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I am wanting to run power to my shed/workshop. I will have 8 outlets running table saw and such not everything at same time also will have shop lights and a separate 20 amp circuit for pool pump. What size breaker in main house panel should I use to connect to subpanel in shed that is approx. 90 feet from the main panel?

  • What are the dimensions of the shed? Is the table saw the largest tool you'll be using? Do you ever plan to add dust collection, or start welding? – Tester101 May 11 '17 at 15:02
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    Also, even though you won't have everything on at once, it's much safer to assume you will than to risk tripping breakers by using multiple outlets. And to build in headroom if you want to add stuff later. – Chris M. May 11 '17 at 15:05
  • Shed is 10x14 and no on the welding or dust collection. – Scott Lewis May 11 '17 at 15:16
  • For the minimal price difference between options, it's likely worthwhile to go with a 100A box. Saving $20-30 on the initial install isn't going to be worth it if you have to rewire it in 3-5 years to accommodate a new toy/hobby. – Myles May 11 '17 at 18:24
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I've said it before, and I'll say it again. It's tough to beat a 60 ampere sub-panel. You might be able to get by with a 40 amp setup, but it's always nice to have future expansion possibilities.

At 90 ft., you're probably okay to use 6 AWG copper, or 4 AWG aluminum conductors. You'll have to pull four conductors, either through conduit or via a cable.

Since it's a separate structure, you'll need a grounding electrode system. Which you'll have to bond the grounding conductor from the feeder, and the grounding conductors within the building to. You'll keep the neutral conductors isolated from the grounding conductors, which will likely mean removing a bonding jumper within the panel.

  • Thanks that was what I was thinking of doing. On the subpanel do I need a 60 Amp breaker for disconnect or will I be fine with just the 3 20 Amp breakers serving as individual disconnects. – Scott Lewis May 11 '17 at 15:41
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    You should be fine with just the circuit breakers as the disconnects. NEC 225.33 allows up to six circuit breakers to serve as the disconnecting means. However, if it were me, I'd probably install four 20 ampere circuits. One GFCI circuit for the pool equipment, one 15 or 20 ampere circuit for lights, and two 20 ampere circuits for receptacles. You might even consider doing the receptacle circuits as a multiwire-branch circuit, and splitting all the receptacles between the legs. So the top of each receptacle is on one circuit, and the bottom is on another, but the neutral would be shared. – Tester101 May 11 '17 at 15:50
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    You could even do each individual receptacle as a multi-wire branch circuit and never have to worry about breaker trips from 2 things plugged in at once. Of course this presumes ample spaces in the panel, kinda thing I would do :) I prefer subpanels with main breakers so you don't have to worry about the rule of 6 and it's obvious to anyone how to turn it off. Very often people will asume the 2-pole in the upper left is the main breaker. – Harper - Reinstate Monica May 12 '17 at 1:43
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Based on your question, since you will be having a subpanel, you want the breaker feeding the subpanel to be sized for the subpanel or slightly smaller. So if you purchase and install a 200A subpanel, then the breaker should be 200A. If you use a 125A subpanel, then use a 125A breaker. And so on...

If you are wondering on what size of subpanel, which I believe this is the question, then you add up the wattage (or amp draw) of each element or potential element to be used.

  • 8 outlets * 1.5A = 12A
  • Table saw * 15A = 15A
  • Pool pump * 10A (guesstimate) = 10A
  • Lighting (240W?) * 2A = 2A

Lighting is high depending on what you go with, but either way this runs up to only about 40A. Giving yourself some space, you still have plenty to work with a 60A panel and run it all. Outlets of course depend on what you plug in and such, but you know that.

So again, if you run with a 60A panel, then use a 60A breaker in your main.

I'd suggest giving yourself additional space though and install a 100A panel and main.

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    No. There's no reason for the breaker in the main panel to be same as the subpanel rating. If you want to feed a 200A subpanel with a 60A breaker, that is fine. The big mistake is getting too small a subpanel (e.g. Because you think it must match the supply breaker) and then running out of spaces. OP should get the size of panel that leaves him plenty of room, and not worry about the subpanel rating (long as it exceeds the supply rating of course). – Harper - Reinstate Monica May 12 '17 at 1:13
  • @Harper You are right, but that wastes money on capacity to save on breaker and wire size? Just match the panel or "slightly smaller". If you want a larger capacity for spares and spaces, then run the breaker and wire to match. – TFK May 12 '17 at 1:21
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    In my years I have never had to go back because a panel was two big but I have upgraded quite a few to larger panels, some package deals have breakers in the panel but GFCI breakers or outlets will be needed so not much savings there. Go larger it is only a few bucks and you will be good when you decide to add outdoor lighting and other things that could be powered like fish ponds. – Ed Beal May 12 '17 at 1:33
  • You're thinking about it upside down. Panel space is cheap. In the history of this forum, no one has ever said "I am so glad I got the 12 space instead of the 30 space panel, look at this cool $20 bill I have as a result!" Nosirree, they say "Help, I'm out of space in this panel and really don't want to have to tear everything out and start over" and our advice costs way more than $20. Often they got the too small panel because they thought they had to. – Harper - Reinstate Monica May 12 '17 at 1:34
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    @TFK varies case by case, depends on the cost of wire, funding, upgradeability etc. Often breaker spaces aren't about current capacity but circuit capacity because it's humanly impossible to run more than a few loads at once, e.g. Heater, dust collector and (insert one of nine wood shop tools here). – Harper - Reinstate Monica May 12 '17 at 1:57

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