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I have a shed roughly 70' from house. I plan on running a ac, small air compressor, and maybe a small welder, along with battery chargers and lights. I just need to know what size wire I could run. Could I use 4 AWG THHN wire?

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    At 70', voltage drop is not an issue. So it comes down to the total expected load, the total expected continuous load and the type of wire (copper 4 AWG is rated for 85A @ 75C, aluminum 65A @ 75C, with aluminum a lot less expensive than copper). AC could vary a lot depending on the size, which depends on the size of the shed. Welder can use a lot. Far too many unknowns to guess whether 4 AWG is enough or not. Dec 14, 2022 at 15:59
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    Yup, this definitely needs a lot more detail to get a good answer. Otherwise, the answer is "Check the NEC charts and figure it all out yourself", and I'm sure that's not the answer you're looking for.
    – FreeMan
    Dec 14, 2022 at 16:16
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    You need to run armoured cable. Plastic sheathed is not sufficient.
    – Tetsujin
    Dec 14, 2022 at 16:52
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    @Tetsujin US (NEC) rules allow for conduit instead of armored cable. Dec 14, 2022 at 23:37
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    @Tetsujin -- also, North American armored cables (BX/AC) are unlike UK SWA in that they are not wet location or direct burial rated Dec 15, 2022 at 4:34

2 Answers 2

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Feeder sizing is kinda funny

A lot of people think "well my 15A circuits are copper #14... and my 20A circuits are copper #12... so for bigger and bigger circuits I just keep going copper # (smaller and smaller number)".

That's actually incorrect because another thing enters the picture: Aluminum heavy feeder. Now I know what you're saying, "that stuff was tried for 15A and 20A circuits and it was a fiasco". True, but it had also been used all along for heavy feeders and transmission lines, and that stuff never gave any problems at all.

So there's a point where we want to crossover from copper to aluminum feeder. Where is that point? Enter Money. #2 aluminum (90A) is roughly the same cost as #10 copper (30A). So really, the crossover point is 30A. Above that you want #2 even if it is overkill for your needs - it's just cheaper.

#2 is at a pricing "sweet spot" because it's very widely used for 100A services to things like mobile homes. (whole house services get a slightly favorable wire size derate; otherwise #2 is 90A).

Anyway, that simplifies the question for any feeder size from 30A-90A. #2 aluminum and done.

90A should satisfy your requirements, since generally you'll only be using 2 tools at once:

  • dust collector and some woodworking tool
  • compressor and some metalworking tool

And welders get a significant favorable derate because of their short duty cycle.

For A/C we strongly recommend a "mini-split heat pump". Why? It can shift gears and become a heater, which means you don't need one of those terrible resistive electric heaters which explodes your utility bill and also your electrical panel lol. Heat pumps are 300-600% efficient (depending on outdoor temp) compared to electric heaters which are 100% efficient by definition.

Heck, heat pumps are more efficient than gas (as in: making the same heat takes less total gas if you burn the gas at a power plant and send the electricity to you to power your heat pump). One heat pump admirer did a whole video on that (vis-a-vis using less gas from suppliers who aren't very nice people).

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There are lots of variables here, so some more information would be helpful (including what you are coming from).

That said, if you have to ask these questions, I'd recommend hiring an electrician. You can always trench and run the conduit to save a substantial amount of money (then they just have to pull the wire and install the panel)

The considerations you need to take:

  1. You need to be able to bury the run safely. Your options are to bury direct-bury wire 2ft or greater, PVC Conduit at 18"+, or Armored/Metal conduit at >12"

I would recommend burying/trenching PVC Conduit (you can always upsize) as that tends to be the most cost effective. Rent a trencher.

  1. You're likely better off running 240V to a subpanel, and then splitting your load there, then trying to run directly circuits for each item, since that allows lower amperage. Do you have the space in the main panel?

  2. To determine wiring size means you need to determine the AMPs and the wiring type. The power usage of the chargers, lights, and other things are going to be inconsequential. Your big power consumers are going to be the welder - check the amperage. It will likely be 20A-30A for a small 120V one and 30A-50A at 240V for a larger one. Your AC is also going to have an AMP rating, which could be 20A for a small one to 50A for a larger one.

Once you know total amperage, you can plug that into a wire size calculator such as http://wiresizecalculator.net/ or http://www.paigewire.com/pumpWireCalc.aspx to determine what size wires, then plug that into a conduit fill calculator (https://www.southwire.com/calculator-conduit or http://conduitfillcalculator.com/) to determine what size conduit.

Based on what you describe, it sounds like 1.5" is adequate conduit size, with but I don't have enough details to confirm for sure - check your numbers!

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