Sizing OCP for motor

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Cleveland Apprentice

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Cleveland, Oh
Hello,

It's been awhile since posting here and I am stumped on sizing overload protection for an air compressor motor and checking on an existing install by someone else.

Specs on motor:

230v
5 HP
Code: L
FLA: 21.5
Ph 1
Cont. Duty
SF: 1.15
SFA: 24.4

I am re-checking an installation done by an electrician of a motor running an air compressor. The 30 amp I/T CB keeps tripping literally 1 second after turning the breaker on due to the inrush current. I got a reading with my meter of amperage upwards around 120-130 before tripping out. This CB was installed at the compressor as the overload protection device.

I have used the code book to check the SC protection and came up with FLC of 28 amps based on 5 HP @ 230 volts. I multiplied 28 at 125% and came up with 35 amps. There is 10 awg THHN cu. and multiplied the 35 amps @ 250% and came up with 87.5 amps, protected by a 90 amp I/T CB. The SC protection is correctly installed by the previous electrician I believe.

I am stumped with the overload protection (part 3) on why the previous electrician used a 30 amp I/T CB.

I believe he took the FLA of 21.5 amps and multiplied that by 125% and got 26.87 amps. There is a 30 amp I/T CB as the overload protection which keeps tripping on inrush.

Would dual element time delay 30 amp fuses hold up better on inrush current than a 30 amp I/T CB?

I understand why SC protection using an inverse time CB would be sized @ 250% but cannot understand why it's permissible to be sized at 125% as overload when keeps tripping.

Any assistance would be greatly appreciated.

Thank you.
 
Did it ever work?
No?
Does the compressor motor have a built in thermal overload device? Usually a red button somewhere on the motor. The label should also indicate one if it is there. If so, you only need branch circuit short circuit protection.

CB is too small.

Yes?
Check capacitors. Is the unloader valve working? How cold is the compressor? What has changed lately?

IMO, the CB is still to small.
 
I second what Pton said.

I will add if the compressor has never been started (no pressure in the system) that takes the unloader issue off the table. Zero system pressure = unloaded.
 
Now that I’m thinking of it, it is thermally protected and didn’t think to look for a reset button. The unloaded valve does work and motor is relatively new. If you believe the CB is still small for the overload protection then I don’t understand why it is supposed to be sized at 125% of the FLA of 21.5 amps or must there be dual element fuses? Thanks
 
Cleveland Apprentice said:
If you believe the CB is still small for the overload protection then I don’t understand why it is supposed to be sized at 125% of the FLA of 21.5 amps
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This is why it’s usually split between SC/GF and overload among two devices. Mike Holt has a ton of good graphics and articles on sizing. In essence it’s hard to have a breaker sized small enough to work as the overload and also allow starting current.

So what is typically done is FLC x 250% for a TM breaker and that’s your SC/GF protection. (You can see why we’re saying it’s low now). FLC x 125% for conductors, and then FLA (nameplate) for overloads while using provisions for the service factor.

Another note is that it’s possible sometimes to size something to Code, but it doesn’t work. It trips. Code is about safety and (should) not be a design guide.
 
I have looked into some unusual contactor failures of similar RLA air compressor circuits and found unloader problems underlying. In this case the unit was outside and the unloader outlet was frozen. They can be heat cabled and insulated in this kind of situation in most cases.
I am running a trial with some electric unloaders under the same conditions.
This will create massive starting problems. I recently found an inrush calculator and ran numbers for the 7.5 HP / 208V 3PH motor I was looking at. The calculated inrush by that method was significantly higher than real world measurements which came out around 138A on start and is fairly consistent between 2 machines of the same exact configuration. In this case that inrush figure is fine on a 30A 3PH breaker.
This is a different calc that I found today but works similar if you have the correct nameplate data:
They may only be good for 3 PH motors.
 
Cleveland Apprentice said:
Now that I’m thinking of it, it is thermally protected and didn’t think to look for a reset button. The unloaded valve does work and motor is relatively new. If you believe the CB is still small for the overload protection then I don’t understand why it is supposed to be sized at 125% of the FLA of 21.5 amps or must there be dual element fuses? Thanks
Click to expand...
There are two kinds of protection necessary for motors. Seldom does one device provide sufficient protection for both kinds.

Short circuit and ground fault protection - with inverse time (standard breakers) breaker NEC allows 250% of motor current , and has allowances to increase that if motor still can't start without tripping it. With time delay fuses it is 175% and again with allowances if it can't start it. Motors have a high surge of current when first energized that rapidly decreases as the motor magnetic fields are building, plus there is higher current while accelerating the motor that also drops off while approaching full speed. These device need to handle that starting current without tripping but yet are necessary to open the circuit rather quickly should there be excessive current due to a short circuit or ground fault.

Then there is motor overload protection - that is a slower response time protection that actually protects the motor from overloading at levels too low for the SCGF protection to reliably detect. Motor can take overcurrent for short times, and in fact does every time it starts, this overload protection comes in different classifications. You can still have a class 10 trip curve that won't hold long enough to start a motor that has a higher inertia load that takes time to accelerate and need class 20 or 30 trip curve device to allow starting of those motors.

Nice thing about motors with internal overload like you have (and this is most common on single phase motors, and sometimes a few smaller three phase motors) is they are basically tamper resistant compared to an overload that someone untrained to know better can change settings or elements, which results in higher motor current and eventually sooner motor failure, or even protects from the guy that wants to do right but don't know how to properly set it or select correct elements.
 
Even if the motor has internal protection I would us a manual motor starter. It will have overload heaters to protect the motor.

If the internal motor overloads are manual reset then that's ok. If they are automatic reset use the starter. If you have problems with the motor tripping most of the time with auto reset it will burn up from cycling before anyone notices.
 
So start here; what do YOU mean by an "I/T breaker"? That is not a standard way of denoting a breaker type. There are, as defined in the NEC, article 430.52:
  • Instantaneous Trip, often abbreviated I-T or IT, also known as MCP (Motor Circuit Protector)
  • Inverse time, basically a "standard" thermal magnetic circuit breaker with magnetic AND thermal trips, also known by I2t circuit breaker, bcause the "thermal" element trip is based on the current squared over time, Thermal-Magnetic (T-M) circuit breaker or even just MCCB (Molded Case Circuit Breaker)
So if by "I/T" breaker, you mean "Instantaneous Trip", that breaker CANNOT be used to protect the motor from Overload, it has NO overload trip elements. An IT breaker only has Short Circuit (magnetic) trip elements. Legally, it can ONLY be used as PART of a FACTORY ASSEMBLED AND LISTED combination motor starter where there is also an Over Load Relay. Any other use was and is a code violation.

If by "I/T" you meant "Inverse Time", then it could be fine for protecting the motor, but ONLY if the thermal element falls within the narrow range of acceptability for thermal protection of the motor per 430.32(A)(1), which in this case would mean a trip setting of 26.875A. But because you are trying to "kill 2 birds with one stone" by using ONLY the T-M breaker to satisfy both 430.32(A)(1) and 453.52, you do NOT get to "round up" to the nearest size, because that does NOT meet the requirements of 430.32(A)(1), which says MAXIMUM.

So either way, that breaker, by itself, is incorrect and always was. You just need to determine which flavor of incorrectness it is so you can fix it correctly.
 
I will sum this up for you.
The breaker protects the wire, the overload protects the motor.
The sq d motor data calculator is very usefull for questions like yours, it recommends a 30 amp tm breaker, but you can go up to 250%. I would typically use 25 amp time delay fuses.
 
tom baker said:
The breaker protects the wire, the overload protects the motor.
Click to expand...

Can I disagree, respectfully? The breaker protects the wire and motor from SC and GF, and the overload protects the wire and motor from overload. They both protect both, but in different ways and for different purposes.
 
bwat said:
Can I disagree, respectfully? The breaker protects the wire and motor from SC and GF, and the overload protects the wire and motor from overload. They both protect both, but in different ways and for different purposes.
Click to expand...

If the motor short circuits or ground faults, it’s too late to protect it.
 
ptonsparky said:
My SQD slide calculator says a 60A TM breaker or 40 amp fuses for SCGF protection of a 5HP single phase 240v motor.
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How about that, so does mine :) What code does yours say it is based on? One in my desk here says 1996 code, hope none those NEC tables have changed since then or I may get something wrong sometime. I think one in the truck is newer but bet it is at least 2005 or maybe 2008.

retirede said:
If the motor short circuits or ground faults, it’s too late to protect it.
Click to expand...
If a winding faults, yes, if a capacitor, starting switch, or other easily replaceable component fails it possibly did limit damage.

ptonsparky said:
They limit the damage. Hopefully.
Click to expand...
 
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