Question for everyone running a 60 gal/5-6.5hp compressor...

[Ingram306]

I'm running a Quincy 5hp two stage on a 30A breaker with no issues

 

[ZRX61]

You would need a 60 for a 200 synch,,, I don't recall if its 8 or 10 wire @60, but its minimum size is for single circuit in pipe and the other machines are designed to run on 50A max circuits. I am pretty much always plan to run a wire dediucated for these type of machines, 2 circuits here, one for the synch, one for the air and might share the plas and the 210.

ok, scratch that plan

 

[marinusdees]

Volts times amps equals watts. One horsepower is nominally 750 watts. 748 exactly I think. I ain't rocket science. At 240 volts you use half the amps as at 120 volts. Ergo, you can use a breaker half the size and wire accordingly sized. You use the same wattage at either voltage and your electric meter measures (kilo)watts and reads the same and you pay the same no matter the voltage you use. The advantage of using 240 volts is to be able to use smaller wire(less current draw) and therefore a smaller breaker. I can't begin to believe the amount of verbiage (and misinformation) that has been expended on this subject. If you have read between the lines, you will have discovered that (approximately) 7 amps at 120 volts is about one horsepower. Be aware that modern electric motors are rated at "locked rotor horsepower". This means that the rotor is locked ( prevented from rotating) and power applied to the motor. The highest amperage recorded just as the motor is enveloped in flames is used by the above formula to calculate the (theoretical) horsepower. In other words horsepower ratings are NOT realistic. Overrated and underpowered as we used to say about "Job Rated" Dodge trucks. Probably before anybody's time but mine.

 

[Durka]

The load is a pinch above 20, no matter what happens it wont overheat the wire, the thermal on ther motor will open first. When you are doing something like this the breaker cant be above the short circuit rating of the wire,,, (poorly explained and only a generalization) but 14 for a 30, 12 to 50A and 10 at 60.

Having said that,,, although it doesn't make much real difference I also prefer to break closer to the rating and I can,,, and have breakers on hand etc so I put a 30 on. It cant hurt as long as it doesn't trip during starts.

Makes sense. -That's what I'm going to do...Swap out that 60 for a 30 and see how it goes. Planed on two two poles and happened to find a 30 sitting on on top of the box. Use that 60 for a Synch and/or a 210...Yea, love to!

 

[Durka]

Ref: post #33 by Durka..................... not the OP.

Section 430 of the NEC has a table for single phase motor amperages based on hp and voltage. It says that a 5 hp motor at 230v is 28 amps.

The code says to size the breaker and circuit using the table in the code, and not the FLA on the motor, even though it will nearly always be lower. Use the table amps to size the wire, hence, #10 or you could go #8 (depends on the type of wire you are using). To size the breaker, you are allowed to go as large as 250% of the table amps, thus 70 amps max on a breaker, or 175% on time delay fuses, 49 amps, or 50 amp TD fuses. The smaller the breaker, the better, so long as you don't blow it on motor startup. It stands a better chance of protecting the circuit. Since you have a 21.5 amp FLA I would use a 30 amp breaker and see how it works. They are cheap, $7 or $8, so why use the larger one?

Also remember, you must have a disconnect at the compressor if it is more than 50 ft straight line distance, or is not in line of sight from the breaker panel. (code actually says line of sight, however, code defines "line of sight" as out of sight or a distance of more than 50 ft.

In this case, a plug will not suffice as a disconnect, since there are no typical flat blade plugs that are motor rated at more than 3 hp, even though they may be more than the necessary amp capacity.

Charles

Thanks for the info Charles. In bold, yea I wasn't aware of that. Under the impression that both amp #'s were on motor plate. Not impression, a good guess that was not so good. The motor has the FLA @ 21.5 as you know. It's the SFA number I must of mixed up. Isn't SFA Start Flow Amps 24.4 on the bottom of the motor plate valid ? At least that's what thought "SFA" meant. Either that or Service Flow. So the NEC book has the numbers,- 28 table amps is the safety.



Line of site was okay...Yea, didn't know that was code either.

View media item 40259
Thanks for the Info...Saved, -good stuff.

Hope the OP can use some of it.

 

[gearhead1]

30A breaker (10 gauge wire) on a 5hp 60 gallon compressor. I bought it about 18 years ago, I assume it's a true 5 hp. I looked at the data tag on the motor and it was 20 or 21A so I went up to 30 A for the breaker and have had zero issues.