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I have an old barn to which i'd like to provide electricity.

The problem is that the barn is about 300 feet away. I understand that voltage drop over 300 feet is fairly significant. Buying thicker wire to accomodate voltage drop of that magnitude may be very costly, and I'd have to figure out how to terminate the end in the panel at such a thick gauge.

Is using a transformer at both ends a viable/smart solution? Would using two conductors in parallel be a better or safer option?

How is something like this done normally?

  • Is it cheaper to buy thicker wire, or buy a transformer (for each end) – KyranF Apr 7 '15 at 1:14
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    but basically yes, if you use a transformer pair to step up even just x10, to 1.2KV, you will drop the current to only 3A. This would drop far less voltage! – KyranF Apr 7 '15 at 1:15
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    @KyranF - Have you considered how much it will cost to get wire with 3 kV insulation? – WhatRoughBeast Apr 7 '15 at 2:15
  • Unless you have one device that needs 30a, run 240v plus a Neutral (3 wire). That reduces current per wire. – Optionparty Apr 7 '15 at 2:36
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    120V/30A is NOT a logical circuit to run. First off, you CANNOT have general use lighting and receptacles on a 30A circuit, and the VD for a 120V circuit is far greater than for a 240V circuit. I'd run a 120/240V feeder and keep the load as close to balanced as possible. Any tools or motors that can run on 240V you should do so. – Speedy Petey Apr 8 '15 at 1:25
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Using Table 8 in chapter 9 of the NEC along with Ohm's Law (the max drop generally permitted by good practice on a branch circuit is 5% or 6V on a 120V circuit) gets you a minimum wire size of 6AWG in copper or 4AWG in aluminum.

Considering that 6/2 UF is $3.00 a foot at the blue borg and 14/3 UF is $0.87 a foot at the blue borg, the transformer idea looks like a good option, until you realize how much 3.6kVA of transformer will cost you -- almost $600 via McMaster-Carr, which is the cheapest supplier I could find, with an alternative part available for about $650 via Platt Electric Supply.

Even with 240V being tapped at the house (source) end of this instead of 120V (so you only need one transformer) -- you'd still be out a good $850, which is comparable to simply burying 6/2 UF. A further drawback created by the transformer idea is the extra labor involved with creating a separately derived system in the barn (the equipment grounding arrangement gets...funky, see NEC 250.30 for details), so simply burying 6/2 UF is going to be your best option.

As to terminating it in the panel? Don't worry about it! The main lugs on a panelboard will easily accept 6AWG! If you're not using a panelboard (as this is a single branch circuit feeding an outbuilding), simply use appropriate wirenuts to pigtail the 6AWG UF to a length of 10AWG for the indoor branch circuit in the barn.

  • Under the NEC there is no such thing as "allowable" voltage drop. There are NO codes requiring that voltage drop be countered, only suggestions to deal with it for best performance. – Speedy Petey Apr 8 '15 at 1:23
  • Yeah -- the FPNs on the topic are merely recommendations, albeit good ones. – ThreePhaseEel Apr 8 '15 at 1:55
  • Thanks, and this answered my other question, how much drop should I allow?... less than 5% – Ace Apr 8 '15 at 18:52
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Concentric-Neutral cable is relatively inexpensive and is intended for direct-burial. Personally, I'd be getting the 2 AWG stuff but that may be more money than you want to spend.

Note that you will need to drive ground rods at the barn. Two rods a minimum of 6 feet apart should be good.

Be sure to both bury it deep enough and put a pressure-treated board over top of the cable before you pile the dirt back on top.

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I believe you are complicating this issue somewhat. It would have been helpful to know the load distributions at the barn, lights, motors, number of circuits etc. Generally running two #4 wire for power and neutral and #10 for ground should work very well for conventional loading. There maybe cheaper solutions but more details would be required.

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You could consider distributing 240V to the barn. At the barn, use a 240-120V isolating transformer. Halving the current eliminates 3/4 of the power loss. without much complexity.

In the UK these transformers are commonly available, so reasonably priced - usually in tough yellow waterproof enclosures, used on hazardous environments like building sites. This allows the use of 110V tools to minimize the danger of cut or crushed cables. If these are available local to you, they sound ideal for a barn.

  • Unfortunately, a 1.8kVA (continuous) transformer is about half the size he needs, and he'd also need to re-rig the grounding to be acceptable to the NEC as per 250.30 on grounding separately derived systems. – ThreePhaseEel Apr 8 '15 at 1:07
  • The linked one was 3.3kVA rated - just not continuously. Whether it would meet the OP's needs would depend on his required duty cycle, which was not stated. It was an example, not a specific product recommendation, my apologies, I thought that would be self-evident given the geographic difference. In any case the basic point stands : he could reduce losses by 75% with a transformer at one end only. – user35376 Apr 17 '15 at 17:48

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