motor sizing conductors?

[Jamesco]

View attachment 19506
I am trying to size the conductors supplying this motor. it is for an air compressor which would turn on automatically as pressure gets to a certain psi. it is for a small shop so the compressor would only kick on maybe once or twice a day. I would consider this motor a non-continuous duty motor and would refer to table 430.22(e). (i'm using a 2008 code book right now because phila is still using 08' code) am I correct in referring to this table? and if so i'm confused on what classification of service I would use...(varying duty?). and the second part of this question is, this motor is rated 5hp @230v. 430.6 says to size conductors according to table 430.248. my supply voltage is 208v (which will lead to another question, 230v rated motor on 208v supply) if trying to find the FLC of the motor would I use the 208v column of the chart if the motor is rated at 230v? hope the picture I posted is readable...if not the nameplate states: 5hp single phase RPM 1740, SF 1.15 , 230v 25amps

Hi, new here.

First off if the motor data plate says only 230 volt I am not sure I would run the motor on 208V. The SF being 1.5 may be blessing though if you try running it off low voltage 208V, jmho.

I agree with others, check with the manufacture first. Just a guess the motor is pretty old if it is only rated for 230V with a SF 1.5. I would also recommend you get his/her answer in writing by an email if possible.

Safest way to fed the motor would be with a buck/boost transformer.

As for your question.

I would use the 208V voltage on table 430.248 for the 5HP motor for sizing the wire. Remember NEC is bare minimum.

Table 430.248
5HP 208V, 30.8 amps X 125% = 38.5 amps. #8 cu wire. (Adjust for VD if needed).

Fuse size? I would probably use the FLA on the motor data plate.

 

[don_resqcapt19]

ironically, the person he purchased the motor from bought this compressor in 1985! and it was run at supply voltage of 208v which leads me to wonder how the motor ran this long...

Older motors seemed to be better built and had less issues with low voltage than new motors.

 

[Cow]

I can't read the nameplate from your picture.

If this is for a customer, if it has no mention of "usable at 208v" on the nameplate and is only shown at 230v, I would install a buck boost and size the conductors appropriately from the NEC tables.

Or replace it with a 208v rated motor.

If this was for a friend or family member, I would hook it up as is and let them know when it burned up, to buy a proper 208v rated motor.