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I am losing my mind trying to figure out what type of service I need to send to a remote office I built in the yard. I have worked as an electrician's assistant so I have experience in perfomring the work but never really planned jobs.

I am trying to figure out what gauge wire I need as well as how many amps to run out there. I will have lights, computer, amp, tv, and wall AC unit out there. Basically a bunch of electronics and lights. The only big draw would be the AC.

Should I be looking at 100 AMP service or is that overkill? Until this point I have been looking at running 100AMP with 1 AWG wire. I have 100 foot run under the house and 100 foot run underground (200 feet total). Any suggestions would be great.

Thank you!

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    What is the A/C rated at (amps)? You'll also have to calculate voltage drops over that distance when sizing the wire. – JPhi1618 Feb 9 '18 at 18:36
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    These two answers provide a lot of good information, and probably make this quesion a duplicate - 100 amp panel with 60' run and 70 amp panel at 300' – JPhi1618 Feb 9 '18 at 18:38
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    Harper's Rule: don't forget the dust collector. Here that means don't get blindsided by a laser printer, the heater you use when the A/C isn't called for, and amp[lifier]s can be surprisingly big. Do a careful and certain energy assessment first, and you can select wire size with confidence. – Harper Feb 9 '18 at 23:30
  • How many square feet is the office-shed? – ThreePhaseEel Feb 9 '18 at 23:32
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Do some research on how to do a load calculation on a building, that should give you a good idea on what size service you need. Once you've got that, you can use tables and voltage drop calculations to determine wire size.

Here's an answer, that describes an easy way to calculate the load for lighting and general use receptacles. On top of that, you'll want to calculate the load for any additional items. You mention an A/C unit. You'll want to check the nameplate on the unit, and add the A/C load requirements to your total load. Do the same for any items that are larger than you'd plug into a typical wall socket.

This answer gives some good information about how to size the conductors, once you've determined what size service you'll need. This answer will show you how to calculate the voltage drop, so you can determine if you need to use larger wire or not.

If you can get your hands on a copy of the National Electrical Code, article 220 is a good place to start for information on doing load calculations. NFPA offers free read-only access to the NEC, all you have to do is sign up for an account.

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