Three phase power refresher question.. 1

[PEMBA]

Sorry, I am having a lapse of memory.
I used to perform panel calcs a few years ago and I need someone to verify I am calculating the kva load per phase on 5.5 AMP three phase 208 volt motor correctly.(I can't find my old notes)
I used:

Load per phase=(5.5AMPS/3 Phases)x1.73x.208KV
=.65 KVA per Phase and I specified a 20 AMP three phase circuit breaker for the motor and used #12 conductors, #12 ground, and a 1-1/4" conduit for the wires to the motor. Can anyone who does this everyday verify I am going down the right path?

 

[birddogger]

I get .66 KVA myself, but close enough. Yes, that is the correct procedure.

However, why such a big conduit? You only need 3/4"C for up to 16 #12 conductors, assuming rigid metal conduit.

Also, just to make sure - are you getting that 5.5A number off of the nameplate or shop drawing data? It's a bit of an odd number for a 208V/3-ph motor, falls somewhere between 1 and 1-1/2 HP.

Either way, you're good using #12's up to a 3 HP motor at that voltage.

 

[PEMBA]

Thanks birddogger....
I agree with the 3/4" CDT, but the Electrical Contractor recommends room for growth in the furture, so I put the extra big conduit. It's nice to know my memory served me well in this case, us Mechanicals need a little help once in a while...

 

[busbar]

‘Sanity check’ rules of thumb are ~2.8 amperes/kVA for 208V?3ø and ~1.2 amperes/kVA for 480V?3ø.

 

[busbar]

I get 5.5A[•]208V[•][√]3 = 1.98kVA

 

[rbulsara]

busbar is correct......

5.5A, 208, 3phase load by definition means 5.5A in "Each of the three phases or lines).

Load (current ) per phase is 5.5A.

Total 3 phase kVA = sqrt (3)*V*A
where V is line to line voltage (208 in this case)
and A is ampere per phase (line) (5.5A in this case).

Load is kVA per phase will be total kVA/3

If you know kVA per phase, to get the current per phase you need to divide it by V per phase which is 208/sqrt 3=120V in your case..


or in otherwords load in kVA per phase will be
currentp per phase *voltage per phase

=5.5A*208/sqrt 3 =5.5A*120V=660VA or 0.66kVA

Total of 3 phase is 3*0.66=1.98kVA as busbar's calculations indicates.

 

[SomeYahoo]

Very clear and concise reply rbulsara!

On a related note, if you see "5.5A" specified on a 3-phase motor, or any piece of equipment for that matter, is it safe to assume that is the per line amperage? Are there cases where the specification will show "total amperage" (line amperage x 1.73)?

 

[rbulsara]

You are welcome gensetguy.

Regarding your last post: Answer to your first question is Yes. Answer to the second question is No.

Good luck.