I am looking to install a 100 amp sub from my main. The run is aprox. 225' from the main panel down into my lower front yard.

The panel will be used to power a:

  • 30amp service for my RV
  • provide power for my recording studio/office in a 12x16 shed with:
    • 10 15 amp receptacles
    • a 120v electric wall heater
    • a 5k BTU 120v window ac unit (later I will switch to a mini split).

I am trying to figure out what size wire to use and the breaker I need for the main panel to supply power to the sub.

I have 1 unused 20 amp breaker/spot open in the panel (old electric dryer line no longer in use available).

I will not be buying a larger RV and there will be no shop tools used in the shed

THANK YOU to everyone who has commented, I really appreciate the input!!!!

  • You indicate a 30 amp service for the RV and 120 volt service for the shed HVAC. What voltage for the RV and what amperage for the heat/AC?
    – FreeMan
    Sep 18, 2020 at 11:28
  • Is the dryer a double pole breaker? 240 Volt?
    – JACK
    Sep 18, 2020 at 12:44
  • Think about a 150 Amp panel at least...
    – JACK
    Sep 18, 2020 at 12:49
  • 30 amp RV circuits are 120v there are 50 amp. RV those are 50 amp 240v. I would be looking at Possibly 75 or 90a , 10ea 15 amp receptacles can go on 1 circuit I would probably break into 2 20 amp branch circuits. But 75 would be enough so it’s best to jump up to 90 or 100. 75a = 2 awg wire with 2.56% voltage drop this would also handle 90 amps to go up to 100 amp 1 awg wire would be needed. With older panels finding anything larger than is doubtful. With max loads listed #2 and a 90 amp breaker would give additional room
    – Ed Beal
    Sep 18, 2020 at 14:04
  • all 110/120v, the trailer and shed will never b run at the same time.
    – Rich
    Sep 18, 2020 at 20:26

3 Answers 3


Yikes. All your load is 120V.

The 30A RV is almost certainly a "TT30" which is 120V/30A.

Receps are all 120V obviously. The good news is, receps are 0 amps. The bad news is, things which plug into receps are various amps, and since you haven't discussed what those will be, we have to punt over to the usual assumption of 180 VA per recep, or 1800 VA the bunch, or 15A.

120V electric wall heater will be 12 amps typically. However presumably you will not be running it at the same time you are maxing out the RV. You won't be running the A/C at the same time as the heater, and it's less than 12A, so we'll count it as part of the heater's allocation.

There are too few simultaneous loads here to be able to effectively balance them on the two 120V phases included as part of 240V. Therefore while we can try to balance them on the 240V, we can't count on that balance existing. Therefore we must think about voltage drop for a single load. The worst case (for balancing and voltage drop) is the RV is maxed out @ 30A and nothing else is on. That must be the basis of our voltage drop calcs.

(so for folks who do voltage drop calcs... just plugging 240V and breaker trip into your friendly neighborhood voltage-drop calc won't tell the whole story. Loads this lopsided require thinking about drop in instances of pure 120V load.)

It's a pity; if only we had a way to guarantee balancing of the 120V loads, we could do our calcs based on your 240V draw, and the wires would be a lot thinner. A 10 KVA or even 5 KVA transformer could do that... but realistically, unless you get lucky on Craigslist (and know exactly what you are buying), the transformer would be more expensive than fatter wire.

Failing to run this 240V would be nuts

So you might think "Why not run it as 120V-only?" Because your future mini-split won't like that. And you may get a bigger RV someday.

I think we need to plan for 40A @ 240V of service... but also watch our voltage drop at 30A @ 120V. (24A [80%] for the RV and 6A for other misc loads.) 3% is a wire salesman's ideal but try earnestly to keep it under 5%.

So this looks like #2 aluminum.

#3 aluminum would work, but it's generally a unicorn.

And the limiting factor is the ability to support a lopsided 120V load based on the RV being most of it.

On the upside, since you're not in Canada, you could breaker the #2 aluminum at 90A. That means it will be totally ready for that mini-split and larger 240V/50A RV should you ever get one -- at 60A actual, voltage drop @ 240V will only be 3.3%.

If you wanted to super-chintz this thing, you might be able to swing #4 aluminum, but under certain conditions voltage drop would be noticeable. The cost differential #2 vs #4 will be tiny compared to total project cost.

Also, get a BIG panel

This wire can be breakered at 90A, which is enough to run a pretty big house. Given how ridiculously cheap breaker spaces are, and how expensive and project-blocking running out of spaces is, a 30-space panel is not excessive. Disregard number of "circuits" in a panel spec; that number is useless. So a 16 space/32 circuit panel is only a 16.

  • the trailer and shed are 110/120v... they are never run at the same time.... there are many small mini splits that are 10/120v sono worries there and that's a year or two away if ever lol ...100amp sub panel?
    – Rich
    Sep 18, 2020 at 20:28
  • 1
    For future proofing most mini splits also heat so I would plan on the heat heat issue so both are used at the same time. For receptacles in a garage I think 15 amp is foolish yes you can put them all 10 on 1ea 15 amp branch circuit but that is foolish especially in a garage where power tools like chop saws and skill saws trip 20 amp circuits in cold weather. Just the view from someone that actually installs things like this, the lighting that is another small load.
    – Ed Beal
    Sep 18, 2020 at 21:15
  • Ed Beal , I understand you concerns but this shed is not a shop.No tools just my recording studio and computer/ cameras for recording videos but i will look into this further
    – Rich
    Sep 18, 2020 at 22:12
  • @Rich The problem is the tyranny of voltage drop on 120V. You're thinking "Hey if I just wire it 120V, that'll make it cheaper". Actually, 120V makes voltage drop four times worse. That is the driving factor for such heavy wire. If we could deliver power at 240V, we could do it with #8 Al. Sep 19, 2020 at 7:09

https://www.cerrowire.com/products/resources/tables-calculators/ampacity-charts/ Here is a Chart on wires needed for this job.The longer the distance from electric source the more heavy duty wire youll need.

  • 3
    Please edit your post to include a specific answer to the OP, as well as maintaining the link where you got it from. Unfortunately, links die and should that happen (right now or in 5 years), this answer will no longer be useful. If you've copied the relevant bits, then this answer maintains its usefulness even if Cerro Wire goes totally out of business.
    – FreeMan
    Sep 18, 2020 at 13:44
  • Also for newbies it is better in my opinion to get the total load and use a voltage drop calculator.
    – Ed Beal
    Sep 18, 2020 at 13:49
  • 1
    Yeah, a "link only" answer is always bad. Also, that chart (310.15(B)(16)) is complicated, and you need to walk people through how to use it properly. And you need to talk about voltage drop. Sep 18, 2020 at 17:52

The problem with trying to calculate voltage drop is the actual load must be used to determine loss, and you don't really supply enough load to justify a 100A panel, or calculate voltage loss that would result in significant voltage loss.

With the load you show you could easily get by with 75°C #4 copper (#2 aluminum) on a 240v 60A feeder loaded at 45A and still only get 5% voltage loss even in the most imbalanced scenario. That's the 30A RV, and shop non-simultaneous heat/cooling on the one leg and everything else connected to the other leg with nothing turned on.

Putting your 120v 30A RV on one leg and the shop heat/AC on the other leg then split your receptacles between two 15A or 20A circuits would provide better balance.

For a 100A feeder #3 copper or #1 Aluminum if using 75°C conductors, or next size larger if using 60°C rated wire or source terminations. Those are the minimum sizes allowed by code for a 100A feed and nothing you present is enough load to justify upsizing.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.