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I am getting so many different answers on what to run to my detached shed. I am going to pull permits soon and have it inspected before hand - but im trying to blend in the codes for what it is allowed in Colorado

So, I am building a 200 sqft shed that will power up around 5 or 6 outlets along with 3 security cameras, power tools such as table saws, circular saws, battery chargers etc.

ive been told to use a double pole 30 amp with UF 10-3 wire to feed my sub-panel (which will be 70 amp sub panel) The sub panel is going to have 2 x 15 amp circuits and 1 x 20 amp circuit for my tools.

I also been told to run a 70 amp with number URD #4 cable (4 wire) to feed the 70 amp sub panel. I am not sure if thats overkill, or if that will give out TOO much power to my 70amp sub panel and if that will be too dangerous?

So, my shed will be about 70 feet from my main panel, so I will have to dig a trench. I was also told not to use a conduit (pvc pipe) for the UF rated wire - instead it will be bare burial. is that code?

  • What size breaker are you going to use to feed the 70 amp sub panel is the first question that needs to be answered.if you only want to use a 30 amp breaker from your main panel then #10 would be correct. If you want 50 use a 50 amp breaker and #6 wire, for the max a 70 amp breaker and #4 wire. UF wire direct burial is ok but the wire going into the ground and where it exits will need to be protected with conduit schedule 80 PVC would be the easiest way to go here. You will need 4 conductors 2 hot 1 neutral and a ground. Also a ground rod at the shed. – Ed Beal Dec 11 '17 at 22:37
  • Thank you! So, actually I was trying to figure what I can use to feed the sub panel 30amp double pole with UF-B 10-3 wire to feed the sub panel or 70AMP with #4 wire URD to feed sub panel – willcha65 Dec 11 '17 at 22:42
  • How far away is the shed from where it's getting power from? – ThreePhaseEel Dec 11 '17 at 23:15
  • I would go with 70 amps since that is the panel you have, at 70' the wire sizes I listed above will work but you will need 4 conductors 10-3 with ground would work but is way two small, I personally would not go smaller than 50 even though it is only 200 sq ft a nice aircompressor and a chop saw or table saw used at the same time with a few lights and a heater and you are out of power. – Ed Beal Dec 12 '17 at 2:33
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Use a fat conduit and appropriately sized THHNs instead

Direct-bury wire (UF) is vulnerable to excavation damage and difficult to upgrade if you want more power at the shed later. Hence, it's better to slap a fat PVC conduit in now while you're digging the trench for the first time -- conduit's cheap compared to the cost of renting a trencher. Inside the conduit, you'll use 3 THHNs of appropriate size and color for 2 hots and 1 neutral (voltage drop control will likely mandate a larger size than the 10AWG minimum you were thinking of) as well as an appropriately sized bare copper ground (10AWG for a 30A feeder, 8AWG for a bigger feeder up to 100A) wire.

Don't skimp on the subpanel either

Buying small electrical panels is penny-wise and pound foolish as a larger panel now is always going to be cheaper than upgrading later. I'd recommend a 100A/24slot subpanel at a minimum here (you can always feed a large subpanel with a small feeder, just not the other way around -- remember that the protecting breaker must be the bottleneck so it can keep you safe from overloads). You'll also need a main device (it can be a breaker or a molded-case switch) at the subpanel to serve as a disconnect switch for the shed in addition to the breaker in the main panel protecting the feeder. (That's so you, or the FD, can shut power to the shed off quickly if it catches on fire -- "firemen don't deal with wires" after all.)

Last but not least, you'll need a ground electrode (Ufer ground if you haven't poured the foundation yet, rod if you have and need something you can put in now) in addition to the grounding wire -- the grounding wire provides a fault path (safety drain) back to the source while the ground electrode dissipates built-up static charges, induced voltages from lightning, and the likes. Don't forget to pull the bonding strap or screw from the subpanel when you put it in!

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For my money, first off, I am notorious for speccing large panels, because spaces are cheap and running out of panel space isn't even stupid. I would go at least 24 spaces and would watch out for 40- or 42-space panels that come with a bunch of free "bonus breakers" and that makes it a net win. This could be as high as 225A. It must be >= the breaker feeding it, so 225A panel fed by 20A breaker is fine, 60A panel off 70A breaker is illegal.

"40 spaces" sounds like a lot until you start hooking up 240V loads, which I highly recommend if you're carrying power any distance. They gobble up 2 spaces each.

Using "2 breakers, 1 space" double-stuff breakers isn't possible if they need GFCI or AFCI protection.

As far as conduit vs. buried cable: on one hand, the risk of damaging the cable somewhere underground is very real. And hard to trench, requiring typically 24" rather than the 6" required for expensive rigid conduit. On the other hand, wire in conduit is easy to steal.

Yes you can put UF in conduit... it's just dumb. There are better wires to use in conduit. Anyway, when selecting cable, do it in this order:

  • conduit vs direct burial
  • amps you want the circuit breaker to be
  • metal you prefer... Most people are drawn to copper because they heard some stuff about aluminum that doesn't apply at all to feeder. Aluminum is more correct for feeder since the lugs are aluminum and this will reduce dissimilar metal corrosion. You may recall that the kerfluffle about aluminum is actually about dissimilar-metal corrosion, and the real message isn't "aluminum ooga booga" but rather "don't mix metals carelessly".

If you we going much further, you'd want to do some voltage drop calculations based on your usual load (NOT breakered max) but you don't have that problem for 70'.

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