Conductor Sizing Question 1

[timjturner]

Hello,

I'm having trouble figuring out what exactly is the legal way to size conductors in one of my typical motor circuits. My typical circuit includes an MCP (motor circuit protector) and overload protection inside of the MCC and a fusible disconnect switch near the motor. Logically, I would size the conductors based on the FLA of the motor (1.25x), because the overload protection would not permit the circuit to reach a higher ampacity. But is this the legal way based on the NEC? If not, how should I go about this?

Here is an example: MCP = 30A, DISC = 30A, FUSE = 25A, MOTOR = 10HP (14 FLA), OL set to trip at just above 14FLA.

Thanks!

 

[dpc]

Yes. Size the conductor at 125% of motor FLA and size other protective devices per Article 430 in NEC. There is no direct relationship between the feeder size and the breaker/fuse size for motor circuits.

This is not the same procedure used for general purpose feeders.

 

[rbulsara]

Motor branch circuit sizing is one the exception permitted in NEC where OCPD's ampacity can be greater than that of the conductors to allow for motor starting current.

The motor branch circuit breaker or fuses only provide short circuit protection. However, there is overload protection in form of the motor overload protection. protection. Do as what dpc said.

 

[waross]

Hi folks: I am not in the US and I don't have a current copy of the NEC.
Question: Have the Rules been changed? I remember the NEC requiring most motor conductors to be sized based on the horsepower with the FLA being determined from the tables rather than nameplate rating. This ensures that the wiring for a given horsepower motor is adequate for all motors of that horsepower, should a high efficiency motor be replaced with a lower efficiency motor.
Thanks
respectfully

 

[dpc]

waross

Basically, yes - you are supposed to use the FLA data in the NEC Tables. It hasn't always been that way, but it is now. It's Article 430.6.

For motors less than 1200 rpm, and where the nameplate FLA is higher than the table, you have to use the nameplate value.

 

[waross]

Thanks dpc
Respectfully

 

[peebee]

I'm a bit surprised that no-one asked, "just where in the NEC does it permit you to size motor branch circuit conductors any differently than any other conductor?!?" There's so much misinformation thrown around with regards to Code requirements, you should never accept what anyone tells you about the Code without asking for a Code reference (or finding it yourself).

Anyway, NEC permits you to size motor conductors (and a few other specific types of feeders and branch-circuit conductors) differently from most other branch circuits in 240.4(G).

 

[timjturner]

Ok, glad to hear i've been doing my job correctly. Thanks for the code reference peebee. (was asking that question in the back of my mind)..... I'm also assuming that the conductor sizes of the other types of most common circuits I run across are based on protection devices. This mainly includes heaters and panel/receptacle feeder circuits.

 

[dpc]

You should read all of Article 240 in the NEC.

In particular, 240.4.

The NEC Handbook (NFPA) is also a good investment - although you should wait for the 2008 version at this point.

 

[rbulsara]

peebee and timjturner:

Article NEC 430.

 

[peebee]

rbulsara: that's just what 240.4(G) says!

 

[rbulsara]

Peebee:

It better!!

 

[rnpe]

Timjturner:

Since you have a fused disconnect switch next to the motor, The feeder from MCC to Fused disconnect switch falls under branch circuit (not a motor circuit/feeder) and therefore, the correct wire size for your example is #10AWG. Now, if you use a non-fused disconnect switch, then the whole circuit will be considered a motor feeder/circuit and NEC allows using #14 AWG wire on it (based on 125% of FLA). Hope this helps.

 

[peebee]

rnpe:

I disagree. I don't see how adding a fuse to a circuit would result in a requirement for a larger conductor size.

I follow your logic & reasoning, I just don't agree with that interpretation. There may well be plenty of AHJ's who do agree with you, though.

 

[resqcapt19]

waross,

Hi folks: I am not in the US and I don't have a current copy of the NEC.

While not the easiest to use and you can't cut and paste or print, you can view the NEC online here.

http://www.nfpa.org/freecodes/free_access_agreement.asp?id=7005SB

The NFPA has made all of there codes and standards available in this manner.
Don

 

[ducks13]

Where does the 125% come from? I think it is only required when the motor is likely to be used in continous operation i.e. more than 3 hours.

John

 

[peebee]

ducks13 -- If you fall under the jurisdiction of NEC 430.22, you'd be thinking wrong.

 

[dpc]

There are some exceptions to the 125% rule for intermittent duty motors, but basically, every motor is considered continuous duty unless it fits the few categories listed and you can prove it to the AHJ.

The 3 hour concept is for general purpose feeder loads and has nothing to do with sizing motor feeders.