Air Compressor 460 volt 20 amp on an 30 amp Breaker

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powerplay

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Hello all,

We are connecting an Doerr Electric Corp. “3N232G” Three Phase 460 volt 20 amp Air Compressor to an old 480 volt FPE Panel. There is an available General Electric TED134030 Breaker with approx 180’ of #6 copper wire to beside where the Compressor is to be installed. Is the old breaker likely able to handle the Compressor starting under load ?

The used Air Compressor comes with a fused Disconnect with one time use Bussman NOS-60 amp 600 volt fuses and an motor starter which we hope to mount beside the equipment for lockout and control.
 
460V 20 Amp, I assume it is a 15 HP motor. (430.250), 430.251(B)
Based on the fuse-Breaker type (430.52)
Crunch some numbers.
AWG#6, copper or aluminum, insulation type, terminal connection rating, 310.16, 310.15(b)(16)
Voltage drop: VD = 1.732 x K x I x D/CM
 
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I would be surprised to see the breaker hold especially on a compressor...as Teaser2 notes..see Table 430.52
 
Teaser2 said:
460V 20 Amp, I assume it is a 15 HP motor. (430.250), 430.251(B)
Based on the fuse-Breaker type (430.52)
Crunch some numbers.
AWG#6, copper or aluminum, insulation type, terminal connection rating, 310.16, 310.15(b)(16)
Voltage drop: VD = 1.732 x K x I x D/CM
Click to expand...
Correct, it is an 15 Hp motor.

I’ll confess I’m up in Canada, but found excellent feedback from a variety of experience on this NEC Site.

I’ve used our Voltage drop chart from the CEC code book, but our local Inspector suggested using the online V.D. calculator and it appears to work very well b
 
augie47 said:
I would be surprised to see the breaker hold especially on a compressor...as Teaser2 notes..see Table 430.52
Click to expand...
As “confessed”, I’m a Canadian looking for feedback from diverse experience, and have enjoyed your NEC Forums…. We’re apparently working towards meeting NEC, and it’s still P= E x I up here as well !

The motor FLA is 20 amps, and at 125% the 30 amp breaker is minimum size, and hoping an air compressor doesn’t start under load unless the pressure is set very high ? Our Canadian Table 29 suggests based on the type of motor, 200% permitted if motor not starting.

The breaker is pricey, so wondering if it will hold on the compressor, but tgey don’t want nuisance tripping and will flip the bill for a larger breaker if needed.
 
powerplay said:
... and hoping an air compressor doesn’t start under load ...
Click to expand...
Don't just "hope" for that. Assure that an unloader is installed, adjusted correctly, and maintained.

With an unloader, the starting current will reflect only the inertia of the moving parts, with the work/torque of compression coming into effect only after the machinery is up to speed.
 
Whether it holds during starting really will depend on conditions. Having 6 AWG for circuit conductors works against it holding, you won't have much VD (which also helps with current limiting during that starting surge) in the circuit conductors unless it has some significant length to it.

Size and impedance of the source factor in as well as size and length of service and/or feeder conductors. But in many cases a 30 amp breaker is probably considered marginal for this. A 40 amp breaker is likely to hold and would be fine to leave as is if that were what you had, even though NEC would allow you to go up to 60 amps.
 
retirede said:
I was once a product engineer for a compressor manufacturer. If you have any specific questions, don’t hesitate to post them. Or DM me.
Click to expand...

Thanks for the option… my interpretation on the scenario is that an 480 volt 30 amp may trip depending on the back pressure when air compressor starts before rotating. I had an friend mention the twin motor is not likely to have an “unloader”, which I assume is like an clutch that disengages to allow motor to work before engaging the load and it will require closer to the allowable breaker size of 250% (in canada) so an 40 amp one is the wise way to go…. We were working to utilize existing breaker to save cost but downtime will be costly and painful for me !
 
powerplay said:
[The circuit breaker] may trip depending on the back pressure when air compressor starts before rotating. ...
Click to expand...
If the unloader valve is working properly, and adjusted properly, there won't be any back pressure on the pump when starting.

powerplay said:
... the twin motor is not likely to have an “unloader” ...
Click to expand...
This air compressor has two motors? That's an important detail.

If it truly doesn't have an unloader valve, install one. Or two, if there are two air pumps.

An unloader valve releases the air pressure from the air pump while stopped, enabling it to start under no {pneumatic} load. (it will still need to overcome friction and inertia when starting) This will greatly reduce mechanical shock loads and electrical inrush currents, greatly improving the reliability & longevity, and modestly improving noise & vibration while starting.

If the pressure switch has two pneumatic fittings, it has an unloader valve. One will be piped to the reservoir, downstream of the check valve, (often mounted directly on the reservoir) and the other will be piped to the pump, upstream of the check valve.

Post close-up photographs; we'll try to help. At least one should be of the pressure switch.
 
powerplay said:
Thanks for the option… my interpretation on the scenario is that an 480 volt 30 amp may trip depending on the back pressure when air compressor starts before rotating. I had an friend mention the twin motor is not likely to have an “unloader”, which I assume is like an clutch that disengages to allow motor to work before engaging the load and it will require closer to the allowable breaker size of 250% (in canada) so an 40 amp one is the wise way to go…. We were working to utilize existing breaker to save cost but downtime will be costly and painful for me !
Click to expand...

What technology is the compressor? Reciprocating piston, rotary screw, other? The unloading techniques will vary with the type.
 
powerplay said:
Thanks for the option… my interpretation on the scenario is that an 480 volt 30 amp may trip depending on the back pressure when air compressor starts before rotating. I had an friend mention the twin motor is not likely to have an “unloader”, which I assume is like an clutch that disengages to allow motor to work before engaging the load and it will require closer to the allowable breaker size of 250% (in canada) so an 40 amp one is the wise way to go…. We were working to utilize existing breaker to save cost but downtime will be costly and painful for me !
Click to expand...
Twin motor? Guessing maybe you talking about a two stage or basically two cylinder piston type compressor. The unloader valve is part of the pressure switch that controls starting/stopping, when it opens contacts to stop the unit it also has mechanically operated valve that releases head pressure, this allows not having to start against that head pressure next time it closes the control switch to start. It is connected between the compressor and a check valve at/near the storage tank that check valve keeps it from bleeding pressure from the storage tank and just lets it release pressure from the compressor head only.
 
kwired said:
Twin motor? Guessing maybe you talking about a two stage or basically two cylinder piston type compressor. The unloader valve is part of the pressure switch that controls starting/stopping, when it opens contacts to stop the unit it also has mechanically operated valve that releases head pressure, this allows not having to start against that head pressure next time it closes the control switch to start. It is connected between the compressor and a check valve at/near the storage tank that check valve keeps it from bleeding pressure from the storage tank and just lets it release pressure from the compressor head only.
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They use the "twin motor" units for where there is only single phase power available and you need higher HP. I used to see them alot but not so much anymore. All of them should have a starting unloader of some type though
3112dl__42784.1533737328.jpg
 
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kwired said:
Twin motor? Guessing maybe you talking about a two stage or basically two cylinder piston type compressor. The unloader valve is part of the pressure switch that controls starting/stopping, when it opens contacts to stop the unit it also has mechanically operated valve that releases head pressure, this allows not having to start against that head pressure next time it closes the control switch to start. It is connected between the compressor and a check valve at/near the storage tank that check valve keeps it from bleeding pressure from the storage tank and just lets it release pressure from the compressor head only.
Click to expand...
 
powerplay said:
Oops… I “tried”to attach an picture off my phone but too big an file apparently, but an friend commented that “That it’s a twin compressor, so likely no unloader valve “
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If it dosen't have an unloader you can easily add one. They have pressure switches with built in unloaders. You run a tube from that to the compressor side of the check valve on the compressor discharge line leading to the tank
 
Joethemechanic said:
If it dosen't have an unloader you can easily add one. They have pressure switches with built in unloaders. You run a tube from that to the compressor side of the check valve on the compressor discharge line leading to the tank
Click to expand...

A compressor this size cannot start into a pressurized system without some unloading mechanism. Normally, once you get above 10HP, the compressor will have suction valve unloaders.

Relieving the pressure from the discharge as you describe is viable with a couple caveats: 1) If the compressor was designed to use suction valve unloading, there will likely not be a check valve in the discharge. 2) Pressure switches with an attached unloading vale are only used in cheap compressors because if the compressor is manually stopped or there is a power interruption while running, the compressor does not unload. A normally open solenoid valve is the preferred option.
 
powerplay said:
Hello all,

We are connecting an Doerr Electric Corp. “3N232G” Three Phase 460 volt 20 amp Air Compressor to an old 480 volt FPE Panel. There is an available General Electric TED134030 Breaker with approx 180’ of #6 copper wire to beside where the Compressor is to be installed. Is the old breaker likely able to handle the Compressor starting under load ?

The used Air Compressor comes with a fused Disconnect with one time use Bussman NOS-60 amp 600 volt fuses and an motor starter which we hope to mount beside the equipment for lockout and control.
Click to expand...
Have connected my share of air compressors . Preferred to use a fusible safety switch with quality 200KIAC rated fuses. Can not remember ever having a problem with any air compressors that shut off at 120 #'s air pressure and only using fuses no more then 150% higher then motor FLA. In some commercial garages they needed at least 135 to 140#'s of air pressure to operate air jacks to lift up loaded trailers so they did not cut off until 150 #'s . In that case had to use a time de!ay fuse rated 175% higher then motor FLA. Had trouble with a couple of jockey air compressors on sprinkler systems where they had the cut in & cut off too close together and air compressors would short cycle ( on & off several times a minute ). You should not have any problem with the existing #6 copper wire. In larger plants where they had several air compressors running at the same time we had tech install think it was an unloaded on at least one air compressor that ran entire production time. At end if day they flipped a switch to return it so air pressure switch controlled it. Had one one a 200 HP screw compressor.
 
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