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We are currently building a new house that will have an upgraded 400amp main panel in it. The builder is going to put a disconnect switch on the outside of the garage where the main panel is located so all I have to do is hook into it. I will also be building a 24'x24' workshop in the back yard. Reading online, it seems like a 100 amp panel in the workshop would suffice. The workshop will need about 180-200' of wire ran. I plan on putting down conduit for the wiring (probably 3" to be really safe). Just have no clue on what size wiring I would need.

I am a DIYer at home and feel I have good electrical common sense, just never ran something this far out or for that large of a panel.

Any help would be great, thank you!

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  • If you are there when the electricians are I'd ask them what the code would require. – Micah Montoya Nov 19 '18 at 16:36
  • Honestly didn't think about that. The house is only down the street from where we live now, so will be searching for them. Thanks. – Ken Nov 19 '18 at 17:06
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First, forget copper wire at this distance and size, unless you own a copper mine. Aluminum is a better conductor (by weight and cost), and is a better match to the zinc plated aluminum lugs you'll be attaching to. Use the dielectric paste. Yes, there was an issue with aluminum in small branch circuits (1350 alloy on receptacles and switches rated for copper only), but this is feeder. The power company still uses 1350 alloy on service laterals, and if you let them install this feeder they will use it. But you're supposed to use AA-8000 alloy.

Of course you know to use individual THWN-2 or XHHW wires instead of a balky cable.

Putting in 3" conduit is brilliant and will increase your ease of DIY. Just keep in mind you won't be able to put data cables in there. Only certain (non-conductive) fiber optic cables are allowed. You can put other power wires in there, say for a switched load like a light or well pump, but if you do, you'll need to derate all the conductors including the big set. Keep that in mind, this whole conversation is about derating.

Your minimum conductor size will be #1Al. If you install that, you are legal because any segment of wire won't get hotter than is safe. However you have many segments, and may have some voltage drop. You may want to upsize wire for the convenience of less voltage drop.

The first question about voltage drop is "do we care?" (Or to be more precise, how much do we care). People "chicken little" over a 3% number, but we just had a lost-neutral in our apartment complex where the balance tipped 20 volts (so 100/140V), and nobody noticed for days. The stock rule of 3% is concerned with this segment, with other unknown segments adding other unknown 3%'s cumulating... but in this case, this is the big segment - there isn't anywhere on either end to lose much more. So the 3% rule aims for no more than a 6-8% drop globally, and that's a fair target, given that the in-main and in-barn wiring won't add up to more than a 1% drop. Keep drop under 7%.

Lastly we must avoid the great bungle of most people doing voltage drop calcs. Do the calc on your actual amp load, not the number on the breaker. The breaker is there to protect the wire. If you voltage-drop-calc derate to 150A wire, you'd be perfectly allowed to up-breaker to 150A. It won't change your actual load one bit. Think it through.

Speaking of that, do not fit a 100A panel. The main breaker in the barn panel is redundant, it's just there to be a shutoff switch. A 200A breaker will work just as well as a shutoff switch, and give you a lot more breaker spaces too. Around here, we are fanatics at recommending lots of spaces, so you never, ever come back asking how to squeeze more circuits into a full panel! Spaces are cheap, regrets are expensive.

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When we talk about running long distances, out main concern is voltage drop. So when we start to do our calculations we always need a total wattage or volt/amperage that is going to used at any one time (demand load). This isn't all of the equipment that is installed in a garage (connected load).

If you feel that you must have a 100A panel in your garage and you want to size the feeder to handle a total of 100A. Then you will need a feeder made up of #1 CU or 2/0 AL conductors which would allow a 3% drop, but let's say we are only going to normally use 80% of that 100A load. Then you would need a feeder with #3 CU or a #2 AL conductor.

The whole exercise is to provide good system and satisfy the home owners needs at a more favorable cost.

So I have given you your answer for a 100A feeder, but you might want to consider your actual demand once you price your feeder up.

Hopes this helps and good luck.

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  • Thank you. I just thought a 100 amp would be more than enough. Guess will have to do some calculations of what really is going to be out there for this. – Ken Nov 19 '18 at 18:20

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