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I'm trying to get a few opinions as I have had 1 electrician tell me i need a subpanel but his whole reason was so i could expand in the future and he didn't really listen to what I wanted. I have had another tell me no I can run a direct burial cable from my main breaker on a single 20 amp circuit to the livestock shed to power the light, 1 outlet for a water pump/heater and 1 outlet for a electric fence controller but I wouldn't be able to expand by much more. Given I only need these specific items, i'm looking at a total of 25 watt pump, 500 water heater, 1.6 joule output for electric fence, 60 watt lightbulb I believe I can do it all on a single 20 amp circuit as I'm sitting at less than 600 watts (assuming I converted the Joules correctly) which is nowhere close to 20 amps

90 feet is the distance from the current panel to the shed.

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    Just because you don't plan to exceed the calculated limits does not mean that a future person will know to only use 50% of the 20 amp breaker with 12 AWG wire. – MonkeyZeus Jan 22 at 19:18
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    @MonkeyZeus That's wrong. Southwire sure wants you to buy all that, but they don't write the electrical code. Learn to use voltage drop calcs: #1 override 3% and put a more reasonable value that makes sense for the loads, and #2 calculate based on actual load -- never, ever more than 80% of breaker trip. – Harper - Reinstate Monica Jan 22 at 19:22
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    @Harper-ReinstateMonica "Ask two people the same question and you will receive 5 answers. If you want the right answer then consult the documentation." The Southwire calculator is the only resource I have so if you have documentation from the code book then I think it would make for a great answer. – MonkeyZeus Jan 22 at 19:25
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    Since you're digging a trench anyway, throw in some PVC conduit and run your separate conductors. You could upgrade to a sub panel in the future if needed. – JACK Jan 22 at 19:37
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    Actually, I don't think OP has stated a distance. 100 feet is plausible, but it could be less or it could be a lot more. David, please tell us the distance between the main breaker and the shed. – manassehkatz-Moving 2 Codidact Jan 22 at 20:13
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You state in your comment that you plan to run conduit.

In that case, do NOT use UF cable. Use THWN wiring - the conduit protects it, and it's much easier to pull than UF cable. It may also cost you less, and it can indeed be easily upgraded if or when you or a later owner wants to upgrade.

12Ga should be fine for your planned use. If your use increased, pulling one more wire and changing the feed breaker to a two-pole 20A GFCI breaker and making the circuit an MWBC would double your available power at the building.

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If that will really be the sum total of your load, that is fine. The electrician is suspicious that you are being short-sighted; and will have other, larger loads later, and you will wish you had provisioned capacity for them now. I agree.

NEC does not permit you to plan to load a circuit beyond 80% of ampacity. Therefore calculating voltage drop for any amount larger than 80% is just silly. That's 16A on a 20A circuit. Further, you should calculate voltage drop based on your actual load - 600W is 5 amps @ 120V.

What I just described above is law in Canada. However, unlike Canada, the US does not specify any permissible voltage drop - you have the freedom to use your good judgment to determine what is right for you and your loads.

The upshot is that any reasonable voltage drop calc says you are fine with 12 AWG wire given your planned loads. If you planned to "max out" the 20A circuit (remember you can only plan to use 16 amps), you would have 4.63% voltage drop on #12 wire. Still not bad.

Remember you must use UF cable, not NM cable, which is not rated for outdoors or outdoor conduit. Before you think about 10/2 UF copper or 8/2 UF copper, do yourself a favor and also price 2-2-2-4 MH feeder (which is aluminum). It's around the same price, and will support a 90A/240V feeder, for 180 amps of 120V!

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    thank you - I get the electricians suspicions but its a 12 by 16 building that is housing the animals only. No room within to have a spot where i would need to plug in tools to do work. If I really need that the house is 25 ft away where i already have exterior plugs. so outside of adding security lights to the exterior of the building it's not like its a shed or barn where i could expand its use to a work bench area, etc my actual shed where I will keep all my tools, would someday like to have electricity for working, etc is a separate building that is another project down the road – David Curtis Jan 22 at 21:16
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    Harp: You say you can only use 16 amps on a 20 amp circuit, but isn't that derating only for continuous loads like heaters and lighting? If I had a table saw at 120v and during it heavy cut it drew 20 amps, I believe that's acceptable. – George Anderson Jan 22 at 21:35
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    @GeorgeAnderson Yeah, the system has flex for that sort of thing. On the saw, for instance, you go by the nameplate; if it called out >16 amps you'd be obliged to go with a 25A or 30A circuit. – Harper - Reinstate Monica Jan 22 at 21:37
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I am licensed in Oregon and our use of the NEC is similar (we have reciprocal license agreements).

You can run a circuit 20 amp 120v circuit the length you can is based on the load.

Your load : let’s get crazy and do the math 600w / 120v = 5 amps

Code doesn't state that you protect for the breaker amperage but for the load. The recommendation of 3% & 5% for voltage drop are just that recommendations not code enforceable but let’s stay close.

At 200’ with 12 awg wire @ 5 amps your voltage drop will be 2.89%

Let’s get crazy and look at the affects of a larger load. 10 amps, double your expected load 200’ with #12 will still only be 5.79% voltage drop do you need to go further?

You could increase the distance by running a multi-wire branch circuit but I would not do this because of nuisance tripping.

You can reach out to 325’ 4.7% voltage drop at your expected load of 5 amps. You would like 3 duplex receptacles sure that won’t change the load calculations.

So yes you can do what you want you will want a GFCI receptacle and other receptacle(s) being fed from the load terminals or line terminals and another GFCI at each receptacle this will be the least glitchy for nuisance tripping.

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NEC Article 225 cover Outside Circuits and Feeders, Part II deals with outbuildings, it says:

225.31 Disconnecting Means. Means shall be provided for disconnecting all ungrounded conductors that supply or pass through the building or structure.

In a larger building often a main breaker in a panel is the easiest way to provide the disconnect, but for a single circuit shed a simple permanently labeled (light) switch is all that is required.

I recommend an extra wire using 12/3 (or a 4th wire in conduit) so if needed you can double capacity by creating a Multiwire Branch Circuit, which is two circuits sharing a neutral. You would still not need a panel, just a double pole switch (per 225.30).

100 ft., 10A, #12 awg will result is less than 3% voltage drop, so not an issue. Don't use cable assemblies in conduit, it's difficult and unproductive. Please search "NEC Table 300.5" for proper cover over cable or conduit for the location.

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Instead of UF you could run conduit from your main panel to the location within your shed where you would put a sub-panel if you wanted one using conduit large enough to supply a future sub-panel. Pull just what you need for now, and just put a pull box / junction box at the end of the conduit in the shed. Wire up your shed from there. So you have a single circuit, as you want, but using an underground conduit built for the future. If your electrician's crystal ball turns out to be correct and you want to install a sub panel and more capacity later, you can re-use the underground conduit, pull heavier cable and rewire your shed without having to dig again.

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