Wiring machines, need help

[lilscorpion]

I have a 3 phase lathe that runs on a rotary phase converter. It takes about 35 amps at start up but draw much less while it’s running. Maybe more like 25. It blows the 30 amp breaker nine times out of ten which to me means the total draw is likely in the lower 30’s.

Must the wire always match the breaker or are there situations like this where you could run 10ga on a 40 amp breaker. I’ve read a few places where hvac guys do that but only in certain situations which are similar (and yet not).

Thanks.

 

[sberry]

You can tailor a dedicated circuit, usually hard wired, not using recepticals. The run would need to be under 25

 

[wcnichols]

Do you have photos of the nameplates of the motor and rotary converter? What horsepower is your lathe motor? Is the 3 phase voltage the same as the single phase input or does your converter change the voltage level?

Your phase converter nameplate should have a rated single phase input full-load ampere rating. Your breaker should be no more than 125% of this value.

Since this is a fixed load your full load current, based on the HP of your motor, will determine the minimum size of your conductors. If your 3 phase voltage is the same as your single phase than you multiply your motor full load current by 250% to determine the minimum conductor size. If your 3 phase voltage is different then you need to multiply the current by the ratio between them and then 250%.

** Your overcurrent protection(ie. your breaker) needs to be able to protect your conductors. If you put a 40A breaker in with a 10AWG conductor you must be 100% sure that the conductor insulation and the terminals at both ends have a 90degC rating. Most home breakers do not. If you need to install a 40A breaker you will more than likely need an 8AWG conductor.

 

[Bert_]

Wire is sized to the motor. Breaker can be 250% of motor amps.

For example a 5 hp single phase motor, 28A. Wire needs to be 35A rated, #10 thhn or #8 Romex. Breaker can be up to 70A.

 

[wyliesdiesels]

Wire is sized to the motor. Breaker can be 250% of motor amps.

For example a 5 hp single phase motor. Wire needs to be 28A rated, #10 thhn or #8 Romex. Breaker can be up to 70A.

no wire needs to be 35a rated- 125% of 28a....

 

[theoldwizard1]

I have a 3 phase lathe that runs on a rotary phase converter.

This sounds like a perfect application for a VFC ! The VFC will allow the lathe motor speed to ramp up slowly so there is no big spike.

 

[Bert_]

no wire needs to be 35a rated- 125% of 28a....

Yup. Typed a little to fast I guess.

 

[wyliesdiesels]

Yup. Typed a little to fast I guess.

That happens to me too

 

[lilscorpion]

Do you have photos of the nameplates of the motor and rotary converter? What horsepower is your lathe motor? Is the 3 phase voltage the same as the single phase input or does your converter change the voltage level?

Your phase converter nameplate should have a rated single phase input full-load ampere rating. Your breaker should be no more than 125% of this value.

Since this is a fixed load your full load current, based on the HP of your motor, will determine the minimum size of your conductors. If your 3 phase voltage is the same as your single phase than you multiply your motor full load current by 250% to determine the minimum conductor size. If your 3 phase voltage is different then you need to multiply the current by the ratio between them and then 250%.

** Your overcurrent protection(ie. your breaker) needs to be able to protect your conductors. If you put a 40A breaker in with a 10AWG conductor you must be 100% sure that the conductor insulation and the terminals at both ends have a 90degC rating. Most home breakers do not. If you need to install a 40A breaker you will more than likely need an 8AWG conductor.

Initially I had thought that the 20amps listed on the plate was total amps but according to the mfg, that’s only for the motor.




I can’t be absolutely sure, I’m going to pull the 10ga and upgrade to 8 ga. Thanks for the feedback.


Sent from my iPhone using Tapatalk Pro

 

[wyliesdiesels]

Initially I had thought that the 20amps listed on the plate was total amps but according to the mfg, that’s only for the motor.




I can’t be absolutely sure, I’m going to pull the 10ga and upgrade to 8 ga. Thanks for the feedback.


Sent from my iPhone using Tapatalk Pro

you need to also factor in the load of what the RPC is powering in addition to the idler motor load (20a) since the 2 single phase legs are passed on to power the 3 phase load.

 

[lilscorpion]

This sounds like a perfect application for a VFC ! The VFC will allow the lathe motor speed to ramp up slowly so there is no big spike.

I might get one some day but for now, the phase converter’s ghost is still too strong to make the shift!

 

[nadogail]

You have received good advice.

 

[theoldwizard1]

I might get one some day but for now, the phase converter’s ghost is still too strong to make the shift!

Expect ...

It blows the 30 amp breaker nine times out of ten which to me means the total draw is likely in the lower 30’s.

What you want is a "soft starter". I don't know if they exist for 3 phase motors. They are not that difficult to build (3PDT relay and 3 resistors resistor, plus a time delay trigger), but you would be better off putting your money toward a VFC.