What cable size do I need to feed a 100 amp sub panel that is 550 ft

Question

I'm building a barn that is 550' away from my house and need to power it. I would like to have 100 amp service from my main house board (200 amp). My house is fed with the typical 2 conductors. I am in Ontario, Canada.

I would like to stick the cable in the ground and use some sort of conduit like 2" likely 2 1/2" PVC shed 80 pipe if this is cheaper than an armoured cable.

What I actually Ended Up Doing The local (Rockwood, Ontario Canada) inspector gave me two options. 1) pull 300 MCM or 2) parallel 3/0 Aluminum 3 conductors director bury, both needed a ground back to the main panel (sized to cable). So....... Based on that I took Option 2, Cost of $3,800 for cable and ground + $1000 for Big-O (two runs) turns out my barn when finally located was 650ft, location was selected while I was in the EU. I had to put the cable inside Big-O to protect it because I didn't want to put 6-4" of clear sand below and above the cable.

What I learned My Electrician is at the mercy of the local inspector. They books all said I need to run single pull of 3/0 or at worst 4/0 but, they can't make a decision because the inspector as the say. Simplest means is to get the inspector out and get him to bless the project.

No One would do the Transformer Option, and the Inspector wouldn't approve it. My local Utility provider came up with a plan for about $25K to get me 600 amp service and back feed the house. needing 4 poles, high side cabling, a transformer, and easement and a tone of work by me along with a pad and remote meter. So that went nowhere.

Answer 1

This application has "transformers" written all over it. A big factor in AC vs DC power (Tesla vs Edison) being the ability to haul it long distances. And this right here is where that comes into play.

In every single scenario, your barn needs its own earth grounding system.

Plan 1: Conventional sub-panel (as discussed elsewhere)

Wire cost: $4000. (about a 25% margin over the best prices I can find).

That plan only makes sense if money is no object and the goal is to stay "DIY friendly". But it still won't be, because of the miserable pulls with huge wire (unless you upsize conduit to about 4", not least, because the bends are wider radius.) The only benefit to all this is avoiding transformers. Since this scenario is well covered by other answers, I'll won't elaborate.

Plan 2: Use a transformer to make the barn a main panel.

Wire cost: $2000. Transformer cost: $900

You only need 2 of the huge 350kcmil wires. You feed them with 240V and they go to the primary side of a 25kva transformer. The transformer removes the need to run a "ground" and "neutral" wire (but the primary and secondary sides are different). Your barn is now a main panel. L1, neutral and L2 come off the secondary of the transformer. The transformer and the barn panel are grounded to the barn's grounding system.

You may be able to reduce the size of the wires somewhat, because the transformer has "taps" to compensate for voltage drop.

Plan 3: Also pump up the voltage for the long run.

Wire cost: $300. Really. Transformer cost: $1800

This takes two 4-AWG (maybe 6) AL wires. Instead of one 25KVA transformer, you get two -- with a higher voltage primary, within the rating of the cable you plan to run. So if the cable rating is 600V (most likely), get 480, 575 or 600V transformers (those last two are a thing in Canada). These voltages are serious, but still within the range of common panels and wire. Call an electrician for the final review, megging and hookup of the higher voltage stuff.

You backfeed the secondary of one transformer with 240V from your panel. The primary feeds your long conduit run to the other transformer's primary, which knocks it down to 240V on the secondary. As above, the barn panel is a main panel which gets L1, neutral and L2 from its transformer, and ground from the barn's grounding system.

Plan 4: Tie into the high voltage supply

Wire and transformer cost: ???? depending on voltage rating and delivery.

The power company transports power across farmland at a much higher voltage like 600V or 2400V, and then have a transformer at your house to knock it down to 240/120V. I imagine you've already talked to the power company and asked them the cost of wheeling their high-voltage to your barn and provisioning their transformer and meter, and didn't care for the answer.

I am saying, see if it is feasible to buy their power on the high side of their transformer, and take responsibility for the pole line, underground, transformer etc. It's sorta like "rent the cable modem or buy one". Used transformers can be had. Of course touching their high-voltage is electrician territory, but you can do all the grunt work, so he megs a few things out, connects 2 wires and throws the switch. There's no reason to pay an electrician to trench conduit or punch holes in walls.

Answer 2

Since you didn't include a load calculation for the barn, I used the full 100 amperes for the following calculations.

A 100 ampere feeder would likely have to use 3 AWG copper conductors (1 AWG aluminum), so that's where you'd start at. You'll want to limit the voltage drop as much as possible, which would mean installing larger conductors. A rule of thumb, is to reduce voltage drop to about 3%.

240 volts * 3% = 7.2 volts

Using copper conductors, you'll have to use 4/0 conductors to reduce the voltage drop enough.

2 x 550' x 100 Amperes x 0.0000608 ohms/ft = 6.688 volts

Aluminum conductor means using 350 kcmil conductors

2 x 550; x 100 x 0.0000605 = 6.655 volts

When you start working with this size wire, you're going to need special tools and knowledge. Which means it's likely out of the DIY realm.

If you want 120/240V at the barn, and not just 240V. That will require three current carrying conductors, and a grounding conductor. So you'd be looking at using 2 1/2" Schedule 80 PVC conduit.

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Having a second service drop installed nearer the barn, might be an option.

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High voltage lines and transformers are likely not a good fit for a DIYer, so that's probably not an option.

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Other options would include generating your own power via fuel, solar, wind, water, etc. Though those systems are likely cost prohibitive.

Answer 3

This is one of those perennial questions where the answer is both simple and complex.

If you actually expect to USE 100 amps at the barn, you plug 100 amps and 550 feet into a voltage drop calculator with various wire sizes and materials, shooting for 3% or less voltage drop. But if you plan rather for the actual loads you will be running, the wire size will virtually always be much smaller than if you plan for 100 amps. I get 350 kcmil aluminum for the 100 amp case and a strict 3% or less condition. 4/0 copper gives the same voltage drop (2.8%) for 240V service at 100 amps. If your actual loads turn out to be 67 amps, wire size required drops to 2/0 copper or 250 kcmil aluminum (though 4/0 aluminum is REALLY close at 3.1%.)

The ground (not neutral) can be a smaller size, though it does have to be proportionally increased to match the increase in size due to voltage drop of the current-carrying conductors.

In practical terms, aluminum wire will almost certainly be far cheaper, even though you have to use a larger size relative to copper. While small size aluminum wire as used for a while in branch circuits is a bad, bad, idea, large size aluminum wire is both normal and safe and a lot cheaper than copper. If in doubt, get an electrician in to make the actual connections correctly.

You will need to run 4 wires - Line1, Line2, Neutral and Ground, and Neutral will not be connected to [will be isolated from] ground (and ground will be connected to a grounding system at the barn.)

Answer 4

I sure am glad I don't have your problem. :-)

Besides the choices of using long heavy wire, a second meter, transformers, etc. Another option might be to install a generator.

A capacity of 100 amps at 240 volts is 24,000 watts (24 kw). Costco carries those online. Such generators run about $5,000 with all the options for failover, but if there is no other source of power, you can reduce the cost by perhaps $1000 since no transfer switch is needed.

For an ordinary shop situation without welding, 7 kw should be more than adequate. Around here (Oregon), they sell this one in stores which is quiet, uses propane or gasoline, can run 240 volt devices, and is only $800. Personally, I am trying to find justification for having one. :-)