What size fuse's?

I'm redoing the K&T Horizontal Mill I got last month, It was wired for 440 I have pulled the motor and changed it to 220.

I have a almost new magnetic started thats set up for a 5HP at 220, I just won a S.D. 60 amp fused disconnect off ebay. I know I didn't need a 60 but the price was cheaper than the 30 amp ones.

So what size fuse would I need for a 5HP motor, I guess I need to check and see if-n it states the amps on the plate for the 220...It says 13.8 amps for 220.

So I think we would be looking at about 15 amp or 20 amp fuse's ??

Thanks

Paul

Hi Paul,

The 15-amp fuses should be OK as long as your voltage stays up. If it sags you might need 20-amp fuses.

- Leigh

Thanks Leigh thats kinda what I figured...

Paul

No....30-amp slo-blo fuses minimum. Ideal is that you use a breaker instead of a fuse on a motor circuit, as a fuse is designed for a resistive-load. and inductive load such as a motor has a high initial current. Use a breaker rated 25A, 10,000AIC @600VAC, which will trip at FLA automatically.

Fuses are not practical for induction motors. If you are paranoid, use a 75A 250V fuse inline with the breaker in case there is a direct-short.

According to the NEC, a 5 hp, if it is three phase would be 15.2 So the wire would be 15.2 * 125% = would be 19 amps so #12 thhn copper wire would be suitable. ( all single motors must add the 125% to the size of the wiring method)
The maximum fuse would be ( slo blo) 175%
the maximum breaker would be (standard) 250% * 13.8 = 34.5 so a 40 would be the next higher inverse time breaker, ( standard breaker)



so the maximum fuse would be 175 * 13.8 = 175% would be 24 amps, so the next size up would probably be a 30.

but you must have a motor controller in the circuit to use these higher numbers. You need to do this because of the starting load of the motor is high, higher than the ampacity of the wire. So a larger circuit protection, but then the controller ( mag starter) prevents the motor from every overloading the wire.

I have to disagree with the poster about fuses, as fuses have been the main source of motor protection for the last 100 years. Until the advent of electronic breakers and motor protection, all motor controllers used fuses, or thermal pots for the the motor. Not sure what the fuse and inductive loads have in common, I have never heard of any info on that, but I am never too old to learn.

I would use the 30 amp dual element, and in your motor starter I would use the lowest thermal link or setting on the breaker that allows the motor to start and run properly, so a 13.5 or a 14 amp probably, The lower the setting the more protection you have, but you don't want nuisance tripping. good luck.

I have a new mag starter for it, I'm just up dating whats already there. It will be run from a 3 phase panel.

Thanks for al the info.

Paul

The smallest fuse size for a 60 A fuse clip is 35 amp, in order to use lower rated fuses you will need to buy fuse reducers, see a Buss fuse catalog..

If you have a motor starter with a properly sized OL heater, then the fuse is only there for Short Circuit protection and the recommended fuse for a 5HP 220V motor, using Time Delay fuses, is 35A, so turns out you needed that 60A disconnect after all.
A 15A fuse will probably pop on stat up.

Old Iron said:

I just won a S.D. 60 amp fused disconnect off ebay. I know I didn't need a 60 but the price was cheaper than the 30 amp ones.

Click to expand...

All you need is 25A slow blow. Been using that size with a 5hp for more than 15 years.

This thread being 9 yars old, I expect the problem is long solved.

If there was a way to mark a thread resolved it would be nice, along with the OP thanking people for help. This is not 1-800-free-help.

rons said:

If there was a way to mark a thread resolved it would be nice, along with the OP thanking people for help. This is not 1-800-free-help.

Click to expand...

Now that's just plain silly talk right there.... What's wrong with you?