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Basically i have the electrical service routed down to my main meter, then a 200A breaker box off of that. This is permanently mounted to a service-pole outdoors.

I need to add power from that pole to a small cabin 530' away. There will be a 100A panel at the cabin.

I am worried about voltage drop, but will only need about 50A of service to the cabin.

I will be able to bury the wire 3' without problem I suspect, but I am not sure what gauge wire to use.

  • Can you post a photo of the main panel? – ThreePhaseEel Dec 10 '18 at 12:41
  • Are you in an area with theft risk of copper wire? Not that you'll use copper, but thieves are stupid. – Harper Dec 10 '18 at 17:49
  • Do you have space in your main box (the one on the pole) for a two-pole breaker, at least? – ThreePhaseEel Dec 13 '18 at 1:32
  • Depending on your definition of 'cabin', I expect you can drop that maximum demand by about 30-50%, particularly with things like avoiding electric resistance. – Someone Somewhere Dec 15 '18 at 9:01
  • @SomeoneSomewhere at these run lengths, I suspect that even at 30A, transformers would still be cost competitive – ThreePhaseEel Dec 15 '18 at 16:33
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This is why Tesla won the War of the Currents

The great advantage of moving power around as AC is that one can step up the voltage easily using a transformer to move it a long distance with minimal losses, then step it back down to something more suitable for household utilization when it arrives at its destination. While this is normally the province of electric utilities and heavy industrial facilities, nothing in the National Electrical Code prohibits the application of dry-type distribution transformers in a more...residential setting, so given the length of your run, the use of a transformer at each end to step the voltage up for the run and back down at the cabin is an option here.

I will cover the conventional (no transformers) way of doing it, first, with example pricing, and then show what it takes to use the genius of AC power distribution to your advantage here, again with example pricing. If you are not comfortable with any of the installation instructions here, of course, please speak with an electrician -- most electricians who have worked in the commercial/industrial space should be comfortable with installing a setup of this nature.

Doing it the hard way, first

Without the aid of transformers, we wind up running 240V @ 50 or 60A over the 530' distance. This requires fat wire in order to keep voltage drop from becoming excessive, (3% is the typical limit for feeders) -- a typical choice would be 3/0 aluminum XHHW-2 for 50A (you could use 250kcmil instead for 60A, but the bump of two wire sizes should tell you why this isn't promising already), and you'd need 3 of those + a 4AWG (10AWG is the minimum for 60A, but the upsize for voltage drop forces us to upsize the grounds by the same ratio as per 250.122(B)) bare copper ground for this, as well as 540' (the extra is so we can do the stub-ups at the end) of 2" (the wires in question take up about 465 mm2 of fill, so it's slightly too fat a run for a 1.5" conduit) Schedule 80 PVC conduit and fittings. (We are doing the run in conduit in both cases as it'd be nonsensical to trench direct bury cable this far only to have to dig it all up again if requirements changed in the future.)

As to cost? The PVC costs $3 a foot or thereabouts, so we are out $1690 for the conduit here ($1620 for the conduit, and $70 for the fittings). Adding the phase and neutral wires puts us at another $.85/ft, or $1377 for 540' per conductor (so we have some left over to terminate with), and last but not least, the ground wire adds $.73/ft, or another $395. This adds up to nearly $3500, quite the sum for providing electric service to a humble cabin, no?

It's transformer time!

However, with a pair of transformers of appropriate rating, we can step the voltage up to 480V for the run, allowing us to use 6AWG copper or 4AWG aluminum for the phase and neutral conductors instead. Furthermore, we can get rid of a wire by running the 480V as single-phase instead of split-phase, leaving us with a single hot, a neutral, and a ground, at the cost of needing a beefier overcurrent device in one spot, which isn't that big a deal in the grand scheme of things, as it turns out.

The transformers we need are what are called dry-type distribution transformers. Unlike the oil-filled transformers on utility poles (which can burst into flame under abuse), these transformers have their windings air-cooled and use flame retardant insulation materials, rendering them fire-safe for use in or on buildings of all types. Specifically, we need a pair of NEMA 3R (outdoor rated), 15kVA (15,000 Volt-Ampere, or equivalently 15kW of resistive load) dry-type transformers with 240/480V primaries and 120/240V secondaries, the slash indicating that that side of the transformer can be wired for either voltage. These cost about $650 apiece if you can get a good price on them, particularly if you can find used ones in your area, and you will need two, putting your cost for the transformers at about $1300 for the pair.

With this, we can use 4AWG aluminum XHHW-2 for the hot, neutral, and ground wires, giving us a wire cost of $422. These wires only take up 150mm2 of fill, as well, so we can downsize the conduit to 1.5" and still have ample fill left over. This translates into a conduit cost of $1/ft, or $600 once the fittings ($20 for the pair) and expansion fittings ($40 for the pair) are included atop $540 or so of conduit.

Last but not least, we'll need a way to protect the conductors between the two transformers, and the primary side of the cabin-transformer, from overcurrent. The most cost-effective way to do this is with fuses due to the fact modern fuses give you better voltage ratings for your money than circuit breakers do. A fuse block will also be needed here: the appropriate fuse, namely a 30A, 600V, Class J fuse, costs roughly $10 apiece, and a matching fuse block for Class J fuses adds another $20.

This sums up to a cost of about $2400 including some miscellaneous parts (such as a 1" RMC or EMT run from the cabin's transformer to the cabin's loadcenter), or about a thousand dollars less than not using the transformers. Even if you can only get a bad ($1000 a piece) price on 15kVA single phase transformers, this is still cost-competitive with the straightforward approach.

Implementing the transformer approach

Implementing things The Hard Way™ is a well-trodden path, so I will not cover it further here. The transformer-based approach, though, requires a few different elements, so we will go over the implementation caveats first, and then cover the procedure involved in doing this.

Notes, cautions, and caveats

First off, the transformers will need homes -- at 220lbs a piece, these are the heaviest pieces of electrical equipment you will ever meet, atop being quite bulky (nearly 30" tall, almost 24" wide, and another 15" deep)! If you want to hang the transformer at the pole from the pole, this is possible, but will require quite a hefty crossmember for the transformer's mounting brackets to bolt to and a metal shield attached to the bottom to prevent sparks from starting a fire should the transformer fail; otherwise, a concrete slab with appropriately placed studs can be used to padmount the transformer onto the ground. Likewise, the transformer at the cabin can be bolted to a sturdy crossmember integrated into the frame of the cabin (with a spark shield if outdoors), or set on a studded concrete slab.

Furthermore, you must be able to mount the cabin's loadcenter in a place with sufficient clear working space -- this may require some slightly tricky conduit routing to do so. Otherwise, trenching the conduit in and pulling the wires from the pole-transformer to the cabin-transformer is as you would do for any other underground installation, and so is the short conduit run and pull of 4AWG Al from the cabin-transformer to the cabin's loadcenter.

Note also that you will need to torque all connections (both mechanical screw lugs and bolted joints) to manufacturer specified torques using a torque screwdriver or torque wrench calibrated in inch-pounds. You will also need to identify wires correctly -- 240V wires can use conventional black/red/white phase taping in this scheme if they are not already correctly color coded, but the 480V wires should be identified using yellow for hot and grey for neutral, with green being suitable for ground as always -- this helps prevent embarrassing disasters involving 480V power being fed to 120 or 240V appliances. In fact, you should apply the phase tape to the wires before you put the wires together and pull them through -- this makes sure the wires are properly identified at both ends, which will avoid mixups down the road.

Last but not least, all of this, especially the 480V work, must be done with the power off, and preferably locked out at the pole using a lockoff and cheap padlock fitted to the newly installed breaker in the pole's loadcenter. This is because 480V not only will shock the living daylights out of you, it can blow you to high heaven in what is called an arc flash event.

Finally, the how-to

As to wiring this all -- the wires from the 50A or 60A breaker that will feed this all (using whatever Chapter 3 wiring means you wish -- you only need 2 hots and a ground here, so you can use 4AWG Al XHHW-2 in 3/4" or 1" metal conduit, or a short length of 6/2 UF with cable clamps at each end for that matter) in the pole's loadcenter go to the X1 and X4 terminals on the pole-transformer, while the X2 and X3 terminals should be jumpered together if they are not shipped that way from the factory. Likewise, H2 and H3 should be jumpered together on the 480V side, with H1 jumpered to one of the fuse block's lugs, and H4 jumpered to a connector that also is jumpered to the transformer's grounding stud -- there will need to be two ports left on this connector, as both neutral and ground will connect to it. (It serves as a bonding point for the separately derived system created by the pole-transformer, in other words.)

We then move onto the cabin -- the H2 and H3 terminals are jumpered together on the cabin-transformer, taking care of the primary side for now. The X2 and X3 terminals on the cabin-transformer are jumpered together and also connected to the neutral wire running off to the cabin's loadcenter, which is taped white, while the X1 and X4 terminals each connect to one hot wire running off to the cabin's loadcenter. In the loadcenter, the white-taped neutral wire from X2/X3 lands on the neutral lug, while the wires from X1 and X4 individually land on one hot lug of the main breaker each, and the bonding screw or strap is installed if it is not already present. Again, this is because the transformer is providing isolation from the rest of the mains-world for us, giving us a separately derived system at the cabin.

At this point, if all is going well, you will want to perform a test (or have an electrician perform a test for you) before you make the final connections and energize this configuration. In particular, you or your electrician should perform an insulation resistance test using a special high-voltage insulation tester (often called a "Megger") on the long run of wires before hooking them up to at either end. This will show if the wires were damaged during the pulling process, preventing a nasty breakdown and fault-finding process down the road.

Once the wires pass such a test, you can then make the final connections. The yellow-taped hot wire in the long run connects to the open terminal on the fuse block, while the grey-taped neutral and green-taped ground wires go to the connector in the jumper from H4 to the transformer's ground stud. At this point, the fuse can be installed into the fuse block as well.

Moving on to the final connections at the cabin, we then hook the yellow-taped hot wire up to the H1 terminal on the cabin-transformer and the grey-taped neutral wire up to the H4 terminal on the cabin-transformer. Last but not least, the green-taped ground wire gets hooked up to the ground lug on the cabin-transformer.

As to the rest of the cabin

The rest of the cabin is wired the same way either way. You will need a grounding electrode system at the cabin in any case, and the panel at the cabin must have a main breaker in it to serve as a shutoff, whether it is a subpanel (in the conventional approach) or a main panel (in the transformer approach). Furthermore, there is no sense in being stingy on panel spaces here -- I would use a 100 or 125A, 30 space, main breaker panel at the cabin if I were in your shoes, as adding breaker spaces later is far more expensive.

  • Is there really a need for a shutoff switch on the 480V side if you are able to put a shutoff switch on the 240V side? I would prefer to run the conduit straight from transformer pad to transformer pad inside the transformer's case, long wires straight from H lug to H lug, so you would have to take the covers off the transformer to get anywhere near the 480. – Harper Dec 15 '18 at 8:43
  • @Harper -- the safety switch is mostly used as a fuse housing here. Let me do some research on class J fuse blocks, though...might be possible to do it cheaper that way. (The overcurrent protection in the 480V run is necessary somewhere to protect the primary of the transformer at the cabin, as well as the long wire run -- it can be located anywhere within the run) – ThreePhaseEel Dec 15 '18 at 14:19
  • @Harper -- it seems to boil down to the question "can you mount fuse blocks within a transformer housing?" – ThreePhaseEel Dec 15 '18 at 15:42
  • (Looking in the NEC, it seems that nothing would prohibit it in this case...but it'd certainly be odd.) – ThreePhaseEel Dec 15 '18 at 16:06
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540' with a 50 amp draw would require 1/0 copper this would have 2.87% voltage drop, if using aluminum or would take 3/0 and have a voltage drop of 2.99% . I would probably go up a size to provide more ampacity or do what I have at my farm and get 600v transformers step the 240 up to 600 and much smaller wire can be used in my case it saved much more than the cost of the transformers and running the smaller wire made it much easier. In this case #8 copper wire could be used with a 2.44% voltage drop 15kva transformers would be large enough and would allow a larger load if using larger wire also.

  • "Require" is too strong a word. "Advised" is about right. There is no code requirement for 3% drop, it's just highly promoted by wire salesmen. – Harper Dec 10 '18 at 17:48
  • Not required but mentioned in code fine print note 210.19 so it is not enforceable , however the equipment must be installed within its mfg voltage requirements this is enforceable. – Ed Beal Dec 10 '18 at 18:01

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