FTU'S OCPD

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sam34

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Hello every one. I'm trying to size an OCPD for a Fan Terminal unit, which has the following rating:
480/3p- 9kw, 1/2hp motor-277v. My calculation shows each leg will carry 3KVA. And one of those legs will have an extra 1.127KVA (9.8A x 115v)for the motor load. As you see, one of those phases will draw more AMP's than the other. How we will determine the right size of the OCPD. Thank you.
Sam
 
Re: FTU'S OCPD

Sam
have you looked for a name plate on this unit?I would think the mfg would give you the required wire size and breaker. Is the 9 kw heat? Also the motor amps should be at 277 volts not 120 volts.
The service to the motor may be 120 volts but the circuit will be at 277 volts.

[ December 02, 2003, 12:46 PM: Message edited by: bob ]
 
Re: FTU'S OCPD

Bob, there is no info regarding the name plates at this stage of the design. The 9kw it's for heat. You are correct regarding the motor that will run on 277v, but I used the 5.8A to determin the VA for this Motor. Also, there is another question I forgot to ask; can I feed to units from one breaker since I don't have a fusible disc. sw. as long the ampacity of the feeder will allow it?
 
Re: FTU'S OCPD

Sam
I suspect that the mfg will specify 1 breaker and
1 ckt to feed both the heat and the motor.
An estimate would be as follows:
9 kw at 480 v 3p = 10.8 amps
1/2 hp at 277v = 2 amps approx
total 12.8 a x 1.25 = 16 amps
conductor #12 thhn and a 20a breaker.
 
Re: FTU'S OCPD

Bob, I think thats what I'm going to use because I did come up with the same load that you have. Thanks again.
 
Re: FTU'S OCPD

I don't think Amps at 480V cannot be added with amps at 277V, can't add apples and oranges! Instead add the KVA together 9 + .554(5A*.277V)=9.554KVA for a total of 11.5A at 1.25% that would be 15A fuses. Though the fan will only be connected to one phase the calculations for each phase with out the motor still equates to 15A fuses(10.8*1.25= 13.5A).
 
Re: FTU'S OCPD

Have you had an opportunity to see and inspect this unit? I was in the electric motor business (motor shop) for 17 years and have never heard of a motor rated at 277 volts. Its sure possible I guess but be something quite out of the ordinary. Methinks the motor is actually a fan unit of 120 volts.
 
Re: FTU'S OCPD

I lost a airhandler motor once where the fan physically locked up and the circuit was protected at 40 amps because of the heat elements.
I always thought since that the motor should have had a seperate fuse in the unit.
The motor was 'inherently protected', so I suppose a fire would not have resulted, but it sure sounded terrible.
Is it possible, in your case you can rate 1 phase higher (heaters?) or add a fuse?
 
Re: FTU'S OCPD

Fan Powered Terminals with Electric duct heaters are now manufactured with a 277V fan with a 3-phase, 480V heater. These boxes have a single point connection box so the contractor installs a four-wire, three phase branch circuit to it.
 
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