Size of breakers to use

B

Beef

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Getting ready to install the following equipment in my garage, looking for breaker size recommendations for in the breaker box . Garage has 100 AMP service.
South Bend lathe 1 H.P. will be running off a VFD
Bridgeport mill 2 H.P. also running off VFD
Air compressor 5 H.P. 2 Stage will be using a magnetic starter for this.

Thank You
 
You size your breaker to the diameter/length of the wires running off it. If you have the ability to run wires, you can then size it to the equipment.

The lathe/mill should both be perfect on a 15A breaker (running 220 obviously). The compressor (assuming a 220v supply) needs at least 20 amps.

That said, if you're running wires, go as big as you can. You'll love the ability to run a bigger lathe/mill someday.
 
The VFD manufactures usually have recommendations as to size/type fusing or breaker recommended. There are also some differences as to hard wiring machines which comes under NEC vs. a socket and plug which falls under UL from my understanding. Recommendations for 1 Hp VFD would be 20A with 12 AWG wire/cord (depends on the length), 2-3 Hp VFD they usually recommend 30A breaker with 10AWG wire/cord. I have run 2 Hp VFD's off of 20A breakers, but it depends on the VFD parameters and motor draw amps. You can dial up some VFD's to 180-200% for up to a minute which can push up the load. Breakers are sized to 125% of the rated input current of the VFD, also breakers will have different overload curves depending on the type installed. Air compressor, I have a Champion 5Hp, 2 stage, I use a 50A breaker and a 50A rated extension cord hardwired at the compressor end. Previously I had it hardwired with conduit with #6 THHN pulled through, the motor has a FLA of 23 and a SFA 27. On an extension cord you may be able to get by with 8AWG based on the running load with a 40A breaker. There are different calculations for breaker size and overload device. I went with a 50A so I could also use it for other machines and at the time was considering an electric car, the compressor has its own thermal overload relay sized to the motor on the run contactor.

One other note, hard wiring is not designed to carry 100% of the load all the time, so a 20A breaker does not mean you can pull 19.9A continuously, there are also thermal limitations based on the type of wire, if in conduit, fill factor, distance, etc. I tend to be conservative in this area when possible.
 
My 5hp Baldor 230V compressor motor draws 20.6 amps, and I run it on 30 amp 240 V twin breakers with no issues. you don't want to go too large on breakers, or you can burn up your wiring before they will trip. Wire size will depend on the length of the run and amperage required. There are many wire sizing charts online. Mike
 
Per the electrical code breakers are to protect the wiring, not the equipment. Obviously the equipment governs the current draw. The wiring and breakers should be sized based on the equipment draw. Check the tables for the appropriate wire size for the current draw and size the preaker for the wire size.
 
2nd to breaker is set by wire size, wire limits breaker to match wire but you can use larger witr to reduce voltage drop.

Motors have starting currents so check paperwork for equipment for suggested wire size.

Sent from my SAMSUNG-SM-G930A using Tapatalk
 
jeffkash said:
Per the electrical code breakers are to protect the wiring, not the equipment. Obviously the equipment governs the current draw. The wiring and breakers should be sized based on the equipment draw. Check the tables for the appropriate wire size for the current draw and size the breaker for the wire size.
Click to expand...

Yes, the breaker is sized to protect the wiring because if the wiring is not protected, it can overheat and burn down the building.
 
Not an electrician, Google might be a more reliable source, but here goes.

15 amps, #14 wire
20 amps, #12 wire
30 amps, #10 wire
40 amps, #8 wire
50 amps, #6 wire

That's for copper wire, up gauge one size (#12 for 15 amps) if using aluminum.

1 HP is roughly 750 watts. Power (watts) = current (amps) x voltage

1 HP motor draws ~3.5 amps at 220 volts, about 7 amps at 110 volts.

Bruce
 
BGHansen said:
Not an electrician, Google might be a more reliable source, but here goes.

15 amps, #14 wire
20 amps, #12 wire
30 amps, #10 wire
40 amps, #8 wire
50 amps, #6 wire

That's for copper wire, up gauge one size (#12 for 15 amps) if using aluminum.

1 HP is roughly 750 watts. Power (watts) = current (amps) x voltage

1 HP motor draws ~3.5 amps at 220 volts, about 7 amps at 110 volts.

Bruce
Click to expand...
The wire size AND length matters, but for anything below around 50 feet those are probably about right.

I usually estimate about 4 amps/hp for smaller motors since the older stuff I get isn't the most efficient.
 
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