Help!!! What size wire???

Old 12-03-04, 10:32 AM
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Help!!! What size wire???

I need help. I want to run electric to my shop. It is about 250 to 300 feet from the house. Ameren wants over $1000 to put in a new pole with a meter so I want to run an underground wire. I need to run a 220 compressor (max 15 amp), 110 table saw, various 110 power tools and 3 to 4 lights. I have asked several people about what size wire to run and have been given several different answers. Anyone out there that can help I would appreciate it.
Old 12-03-04, 11:11 AM
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You are probably getting different answers because you are not clear on exactly what your requirements are.

'Various 110V tools' just doesn't say how much power you will be using.

'220 compressor (max 15 amp)' is also quite unclear. Do you mean a 220 compressor that you've not yet selected, with a running load of 15A or less (couple of horsepower) ?

You have to deal with two issues: the maximum current that a particular size wire is permitted to carry on a continuous basis, and the voltage drop produced by a long run of wire.

A good round number for a shop panel is 60A, and if you were to size your conductors at the bare minimum for a 60A panel, you could use #6 copper or #4 aluminium for your conductors. This deals with the maximum current that the wire is allowed to carry.

But that 15A compressor might have a startup current of 75A or more, and you have a very long run of wire, which means severe voltage drop. If you used the bare minimum #6 wire, at compressor start you would probably see the voltage in your shop drop by 10% or so, which is probably unacceptable. The NEC _recommendation_ is 5% maximum for your entire circuit, and 3% for feeders, but this is only a recommendation.

Basically anything thicker than #6 copper conductors will be legal, but I would recommend #3 copper at a minimum for voltage drop reasons, giving a 4% voltage drop with the (guessed at) 75A startup current on the compressor. If you can afford it use #2 or #1.

Old 12-03-04, 11:37 AM
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Hold your voltage drop to 3%!

I realize this may be a shocker but for the loads you are talking about you will want a thirty ampere feeder to your shop. That will allow you fifteen amperes for your compressor, five amperes worth of lighting, and still allow you to start a power tool under load. To get the thirty amperes to the shop over a distance of three hundred feet you will need twelve hundred feet of number three American wire gage single conductor Aluminum type UF cable. That will hold the voltage drop to 3% in the feeder conductors which will leave two percent for the voltage drop in the individual branch circuits in the shop itself. That works out to four #3 AWG type UF aluminum cables suitable for direct burial. That gives you two hots, one neutral, and one Equipment Grounding Conductor.

I suspect that your compressor may be less than fifteen amperes. Can you tell us the actual name plate running amperes? It is conceivable that you could get by with twenty amperes which would lower the cost somewhat.

In any case the feeder Over Current Protective Device (OCPD) should be sized to provide short circuit and ground fault protection but not overload protection. You would size the feeder OCPD as if the feeder were rather short. For the number three UF Aluminum that would be a seventy ampere breaker. At the shop end you would install a main breaker in the shop panel to serve as the building disconnecting means. That breaker would be sized for the ampacity of the feeder at the three percent voltage drop. You do that so that if you overload the feeder the main breaker at the shop would trip instead of the feeder breaker three hundred feet away at the house.
Tom H
Old 12-03-04, 12:25 PM
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Why suggest aluminum for direct burial? I know it costs more to run conduit, but at least you'll know you hit it if you ever dig near your trench line and it provides an extra layer of protection. But you could save your money buy using copper at a smaller wire size than aluminum in the same conduit. (Of course you'll need to have 4 seperate strands rather than one insulated multiwire feeder)
Old 12-03-04, 12:26 PM
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At 300 feet, design is critical. At shorter distance, you could afford to just overengineer it to play it safe. But a this distance, overengineering will be quite expensive. And underengineering will be quite annoying.

The key question to answer, as accurately as you can, is how many KW of power you will need. That requires you to figure out the total wattage usage of everything that you might want to run simultaneously. Of course, you can decide how careful you want to be about what you run at the same time.

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