Air compressor wiring
#9
Hi everyone, I am asking a question requiring a little of the vast pool of knowledge Woodnet posters possess.
I need to replace my air compressor. I have a 60 amp service in the garage. It will have a 5 HP 230 volt motor and the manual calls for a 40 amp circuit.
Are the amps additive? In other words does this calll for a 20 amp breaker on each voltage leg? BTW, it is listed as a 22 amp FLA motor and I am going to run 10 gauge wire (about 27 feet long)
Proud maker of large quantities of sawdust......oh, and the occasional project!
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#10
No, they're not additive.  A 40A 2-pole breaker (takes two full-sized slots in the panel) is used, and current passes from the transformer on the pole, goes through one of the two poles of the breaker in your panel, continues through the motor, and 'returns' through the other pole of the breaker and goes back to the transformer on the pole.  

If either pole of that breaker is overloaded, it will trigger a trip event, and both poles will open.  Of course, since both poles are seeing the same current, there won't be a current unbalance unless there's a ground fault (one of the hot leads contacts a grounded box or conduit or whatever).  But that's how they work.

Does this compressor have its own overload protection?  If so, you can use 10 gauge copper and a 40A breaker, as the compressor's motor control is the overload protection for the motor and for the conductors between the panel and the motor control.  The breaker provides ground-fault and short-circuit protection.  This is per NEC Article 430 (motors, controls, etc.), which is just one of many places where the 15/20/30A max breaker on 14/12/10 gauge copper conductors isn't an iron-clad rule.  "Oversized" breakers are allowed on motor circuits, per a pile of sizing rules, because motors typically draw from 4 to 8 times their rated full-load current (FLA) on start-up, and an "oversized" breaker or fuses are allowed to prevent tripping.

If you already have a 30A breaker, you could try that to see if it reliably starts without tripping.  Common thermo-magnetic breakers can handle a large overload for short duration, so you may be fine with a 30A breaker.  Most 5 hp motors are.
Tom

“This place smells like that odd combination of flop sweat, hopelessness, aaaand feet"
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#11
Thanks TDKPE, 
It was you whom I was hoping would reply. In the online use manual it says the motor has built in overload and thermal protection. I will try the existing 30 amp two pole breaker and see what happens. The circuit is multi use. I have a 180 amp mig welder and a three HP Unisaw on it but only ever run one at a time. I plan to leave the air compressor off unless I am using it alone as well.
Proud maker of large quantities of sawdust......oh, and the occasional project!
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#12
(04-08-2020, 07:01 AM)KyleD Wrote: In the online use manual it says the motor has built in overload and thermal protection. I will try the existing 30 amp two pole breaker and see what happens. The circuit is multi use. I have a 180 amp mig welder and a three HP Unisaw on it but only ever run one at a time. I plan to leave the air compressor off unless I am using it alone as well.

Interesting that you have a welder on that circuit sometimes - welding machines are another application where, in this case, "undersized" wiring is permitted (as opposed to "oversized" breakers or fuses), depending on the duty-cycle of the welding machine (NEC Art. 630).  Cranes are another (NEC Art. 610), and there are many others.

Undersized wiring is permitted when the duty-cycle is relatively low and/or there are multiple welding machines on the same feeder, as the time-averaged conductor heating due to conductor impedance is also lower since the load is only applied intermittently.  That doesn't usually do you much good for a single home welder, but when wiring banks of welding machines on long feeder cables, it starts to add up.  I have mine wired with 10-gauge copper, and can legally use a 50A breaker since it's a 20% duty-cycle machine at max output (230A with 50A input). 

I use a 30A breaker anyway, since I only do relatively light work with it these days (~120A out max), and as that feeder now also serves a small sub-panel with four 15A 120V breakers supplying four 15A duplex receptacles (30A each leg, in other words), plus my 30A 120V TT30 RV receptacle, plus my welder receptacle.  It's a 'temporary' panel/receptacle system on a backing board, plugged into a 14-50 receptacle, that's been in service for 15 years now.  
Laugh
Tom

“This place smells like that odd combination of flop sweat, hopelessness, aaaand feet"
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#13
Check to see if your compressor has a "soft start" option.  I am working from a fuzzy memory here but I believe part of the huge power surge on start up has to do with working against the need to start compressing.  There may be a way to decouple the compressor option or open the compressor tank so the initial start up has to only get the motor running then you can engage the compression.  This might allow you to get buy with a less than recommended wire and circuit breaker.

On a semi-related note.  I recently replaced an electric overhead shop heater and it said it required a 40a circuit.  Checking the math I was not seeing the need for a 40a circuit and since i had a 30a in place I wired it to the 30a circuit and it has not had any problems.
A carpenter's house is never done.
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#14
Maybe you are talking about the unloader valve Photbug. It is my understanding every compressor of decent size has a valve that bleeds air out of the pump when it stops. The next time the compressor starts empty and is not under the extra load of a cylinder full of compressed air.
Proud maker of large quantities of sawdust......oh, and the occasional project!
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#15
(04-11-2020, 01:41 PM)KyleD Wrote: Maybe you are talking about the unloader valve Photbug. It is my understanding every compressor of decent size has a valve that bleeds air out of the pump when it stops. The next time the compressor starts empty and is not under the extra load of a cylinder full of compressed air.

Yep that is what i meant.  I don't know that it happens automatically I thought it was a switched mode on mine.
A carpenter's house is never done.
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#16
(04-11-2020, 03:46 PM)photobug Wrote: Yep that is what i meant.  I don't know that it happens automatically I thought it was a switched mode on mine.

If your compressor discharges a bit of air when it shuts down, a little 'psshh', that's the unloader. Mine is mechanical, got it from Grainger. My compressor came with a pressure switch that had the unloader built in. The switch failed and got a new one, flimsy piece of crap. I could not get it to quit leaking so got annoyed and got a real Square D pressure switch and separate inline unloader. That story ended happily.
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