motor sizing conductors?

Status
Not open for further replies.
J

journeyman0217

Senior Member
Location
Tennessee
Occupation
Electrician
norelli compressor.jpg
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
 
If the motor is only rated at 240v then it is a bad idea to run it at 208v. That's almost a 15% drop in voltage. The motor may run but it will not run at the efficiency that it should.
 
Dennis Alwon said:
If the motor is only rated at 240v then it is a bad idea to run it at 208v. That's almost a 15% drop in voltage. The motor may run but it will not run at the efficiency that it should.
Click to expand...

I definitely agree, but the customer already had purchased this compressor. I told him it was not recommended to use the motor... I did not find anything in the code stating you cannot use a motor with a lower voltage rating than the supply voltage, it only states that you cannot use a motor with a higher voltage rating than the supply voltage.
 
I can't read the nameplate in the picture. IF it is a 230V motor and IF your voltage runs a little high, around 215V, then you are right on the low end of the nominal voltage for a 230V motor and it will run ok.

If it were me, I'd run it and get a new motor on order. A 5hp motor that is usable at 208V is prolly gonna be around $600 and not hard to change.

Edit to add.. that piece of junk Dayton motor is gonna need replaced soon enough anyway.
 
ActionDave said:
I can't read the nameplate in the picture. IF it is a 230V motor and IF your voltage runs a little high, around 215V, then you are right on the low end of the nominal voltage for a 230V motor and it will run ok.

If it were me, I'd run it and get a new motor on order. A 5hp motor that is usable at 208V is prolly gonna be around $600 and not hard to change.

Edit to add.. that piece of junk Dayton motor is gonna need replaced soon enough anyway.
Click to expand...

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...
 
journeyman0217 said:
I definitely agree, but the customer already had purchased this compressor. I told him it was not recommended to use the motor... I did not find anything in the code stating you cannot use a motor with a lower voltage rating than the supply voltage, it only states that you cannot use a motor with a higher voltage rating than the supply voltage.
Click to expand...
110.3(B)
 
I would consider a transformer to get the propper voltage then proceed the calculations from there Mike holt has some discussion on this in his transformer calculations cd where the power company changed the supply. Please remember working and being done correctly are not the same.
 
Brian Baker said:
I would consider a transformer to get the propper voltage then proceed the calculations from there Mike holt has some discussion on this in his transformer calculations cd where the power company changed the supply. Please remember working and being done correctly are not the same.
Click to expand...

For a motor like we have in the OP there is no reason to add a buck/boost and if there were a need replacing the motor would be a more cost effective option in my opinion.
 
180131-2141 EST

There was no mention of distance from panel to motor. Very important relative to voltage drop on starting current. Even more important when the motor is run on a lower nominal supply voltage.

.
 
ActionDave said:
Because the motor rating is for nominal voltage and can vary plus or minus about ten percent and still be within design specs.
Click to expand...
And it may or may not have run a lot, and also depends on how close to full load rating it actually operates at also.

Brian Baker said:
I would consider a transformer to get the propper voltage then proceed the calculations from there Mike holt has some discussion on this in his transformer calculations cd where the power company changed the supply. Please remember working and being done correctly are not the same.
Click to expand...
I'd connect that motor to most any 208 volt system around here, especially if not expecting to see extreme duty cycles. 208 systems here often run ~215 volts when little or no load, still see them hang around 212 quite frequently when there is significant load. If you have one that operates at the low end of acceptable range then you might want to be more concerned with motor voltage.

I'd also probably run that motor until it burns up before I spend half to same price as new motor on a transformer. Might get a buck boost for as little as a third of the motor - still have to install it somehow/somewhere, full capacity isolation transformer - might be about same cost as motor.
 
Run it. At low duty cycles, reduction on motor life will not be noticeable. If you’re worried, check the full load running amps as installed and compare to nameplate.
 
180201-0932 EST

With a constant power load on the shaft of an AC synchronous motor the motor input power will remain almost constant independent of input voltage if the frequency is constant. That means input current is approximately inversely proportional to voltage over some reasonable workable range.

Almost the same for an induction motor except that as voltage drops slip will increase and output speed will diminish, but over a reasonable range if the induction motor has a low resistance rotor, then it is quite similar to the synchronous motor.

.
 
I would use the 208v column, not the 230v column. The current is higher for a given motor HP.
 
Status
Not open for further replies.
Top