motor conductor sizing

[larrycad]

I'm sure this has been discussed many times, so forgive me for asking again:

Code requires for a motor that the MINIMUM conductor size is 125% of FLA. Then, code permits an OCPD to be sized at up to 250% of motor FLA.

Question: if the OCPD is upsized greater than 125% of motor FLA, does the code require the associated conductors to upsized accordingly? In Annex D, Example D8, they first describe conductor sizing, then OCPD rating. No where does it state that the conductor must be sized to the OCPD rating.

My example is a 60hp motor with FLA of 70A. There is a 150A circuit breaker ahead of the starter. Assuming 75C rating, can I use #3 wire, (100A rating), or do I have to use 1/0 awg, (150 A rating)?

I am concerned that the 100A wire would melt before the 150A breaker would trip. Is my concern legit? Where in the code is this explained?

Thanks all for your help.

 

[sky2]

The code permits the branch-circuit short-circuit and ground-fault protective device to sized up to 250% if using an inverse time circuit breaker or fuse. Table 430.52

Overcurrent protection can be found in Part III of the 2002 NEC and I believe it is the same for the 2005 NEC.

 

[larrycad]

The code permits the branch-circuit short-circuit and ground-fault protective device to sized up to 250% if using an inverse time circuit breaker or fuse. Table 430.52

Overcurrent protection can be found in Part III of the 2002 NEC and I believe it is the same for the 2005 NEC.

I understand that the OCPD can be "upsized", and I understand the reasons. My specific question has to do with the conductor ampacity. If I do upsize the OCPD, do I have to upsize the conductor????

 

[eric stromberg]

No, you do not upsize the conductor.


The conductor is 125% of FLA (tabular value)
DE fuses are 175% FLA
SE fuses are 300% FLA
Time inverse breakers are 250% FLA
instantaneous breakers can be 1100% FLA!!!!!! :shock:

The above values are typical, look at table 430.52 for other cases.

The motor overloads are sized at either 115% or 125%. See 430.32 for all the details. (intermittent motors do not have overloads!)

So, in essence, you can consider the conductor to be protected by the overload (in the case of 'slow burns'). The conductor is protected by the Short-circuit/ground-fault protection for catastrophic faults.

Eric Stromberg, P.E.

 

[eric stromberg]

intermittent motors do not have overloads

There. I'm quoting myself!

I need to point out that intermittent/continuous have different meanings in Article 430 than the rest of the Code. It has nothing to do with 3 hours on or time duration.

An intermittent motor is defined as intermittent when it must stop and start in performance of its task. For example, an elevator motor is intermittent because it must come to a stop to perform its duty.

Eric

 

[infinity]

The basic idea is that the CB or fuse only provides short circuit and ground fault protection for the conductors. The role of the CB or fuse is not to provide overcurrent protection. This is provided by the thermal protection built into the motor or as part of the motor starter. So if your conductor are sized at 125% of the FLA and your thermal overloads are sized at 115% the conductors are protected by the thermal overload device.