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Old 07-13-2006, 02:31 PM
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240 volt extension cords

i'm getting ready to buy a 80gal 5.5 hp compresser for my 65 mustang job the contractor wants a boat load to instale the 240 line i need in my shed can i get away with running a heavy extension cord to it i have a 240 outlit on the sid of the house for a welder

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Old 07-13-2006, 03:13 PM
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Doc here,

Sure, provided: The ampacity of the Cord IS NOT exceeded by the current draw of the compressor for the length of the run in footage, AND the Plugs Mate and LOCK when connected, AND the compressor is properly grounded, And meets local code requirements.

You can probably Find a proper cord at A Contractor's Supply outlet.

Doc
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Old 07-13-2006, 03:58 PM
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240 volt extension cord

i was thinking a 10.3 wire 30amp compressor should be 20 amp about 30' or less
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Old 07-14-2006, 04:47 AM
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What size breaker is feeding the existing outlet? And how far from the box is the outlet and shed? If youre mechanically inclined and safety-conscious, you could probably run a new line yourself.
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Old 07-14-2006, 05:50 AM
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Compressor HP ratings can be a bit erroneous. You must provide us with the motor nameplate amperage draw before we can tell you what size wire you need. More than likely the 3/conductor #10 will be adequate for the distance you mentioned though.

Vince
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Old 07-14-2006, 07:42 AM
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"Compressor HP ratings can be a bit erroneous"

That's letting them off easy!, Comical would be a more apt description.
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Old 07-14-2006, 04:18 PM
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Doc Here,

746 Watts is 1 HP...or in your case 4100 watts..

P/E= about 18 amps..

BUT this is in a perfect world..That would assume the compressor was steady and not cycling (High / low Rotor load) and Does not account for startup rotor load...nor does it assume, line loss or switch gear loss..

30 Amp 8 awg for a short distance would be fine..

As was said before if the house and garage isn't too far, and you have a slot for the breaker, you can get some 1 inch conduit, Ell's, 90' Condulets, whatever is needed..rent a bender, and install a permanent location yourself without too much trouble.

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Old 07-14-2006, 04:35 PM
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Ummm, doc, I think youre thinkin of #10 thatll carry 30 amps. For a 5 hp motor, you need to run #8 wire and use a 40 amp breaker, jon.
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Old 07-14-2006, 04:43 PM
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Doc here,

No the reason I specified 8, is the wire is larger than required for the draw because of possible rotor locks, and changing conditions of the compressor, as was mentioned, Compressors are not stable load items..(less heat, more power to the load) and the 30 amp breaker to limit the amount of draw between the line and the load closer to the spike load.

Better safe than sorry..and I am assuming it's under 100 foot run ..

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Old 07-14-2006, 04:58 PM
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Right-O, doc I prefer running larger than needed wiring too, instead of "just enough"...hell, my compressor is only rated at 10-12 RLA and I still ran #10 to it with a 30A breaker. Like you said, better safe than sorry. I would use #8 but with a 40A breaker cuz that 5 hp motor may draw 30 or better on startup, and a 30A breaker will weaken over time with a full load through it and eventually start tripping when the compressor starts up.
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Old 07-14-2006, 05:24 PM
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10AWG conductors should be fine for 240 volt, 40 amps, single phase, copper, at a length of 30 feet. Actually 12AWG is the minimum wire size with a 3% voltage drop. Ampacity limits this to 30 amps though for RUW, T or TW insulation on the cord. If the insulation on the cable was FEP, FEPB, THHN, or XHHW then 40 amps max would work for 12AWG - the limiting factor is the insulation material, not the conductor material (the insulation would begin melting before the copper conductor).

The 10AWG conductors will only drop the voltage 1%, which is not a problem as most line voltages fluctuate way more than that on a regular basis.

If you are still interested in an extension cord, I made one for a 240V mill/drill machine - went to Lowes and bought a leftover length of AWG 10.3 and the proper plugs for each end of the cord. Total cost $28. Properly assembled, this should serve you well.

If you are going to run it outside between two buildings, the underground conduit is good advice. If you place this in a conduit, then 10AWG would be the minimum as the above is for free-air (no conduit) installations. Also, most plastic-sheathed cables should not be exposed to sunlight unless explicitly stated otherwise.
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