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I have a detached workshop 100 feet from my house that I want to run power to. I need help determining if an existing 30a circuit will work, and what gauge wiring to use for the 100 foot run.

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The previous owner of my house had a large air compressor on the corner of the house outside. To power this he had a double slot 30a breaker and 10/2 wire run to a cutoff box outside the house. This location is ideal for running power to my workshop 100 feet away, only one minor 35 degree turn is needed a few feet from the workshop.

In the workshop I think i want two circuits, one 20a to power three outlets for saws and various items on my workbench. The second circuit could be only 10a, i plan on using it to run two exterior LED lights, and an interior LED flood light over my workbench. LEDs are pretty efficient, so I think 10a is enough.

I plan on running 3/4" underground conduit in an 18 inch deep trench (code in my area) and installing an 8 slot 125a sub panel in the workshop to house the 10a and 20a circuits.

My questions are:

  1. Can i use the existing 30a double circuit with 10/2 wiring, and extend off that to run to my workshop?

  2. Is 10/2 wiring the proper gauge to run about 106' from the cutoff to my new sub panel?

  3. The existing 30a circuit uses a 10/2 wire, (1 hot, 1 neutral, 1 ground). Can that be run to a sub panel? Other subpanel install i have seen look like they use a line with 2 hot wires, 1 neutral, 1 ground.

  4. Does the subpanel need a grounding rod? If so what gauge copper line is needed?

My goal is to do the majority if not all of this work myself, especially the conduit and running the wire. I use extension cords today and it's getting a little old. Especially tired of my chop saw not getting up to full power and speed.

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    Is the conduit the current wire is running in metal or plastic? – Nate S. Aug 9 '19 at 18:34
  • Is replacing the existing 30A circuit an option? What wiring method was used to run it? – ThreePhaseEel Aug 9 '19 at 22:50
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You can put in a sub you need 4 wires ground , neutral and your 2 Hots. Since the shop is detached you will need to have a ground rod and #6 wire is normal for you his , I would consider running a larger feeder wire than #10 but if only running 1 saw and your lighting #10 could work. Even though you have the wiring protected at the main panel with a 30 amp breaker your sub can have a total of more than 30 amps if you add the breakers. For example if all single pole breakers you may have 80 amps if you total all the breaker values. The total load is the important thing here as we usually have a safety factor and only use 80% of the circuit capacity. In fact most homes that I check are running at 25% of their main breaker or less. So yes you can but I would go bigger, I have never been asked to reduce the size of a panel but many times I have gone back and upsized the service. At a minimum put in larger conduit so you can upsize later. Added I was realizing that you did say 2 with ground as I earlier mentioned you will need 4 wire to the sub since you will need to add a neutral if in conduit back to the panel this or a new 4 wire run at the largest size you can afford, for example I just put a sub in for a small heat pump it needed a 20 amp circuit , I put in #6 and a small sub, now I have plenty of power to add lighting to my round pen and even a hot tub over there or whatever I might need hope the additional info helps.

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  • Is it possible to run a 120v sub-panel or do they have to be 240v? If 120v is allowed, OP may be able to use the 10/2 that is in place as a starting point? – JPhi1618 Aug 9 '19 at 17:30
  • The problem with 120v would be the voltage drop at over 100’ and that was one of the concerns listed by the op. But you can have 120v – Ed Beal Aug 9 '19 at 18:08
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You need to figure the maximum current that could flow given the size of your feeding breaker, size the wires to match, then use large enough conductors based on the distance they need to run. The are charts that show code permissible sizing for given aperage, esp. over distances.

To wire the remote building, you need to install a sparate ground (like a rod, plate or ring) and bond the cable’s ground to that. Use 4 conductors - 2 hots, neutral and ground. Keep ground and neutral separate except at the first panel with a cutoff switch for your property, usually the main panel where ground and neutral should be bonded. This prevents normal current from using the ground system as the return instead of neutral, which is another way of saying unexpectedly energizing your ground system; that could be dangerous.

If you run conductors in conduit, they need to be separate wires - not a cable. I think you’d look for something like THHN. Be sure to match the type with your intended use (eg damp or water resistant, or heat resistant) as well as sizing for the load and distance.

Here is one basic article that talks about some of this, Conduit 101 - The Spruce

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  • Nothing in NEC requires you to up-size wire for long distances, except that there is a bare minimum floor of voltage drop that is considered intolerable. This is a lot more generous than the 3% drop @ breaker trip current which the cable salesmen would like you to believe. They love making you spend money on wire, and it's unnecessary. Cable in conduit is allowed, just not a very pleasant pull. – Harper - Reinstate Monica Aug 10 '19 at 4:29
  • I’ll have to look for it, thought there were definitely restrictions on ampacity vs distance...wire salesmen, haha, just could be. Again, common knowledge restricting nm pre-formed cable in conduit — really, not so? – Old Uncle Ho Aug 10 '19 at 4:48
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    You can run cable in conduit if you really are a masochist and like difficult pulls... but the conduit fill numbers are pretty punishing - they treat flat cable the same as round wires of the large dimension, and the fill rules for 2 "wires" in conduit are particularly punishing - only 29% fill allowed after we circularize the flat cable! NM isn't allowed in outdoor conduit because NM isn't allowed outdoors period. UF is allowed, but again, more trouble than worth. – Harper - Reinstate Monica Aug 10 '19 at 4:58
  • And UF is so flat that it kills you on the circularization. Somebody wanted a 6/3 and 12/2 UF in a pipe (the deadly "2 wires" at 29% fill) - it calculated out to a 2" pipe. – Harper - Reinstate Monica Aug 10 '19 at 18:08
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Given your 3-wire supply, you could only use that for a) 240V-only service (no 120V), or b) 120V-only service (no 240V). There's a third option that involves a supply transformer, but it's complicated. Running 240V-only is dangerous because "the next guy" will interpret the panel as an old-school 120/240V panel, and insert 120V circuits in it because he doesn't know any better. So the only option is a 120V-only panel.

This uses 3 wires - hot, neutral and ground, so your white wire would have to be converted to a neutral. Now, the distance is nothing to worry about for a 240V run, but taking 120V that long will risk significant voltage drop. Assuming 130' total run of #10 wire, and allowing a 20A circuit at 80% capacity (which is the max you should load it to), and a lighting circuit drawing 2A, I see 4.06% drop which is perfectly acceptable. Even with the 20A circuit at full 20A + 2A lighting, you're still at 5% which is marginal but acceptable.

To wire the subpanel, you separate neutral and ground bars (you'll need an accessory ground bar if it doesn't come with), and then split the 1 hot wire so it goes to both L1 and L2 lugs.

The subpanel absolutely requires a main "shutoff switch" and the cheapest way to get that is use a subpanel with a main breaker. The trip rating of the main breaker doesn't matter.

The subpanel also requires a standard ground rod setup. This is two 8' rods set at least 10'(?) apart, unless you install one rod and it passes a fairly complicated impedance test that requires special equipment. The grounding electrode to the ground rods can be #10 bare copper.

Since you are running in conduit, you would be unwise to use 10/2. Right off the bad you can't use 10/2 "Romex" NM-B cable, because it's not rated for outdoor/wet. All conduits are assumed to be 100% full of water 100% of the time. The only option in cable would be UF-B, which is very flat and wide, and that creates extraordinary demand on the size of the conduit. Instead you should use THWN-2 individual wires. This also allows you to later add a wire for the other L2 phase, so you can have a full 120V/240V panel. Unfortunately you must buy 3 colors - hots can both be black, but neutral must be white and ground must be green or bare. This makes buying 106' of wire awkward, since spools are 100'.

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