Motor Wiring

[mstrlucky74]

All it say here is #6. A few questions:

1. It was 2P so I need 2 phases. I'm assuming 2 hots, neutral and ground. Not sure if I need neutral. How would I know?
2. Would the disconnect switch be fused or unfused? How would one know. Symbol list is showing disconnect but doesn't say fused or not. Thanks.

 

[ptonsparky]

All it say here is #6. A few questions:

1. It was 2P so I need 2 phases. I'm assuming 2 hots, neutral and ground. Not sure if I need neutral. How would I know?
2. Would the disconnect switch be fused or unfused? How would one know. Symbol list is showing disconnect but doesn't say fused or not. Thanks.

1. Motors never require a neutral unless they are 120 volt.

2. Overcurrent and short circuit protection is provided by the 2 pole 45 circuit breaker. The disconnect is just that, a disconnect.

It is possible that the Compressor instructions call for fuses. Ask whoever is providing the compressors.

 

[infinity]

All it say here is #6. A few questions:

1. It was 2P so I need 2 phases. I'm assuming 2 hots, neutral and ground. Not sure if I need neutral. How would I know?
2. Would the disconnect switch be fused or unfused? How would one know. Symbol list is showing disconnect but doesn't say fused or not. Thanks.

What's the voltage? I believe that there is something wrong with what you have posted. The NEC tables do not list a 1Ø, 10 HP motor at anything above 230 volts. If this is 1Ø and the voltage is 208 volts then the FLC is 55 amps.

 

[drcampbell]

... If this is 1Ø and the voltage is 208 volts then the FLC is 55 amps.

If 208 is available, you shouldn't be installing a single-phase motor.

 

[ptonsparky]

Where did 208 come.into play? They are right that the 45 will be too small and most likely the conductors as well even at 240.

 

[infinity]

Where did 208 come.into play? They are right that the 45 will be too small and most likely the conductors as well even at 240.

It was a guess, either mrlucky's building will be 3Ø, 208Y/120 or 3Ø, 480Y/277 or both. I don't see that it would be a 1Ø, 480 volt motor.

 

[augie47]

Northern Tool shows a "10HP" single-phase air compressor with a "motor amps = 40".. still a SWAG but a possibility.

 

[kwired]

Northern Tool shows a "10HP" single-phase air compressor with a "motor amps = 40".. still a SWAG but a possibility.

There are many 10 HP single phase motors out there with nameplate amps of 40 or approximately 40.

NEC table accounts for worst case efficiency and power factor you may ever find.

 

[infinity]

In mrlucky's case the voltage would likely be 208 volts which according to T430.248 {55 amps} is nowhere near 40 amps.

 

[ptonsparky]

OP. What is your voltage?

 

[kwired]

Air compressors (especially single phase units) can fall into the world of power tools that have special ratings on them that usually is a peak power rating instead of a continuous power rating. Actual motor may be built more like a continuous rated 7.5 or even 5 HP but unit is marked 10 HP.

 

[Jraef]

Air compressors (especially single phase units) can fall into the world of power tools that have special ratings on them that usually is a peak power rating instead of a continuous power rating. Actual motor may be built more like a continuous rated 7.5 or even 5 HP but unit is marked 10 HP.

That would be my take as well. What they (compressor mfrs) do, especially on packaged retail compressors, is that the “HP” is referred to by saying it “develops” that rating. They get to that number by using the HP calculation that says HP = torque x rpm / 5250, but instead of using Full Load Torque like everyone else does, they use the peak Break Down Torque, which is typically 200% of FLT. The motor can only sustain putting out BDT for short periods of time without burning up, but the theory is that in their packaged unit, they have total control of how long the motor runs. So it’s more likely a 5HP motor under any other circumstances, but they get away with marketing it as “develops 10HP”.

 

[GoldDigger]

That would be my take as well. What they (compressor mfrs) do, especially on packaged retail compressors, is that the “HP” is referred to by saying it “develops” that rating. They get to that number by using the HP calculation that says HP = torque x rpm / 5250, but instead of using Full Load Torque like everyone else does, they use the peak Break Down Torque, which is typically 200% of FLT. The motor can only sustain putting out BDT for short periods of time without burning up, but the theory is that in their packaged unit, they have total control of how long the motor runs. So it’s more likely a 5HP motor under any other circumstances, but they get away with marketing it as “develops 10HP”.

Right up there with the guy who rated the output of a wind turbine at a particular wind speed by measuring the maximum output of the generator as the turbine used up its rotary kinetic energy slowing down from maximum speed. It could only deliver that for a few seconds and then you had to reduce the load to continuous output or below to let the machine speed up again.

 

[mstrlucky74]

What's the voltage? I believe that there is something wrong with what you have posted. The NEC tables do not list a 1Ø, 10 HP motor at anything above 230 volts. If this is 1Ø and the voltage is 208 volts then the FLC is 55 amps.

208v

 

[mstrlucky74]

Here's the equip schedule for it.

 

[infinity]

Either the HP rating is artificial or the full loads amps are incorrect.

 

[retirede]

That would be my take as well. What they (compressor mfrs) do, especially on packaged retail compressors, is that the “HP” is referred to by saying it “develops” that rating. They get to that number by using the HP calculation that says HP = torque x rpm / 5250, but instead of using Full Load Torque like everyone else does, they use the peak Break Down Torque, which is typically 200% of FLT. The motor can only sustain putting out BDT for short periods of time without burning up, but the theory is that in their packaged unit, they have total control of how long the motor runs. So it’s more likely a 5HP motor under any other circumstances, but they get away with marketing it as “develops 10HP”.

As an engineer who formerly worked for a compressor manufacturer, I can says that this is exactly correct. It is mainly true for smaller (less than 5 true horsepower) machines.
The company I worked for exited this small sized compressor business because it was impossible to ethically and economically compete with the “horsepower whores”.
If you buy a compressor, base it in CFM, not HP.

 

[infinity]

As an engineer who formerly worked for a compressor manufacturer, I can says that this is exactly correct. It is mainly true for smaller (less than 5 true horsepower) machines.
The company I worked for exited this small sized compressor business because it was impossible to ethically and economically compete with the “horsepower whores”.
If you buy a compressor, base it in CFM, not HP.

Sounds like my Craftsman 6.5 HP compressor that has a motor nameplate amp rating of 15 amps which translates to about 1 HP at 120 volts. :roll:

In the case of mrlucky's installation the HP rating of the unit is irrelevant, he has the engineered conductor size of #6 AWG and a 2 pole 45 amp OCPD so he can just follow that.

 

[retirede]

In the case of mrlucky's installation the HP rating of the unit is irrelevant, he has the engineered conductor size of #6 AWG and a 2 pole 45 amp OCPD so he can just follow that.

Agreed. My comment was to add to what Jraef posted. Not too relevant to the OP.

 

[zbang]

Sounds like my Craftsman 6.5 HP compressor that has a motor nameplate amp rating of 15 amps which translates to about 1 HP at 120 volts.

Or my Ridgid shop vac marked "2.5 pHP" (peak, I assume). (Or the ones marked "10 AMPS POWERFUL!!")