Used to know this stuff, but...

I want to run a 5 hp roller mill. The distance is about 70 feet to the breaker panel. What size wire would I require? I'm thinking about a #6 copper.

Any suggestions much appreciated.

Single or 3 phase? Voltage?

single phase, 220 volts,, shoulda thought of that myself..

What amperage is the motor? If you were closer and under 24 amps 10 gauge would work. Going 70' I would use one gauge heaver, two gauges if aluminum wire.

30 amp breaker 10 gauge wire 24 amp constant use.
45 amp breaker 8 gauge wire 36 amp constant use.
60 amp breaker 6 gauge wire 48 amp constant use.

Without knowing the actual full load amps of that motor I will have to venture a somewhat educated GUESS:

If the 5 HP single phase AC motor draws say 28 amps at full load,,,,,,,,,that would require a MINIMUM of 8 gauge (40 amp rated) copper wire and you could "get by" with that.

HOWEVER to reduce the voltage drop over 70 feet of conductors Id probably go one size bigger and use 6 gauge.

If its a straight 230 volt motor load with NOOOOOO 120 volt required, that means you can run 3 conductors:

2 Hots L1 & L2 (Red and Black)
1 Equipment GroundING Conductor (Green/Bare)

If I were designing and specifying the job (like I used to do but long since retired) there at the motor (within sight and 50 feet at minimum but I prefer right near the motor) I would place a "method of disconnection" which at the bare MINIMUM could be a plug and cord from the motor connected into a 2 pole 3 wire grounding outlet and Id prefer a twist lock.

HOWEVER my preference would be EITHER a 2 pole knife blade type "Safety switch" or better a 2 pole "fused safety switch" equipped with dual element Class K5 time delay fuses or IFFFFFFFFFF the motor isnt equipped with thermal overload protection then a "combination starter" which is BOTH a safety switch PLUS a thermal overload protection device (with appropriate sized heaters/thermals/overloads)

NOTE The motor should have thermal overload protection, either built in but if not then as noted above.

NOTE you should use flexible conduit or rubber cord from the motor to the means of disconnection

NOTE if you use 6 gauge wire and a 50 amp 240 volt 2 pole circuit breaker as the motors branch circuit feed out of the panel and it doesnt allow the motor to start???? (may well do it) you can use a bigger breaker if necessary which wont trip due the the initial higher inrush starting current

DISCLAIMER Ive been retired from engineering for yearssssssssss and am rusty on this and the latest NEC so if more current professional electricians or engineers need to add to or correct this PLEASE DO SO

John T

John you still in fl?? your educated guess is what is called SWAG Scientfic wild axx guess

This is how I would figure it;

1HP is roughly 760 watts
Watts = Volts X Current
Motors are about 70% efficient
And you do not want to exceed 80% of rated circuit capacity

So:

5HP X 760watts = 3,800 watts
3,800watts / .7 (motor efficiency) = 5,428 watts
5,428 watts / .8 (rated capacity) = 6,785 watts
6,785 watts / 220 Volts = 30 Amps (Should allow for startup current)

Per my handy dandy link to wire size calculator – 10 AWG copper (8 AWG aluminum)

Depending on the setup it can probably surge at startup to 35 amps per line with a 230 volt motor. #8 copper or #6 aluminum at 70 feet results in less then a 2% voltage drop. So yeah, #6 copper is overkill but certainly fine for the job.

The standard generic motor charts show a 5 HP 230 volt single phase motor draws 28 amps (fairly close to your 30 amp calculation)

HOWEVER when I was a design engineer I never exceeded 80% of the wire ampacity for full load conditions and 80% of 30 amp rated No 10 Gauge wire is only 24 amps THEREFORE Id specify 8 gauge wire at a minimum.

ALSO HOWEVER even if it may be some overkill, to lessen voltage drop over 70 feet of wire Id bump it up yet one more size and recommend 6 gauge.

Since 80% of 30 amps is 24 and the motor will draw at least 28 amps and due to the voltage drop at 70 feet, I just couldnt in good faith recommend only 10 gauge wire.

But hey its his money and hes free to do as he pleases and Im rusty as an old nail as far as the NEC is concerned so if he wants yo use 10 thats his business but I have to stick with remommending at least 8 better yet 6 gauge

Fun chatttin with ya

John T

Yep still here neighbor, will be back around first of April...

John T

measure the voltage on your 220 line,mine measures 242 volts.I run a 5hp motor on #8 wire with 30 amp time delay fuses.