[donf]

Greetings,

My NEC final is next Saturday and currently I am reviewing motor calculations.

My question is, when to apply the 3 phase multiplier when calculating either the Branch Circuit Conductor size or the Branch Circuit OCPD?

Second, is the 3 phase multiplier a factor in determining the Motor OCPB?

[tkrussell]

3 phase multiplier is 1.73 for load calculations, but doe not apply to your question.

For branch circuit conductors:

430.22 Single Motor.
(A) General. Conductors that supply a single motor used in a continuous duty application shall have an ampacity of not less than 125 percent of the motor's full-load current rating as determined by 430.6(A)(1).

Note, does not matter single or three phase.

For OCPD you need to review:

IV. Motor Branch-Circuit Short-Circuit and Ground-Fault Protection

Be sure you do not get short circuit/ground fault protection confused with overload protection.

By your question, I get there is a bit of confusion.
'

[donf]

Motor Calcs are a probably my worst enemy!

[tkrussell]

If you think Article 430 is confusing, try Article 440, Refrigeration.


Everything is calculated by using the tables, OTHER THAN overload protection, which uses the nameplate amps, generally speaking, of course.

This may help:

Motor Calculations Part 1: Motors and Branch-Circuit Conductors

[donf]

TK,

Thanks for the link.

I believe that I am my worst enemy on motors. I just seem to be reading calculations into the mix that are not needed.

Refrigeration did not cause me the grief that motors did.

In fact, by comparison the range table is simple compared to my struggle with motor calcs.

One thing for sure, when it clicks in, I'll have it for life!