ECN Forum Forums General Topics of the Trade General Discussion Area disconect size

hey guys i have an air compressor that needs to be hooked up. Its a 7hp 240 volt single phase motor.The nameplate has an amp reading of 23amps .What is the proper way to size for the disconect? Can you please help this rookie.

Take a look at 430.6(A)(1), and see that it refers you to the tables. Table 430.148 will be the table that governs. It looks like it will be slightly less than 40 amps, therefore a sixty amp disconnect will be required, fused properly to protect the conductors.

[This message has been edited by Ryan_J (edited 03-01-2004).]

why is it the the wire going to the motor are so small? They look to be about 10 awg?

The motor probably has internal overlaod protection. If you look at the running load amps of the motor, you will see that the #10 will carry the load.

The insulation on motor conductors is different from the building conductors, and can handle more current without damaging the insulation.
Table 430.148 shows 40 amps for a 7-1/2 HP motor. Interploating for a 7 HP motor, I get 37.6 amps. Table 310.16 gives me (at 125% per 430.22 that's 47 amps)#8 copper wires. 60 amp disconnects are standard size.
The only place the 23 amps is used is for the heaters. Use overloads in your magnetic or manual starter from the FLC found on the nameplate. See 430.43(C) for allowed oversizing.

Earl

after running the compressor i get about 12 amp per leg....i think i need to go back an install a bigger disco. my wire size is good and so is my breaker but im still having trouble reading the code book . Why cant they make it easiear to understand?

Ryan_J,

Isn't there an exception inf 430.6 for appliances that include motors, where you get to use the nameplate amp rating _of the appliance_ rather than the table value?

drgnz23,

The wire _inside of an appliance_ is rated under different rules than the wire used for running circuits in walls. This is because wire is not limited by _current_, but is instead limited by temperature. The temperature is caused by _heat_, which is produced when current flows, and _heat disspiation_, which depends upon the surroundings. When you have a better idea of the heat dissipation available, you can use thinner wire, and the UL listing (not the NEC) determines what wire is appropriate in an appliance.

If you calculate out the values in table 430.148, the VA of the circuits required is roughly _double_ the wattage of the motors described (1 hp is 746 watts). This doubling is required because of inefficiency, power factor, and overload requirements. With an appliance, all of these factors are better controlled than with a general purpose motor.

If possible, you should consult the manufacturer of the appliance. They may have a recommended or required conductor ampacity, disco size, and OCPD requirement. This should be reflected on the nameplate, but the instruction manual is probably a clearer source.

Lastly, the current reading that you get when running the compressor will depend upon the load on the motor; in particular the pressure in the tank. The higher the pressure, the more work the motor will do, and the greater the current. I can tell you right now that 12A at 240V is _not_ 7hp, but less than half that depending upon power factor and efficiency.

-Jon

winnie . You beat me to it after a long stare at da book i found the exception you are mentioning. Ive been trying to learn and do from the nec code book as much as possible you guys have been great help.

drgnz23,

Before you trust the 'exception' interpretation, please let the more experienced chaps chime in on it. I am here learning code as well

-Jon

For sizing disconnects I usaully rely on the HP rating. Instead of ordering a 60 amp disconnect, ask them for a 7-1/2 HP at 240 volt disconnect. The manufaturer does all the sizing for you.