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I currently have a 200amp main panel that has no room for additional breakers. However I want to add circuits for the following: 1) 50amp circuit for a hottub 2) 50amp circuit for my garage for intermittent use of a welder and air compressor 3) 15-20amp circuit for general use

I plan to add a sub-panel this weekend. What I'm trying to determine, and I seem to get varying answers from the supply stores, is WHAT amperage do I need to supply the new sub-panel with? My initial thought was 100amps just to make sure I had the required power, but appropriately, it was pointed out that not all of these items would run at the same time, so a 60amp breaker may be sufficient to run the sub-panel.

I'd like to get some sound advice on which I should run, and what gauge of wire should be run between the two panels. My thought is to run a 4gauge regardless.

Any thoughts are sure appreciated.

HTK
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  • Are all the breakers in your panel tandem breakers? – Steven Jun 04 '15 at 18:58
  • Does the hot tub pull any load when it is not in use? How about the compressor? – DarthCaniac Jun 04 '15 at 19:31
  • I assume the air compressor does not plug into a standard 3-prong outlet if you want to hook it up to a 50 A line -- otherwise putting a 3-prong outlet rated for 15 or 20 A on a 50 A line would not be up to code. What are the specifications on your air compressor (horse power, voltage, and amps)? – statueuphemism Jun 04 '15 at 19:47
  • No, not all the breakers are Tandem... I guess I could convert a few of them and save myself the effort of a new panel... I'll have to consider that. – HTK Jun 04 '15 at 19:58
  • Also, the Air Compressor requires a 230v plug. The hot tub pulls a 40watt load to run circulation pumps when not on, and then the heater pulls about 30 amps as needed. – HTK Jun 04 '15 at 19:59
  • Essentially, you are looking to have someone do a load calculation (section 220 of the NEC). More information is needed about the types of equipment (arc welder or resistance welder?) and the volts, amps, and horsepower of the equipment. The NEC has specific articles for load calculations and wiring methods for Welders (section 630), motor loads (430 -- this is where the air compressor's motor would fit), "Swimming Pools, Fountains, and Similar Installations" (section 680 -- hot tub fits here, though specific motor loads may fit under 430). – statueuphemism Jun 04 '15 at 20:33

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If you're spending the money to add a panel anyway, you might as well reduce the changes of having to expand it later. Unless you're installing the second panel really far from the main, you're likely not going to spend much more to put in the 100 amp panel over a 60 amp panel. Oversizing the panel will not hurt anything but your wallet, so why not do it?

The installation will be exactly the same either way, the only differences will be the size of the breaker in the main panel, the main breaker in the new panel, and the size of the conductors between the panels.

You can read this answer for more detail, but likely you'll need four 3 AWG copper conductors to feed a 100 ampere panel. Of course the size will change based on the length of the run, and other factors. So make sure you verify the size once you know exactly how long the wires will be, and how they'll be run.

Did a quick search for wire prices, and it looks like it's about $0.20 per foot difference between a 6 AWG stranded copper wire and a 4 AWG stranded copper wire. Then another $0.20 per foot difference between 4 AWG stranded copper wire and 3 AWG stranded copper wire. So you'd be looking at an $0.80 per foot difference between 6 and 4 AWG feeders, and an $0.80 per foot difference between 4 and 3 AWG feeders.

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Tester101
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  • That was my initial thoughts as well. Like stated below, I won't probably be welding while sitting in the hot tub, but on the other hand, the difference in price between breakers is $20... Not worth the discussion. The boxes are literally side by side... so the wire run would be under 5 feet (probably about 3'). – HTK Jun 04 '15 at 21:59
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If this were me - and it's obviously not, but if it were - I would tend to assume you won't be using the welder / compressor at the same tiem you're using the hot tub, but you may be using the welder/compressor along with something on the 20A circuit, so I'd put in a 70A two-pole breaker for the run. I don't have the wire gauge guide handy, but based on my memory, I think it was 6 AWG. Please double-check this. Then, for every ~ 100ft the wire travels between breaker boxes, increase the wire size by 1 - so if you have a 250 ft run, use 2 AWG wire. This helps account for voltage drop along the run between breaker boxes.

In summary - please, for the love of the little people, talk to a licensed electrician.

John
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  • Why 70 amperes? – Tester101 Jun 04 '15 at 21:31
  • I'd hope one doesn't use the hot tub and welder at the same time. That's a recipe for disaster. :) – DA01 Jun 04 '15 at 22:20
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    My wife sometimes soaks in the hot tub, while I'm welding or using the compressor. It's not safe to assume that these loads will not run at the same time. However it is dangerous to weld while in a hot tub, so that should be avoided. – Tester101 Jun 04 '15 at 23:14