal house, with less than 100' from the panel to the furthest load, but on longer runs, like from the house out to the shop, voltage drop can be the controlling factor rather than overheating.
Remember the compressor doesn't need a neutral, so if you're pulling THHN in metal conduit, you only need to pull 2 wires. The conduit will be the ground. Use armored cable or flex conduit to go from a J-box on the wall to the compressor itself. If you use Romex, you'll probably have to buy 8/3/WG or 6/3/WG. I have not seen 6/2/WG or 8/2/WG Romex. That means you're buying 50% more copper than you need, which is money you could put towards conduit and have a tidier and better installation overall.