New compressor - 50 or 60a circuit?

[restorick]

Good morning,

I'm a little torn trying to figure this out. I've always worked under the rule that the breaker/device is there to protect the wire.

I have a new 7.5hp compressor waiting to be wired in. My garage shop has a 60a subpanel.

Compressor install manual recommends a 60a breaker. Motor plate says 31.2a with 1.15SF.

I'm hardwiring the circuit. Will be using 8ga wire (not Romex) through the wall. Compressor is approx 10' from breaker panel and perhaps 3' from disconnect. Will use wire in BX between disconnect and starter.

Appreciate your thoughts on using a 50 or 60a breaker.

Thanks,
Rick

 

[Kevin Essiambre]

**not a nec expert, I am Canadian**

It appears that the NEC says that motor will have 40 amps FLA.

125% of 40 amps is 50 amps, so you need minimum of 50 amps.

I *think* you can put it on a 60, but I'm not a NEC expert. If you're Canadian, then I can help more.

Of course, I'd be inclined to say just put it on a 60 as per the user manual...

 

[MerlinsBeard]

Good morning,

I'm a little torn trying to figure this out. I've always worked under the rule that the breaker/device is there to protect the wire.

I have a new 7.5hp compressor waiting to be wired in. My garage shop has a 60a subpanel.

Compressor install manual recommends a 60a breaker. Motor plate says 31.2a with 1.15SF.

I'm hardwiring the circuit. Will be using 8ga wire (not Romex) through the wall. Compressor is approx 10' from breaker panel and perhaps 3' from disconnect. Will use wire in BX between disconnect and starter.

Appreciate your thoughts on using a 50 or 60a breaker.

Thanks,
Rick

Assuming single phase, you can convert Hp to Amps by the following:

Amps = Horsepower * 746W / (Power Factor * Efficiency * Voltage)

If you make everything 100% efficient (PF and Ef are 1), you get

Amps = 7.5 Hp * 746 W/Hp / 120V = 46.625 A

or with 240V

Amps = 7.5 Hp * 746 W/Hp / 240V = 23.3125 A

A power factor/efficiency less than unity will drive up the amps needed.

The Motor service factor shows that the Hp load could temporarily get as high as 1.15 * 7.5 Hp or 8.625 Hp.

Amps = 8.625 Hp * 746 W/Hp / 120V = 53.6 A

or with 240V

Amps = 8.625 Hp * 746 W/Hp / 240V = 26.8 A

If you note that the max amp is 31.2, I presume that it's assuming a 240V connection. The ratio between the theoretical calculation and the nameplate amp gives an approximate value for the scaling factor.

26.8A / 31.2A = 85% (I rounded down)

Amps = 8.625 Hp * 746 W/Hp / (0.85 * 240V) = 31.5 A

Applying the same factor to 120V

Amps = 8.625 Hp * 746 W/Hp / (0.85 * 240V) = 63 A

If you service has 60A on 120V and you plan to hook it up, you could end up tripping your service panel, especially if you run other stuff at the same time.

You'll likely want a 240V connection if you want to keep your 60A subpanel.

Hopefully someone else verifies my math. Hope it helps.

 

[restorick]

Clarification - yes, this is a 240v circuit.

 

[dogdog]

your motor plate should tell you what you needed... I think most 1PH motor for compressors are 30AMP circuits.. of cause you could be special. There are those motor SPL thing or RLA / FLA rating thing for motors. Worse case, pull a 8 gauge and expect to put in a 30AMP or 40AMP.

 

[Norcal]

The circuit must be sized 125% of 40A from table 430.248, which is 50A since the OP said it was a 7 1/2 HP motor. If NM cable is used 6 AWG or 8 THHN THWN is needed a 60A breaker is the same price as a 50A so would use a 60A to avoid problems.

 

[BBE]

Oversize by one, for future proofing

 

[Bert_]

50a and 60A are the same price. No reason not to use the 60A. Code allows bigger yet but you don't need to.

 

[u2slow]

Good morning,

I'm a little torn trying to figure this out. I've always worked under the rule that the breaker/device is there to protect the wire.

I have a new 7.5hp compressor waiting to be wired in. My garage shop has a 60a subpanel.

Compressor install manual recommends a 60a breaker. Motor plate says 31.2a with 1.15SF.

I'm hardwiring the circuit. Will be using 8ga wire (not Romex) through the wall. Compressor is approx 10' from breaker panel and perhaps 3' from disconnect. Will use wire in BX between disconnect and starter.

Appreciate your thoughts on using a 50 or 60a breaker.

The basic rules for the breaker protecting the wire change for a motor load.

What is the distance and wire size used for the subpanel feed? (off how large a main service?) What is the max allowed branch circuit size on your 60A subpanel?

If you had a 100A (or more) subfeed to the shop, I'd say #8 and a 60A breaker would be fine, due to the short run, and actual FLC of the motor. I'd double check the running current (clamp meter) after install, and voltage sag @ your subpanel, and compressor on startup.

 

[Showkey]

“I have a new 7.5hp compressor waiting to be wired in. My garage shop has a 60a subpanel.”

What going to power the rest of the shop while the compressor is running ????

Not being a smart ***.......but..........not much left to power the lights, saws, welder, sanders, lift, etc

 

[restorick]

“I have a new 7.5hp compressor waiting to be wired in. My garage shop has a 60a subpanel.”

What going to power the rest of the shop while the compressor is running ????

Not being a smart ***.......but..........not much left to power the lights, saws, welder, sanders, lift, etc

One man garage shop. When compressor runs, I'll be running air tools (blaster, sander, etc.). Lights are all LED.

I'm more concerned about startup. The motor will take 32a running. Plenty of juice left over for the rest.

Thanks,
Rick

 

[restorick]

The basic rules for the breaker protecting the wire change for a motor load.

What is the distance and wire size used for the subpanel feed? (off how large a main service?) What is the max allowed branch circuit size on your 60A subpanel?

If you had a 100A (or more) subfeed to the shop, I'd say #8 and a 60A breaker would be fine, due to the short run, and actual FLC of the motor. I'd double check the running current (clamp meter) after install, and voltage sag @ your subpanel, and compressor on startup.

Wire size - 4/4/4, with 6 bare copper ground. Distance from main panel - 42'.

200amp main. I asked the electrical inspector about the max circuit on the sub and he said 60 would be fine.

Thanks,
Rick

 

[u2slow]

Wire size - 4/4/4, with 6 bare copper ground. Distance from main panel - 42'.

200amp main. I asked the electrical inspector about the max circuit on the sub and he said 60 would be fine.

Seems viable. 100A main might be trouble on a long service drop. Not a long run to the shop.

You basically have a healthy 60A circuit for the compressor already, that you're 'stealing' lights from.

 

[alfredeneuman]

The Square D (Schneider) motor data calculator lists.
#8Cu wire (NM #6) with an 80A c/b and 60A dual element fuses in the disconnect.

 

[Terry D]

The Square D (Schneider) motor data calculator lists.
#8Cu wire (NM #6) with an 80A c/b and 60A dual element fuses in the disconnect.

Im just asking, why would you need a fused disconnect?

 

[restorick]

Im just asking, why would you need a fused disconnect?

I'm not using one. My breaker panel is only 10 feet or so from the compressor. Disconnect is old habit, but I'm not using a fused disco.

 

[Lonnies Performance]

For my 7.5HP compressor, the recommended breaker size was 35A. Works fine this way, but I doubt I needed a #6 wire.