Motor Sizing

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abrace

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New Hampshire
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Telecommunications Engineering
All,

Been reading this forum for years, lots of info and expertise here.

Motor OCPD, overload, and circuit sizing is definitely a confusing topic. I am going to be installing a new air compressor shortly, and I want to make sure I have everything correct and want your expertise to make sure I understand everything.

First of all, it is a continuous duty rated 3HP single phase 230V motor with a nameplate FLA of 15A. I have also acquired a 5HP magnetic starter/controller (I realize it is bigger than I need, but it was a good price).

1) The FLC is to be computed via 430.248, which is 17A
2) Since it is continuous duty, the conductors need to be sized at 125% of FLC, which is 21.25A
3) If I were to run NMB cable, I would need to run #10. However, if I pipe it and use THHN, or I use MC, I could get away with #12, since the small conductor rule normally limiting #12 to 20A does not apply to motors, and I can use the 75 degree column for 25A? Do I have this correct?
4) I have a 30A disconnect switch rated for 3.0HP, but since I am only using it as a disconnect, its horsepower rating really doesn't matter anyways and it is only the 30A rating that counts. The disconnect needs to be sized 1.15 times the continuous FLC of 21.25A above, which works out to 24A, so I am good there? Correct?
5) I sized the overload heater based on the FLA of the motor.
6) The OCPD can be up to 250% of FLC...or up to 42A, correct? So, a 45A breaker due to next size up rule?
7) The control wires, which are tap off of the supply side wires coming into the magnetic starter/controller and head to the pressure switch several feet away need to be protected by a supplemental OCPD or must be at least 14AWG per 430.72(b) column C.

Anything else I am missing or not thinking of to make for a nice safe install?

I realize I am making a mountain out of a mole hill here, but I just want to make sure I understand it all.

Thanks to everyone for any feedback.
 
It all looks good to me except the control wire portion. The way I read 430.72, you would need to protect the control wire at its ampacity since it leaves the controller.
 
If you may not know....there are different breakers for different applications. An MCP (Motor Circuit Protector) breaker is an instantaneous only breaker, allowing the overload to protect for overload and the breaker to protect from short circuits. That's all we try to use for pump motors at our water utility. Helps cut down on tripped breakers, which the water techs are not allowed to reset. Specifically designed for motor circuits with motor starters. http://www.eaton.com/Eaton/index.htm
 
meternerd said:
If you may not know....there are different breakers for different applications. An MCP (Motor Circuit Protector) breaker is an instantaneous only breaker, allowing the overload to protect for overload and the breaker to protect from short circuits.
Click to expand...

It is my understanding that those types of breakers may only be used in listed motor starter / breaker combination units.
 
Here is the NEC rule on it.


430.52(C)(3) Instantaneous Trip Circuit Breaker.
An instantaneous
trip circuit breaker shall be used only if adjustable and if part
of a listed combination motor controller having coordinated
motor overload and short-circuit and ground-fault protection
in each conductor, and the setting is adjusted to no more than
the value specified in Table 430.52.

Informational Note: For the purpose of this article, instantaneous
trip circuit breakers may include a damping means
to accommodate a transient motor inrush current without
nuisance tripping of the circuit breaker.


Exception No. 1: Where the setting specified in Table 430.52
is not suffıcient for the starting current of the motor, the setting
of an instantaneous trip circuit breaker shall be permitted to
be increased but shall in no case exceed 1300 percent of the
motor full-load current for other than Design B energy effıcient
motors and no more than 1700 percent of full-load
motor current for Design B energy-effıcient motors. Trip settings
above 800 percent for other than Design B energy effıcient
motors and above 1100 percent for Design B energy effıcient
motors shall be permitted where the need has been
demonstrated by engineering evaluation. In such cases, it
shall not be necessary to first apply an instantaneous-trip circuit
breaker at 800 percent or 1100 percent.


Informational Note: For additional information on the requirements
for a motor to be classified “energy efficient,”
see NEMA Standards Publication No. MG1-1993, Revision,
Motors and Generators, Part 12.59.

Exception No. 2: Where the motor full-load current is
8 amperes or less, the setting of the instantaneous-trip
circuit breaker with a continuous current rating of
15 amperes or less in a listed combination motor controller
that provides coordinated motor branch-circuit
overload and short-circuit and ground-fault protection
shall be permitted to be increased to the value marked
on the controller.
Click to expand...
 
Wow, you guys are great, thanks for all the feedback.

augie47 said:
It all looks good to me except the control wire portion. The way I read 430.72, you would need to protect the control wire at its ampacity since it leaves the controller.
Click to expand...

The way I read 430.72(b)(2) is that the motor's branch circuit OCPD can be used to protect the control wires, obviously assuming the control wires are a tap off of said branch circuit. If the taps stay in the enclosure Column B in table 430.72(b) is used. If they leave the enclosure Column C is used. Since this particular motor would not be protected by an OCPD that exceeds 45A, the way I read it, it does not look like we would require supplementary OCPD provided the conductors for the control wires were copper 14AWG or larger?


ActionDave said:
I don't see any reason to try and get a 45A breaker, just use a 30A. It will be fine and if for some reason it trips you got a problem other than the breaker.
Click to expand...

Completely agreed. I was citing the max breaker size just to make sure I was doing the calculations correctly per the NEC and understand its intent. A 30A should be enough for this, and it could always be increased if nuisance trips result.


Also, FWIW, the breaker would be an inverse time breaker, AKA standard molded case breaker.

This post was really more for my education purposes as motor sizing has been an area I have always been soft in, hence why I am going into so much detail. I want to make sure I get it.


Thanks everyone.
 
You are correct about the control circuit. I have always protected circuits that were outside of the enclosure and misread the Article to suit myself :)
 
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