Motor Full load current and Instantaneous current

Hi,

I am using 3 motor pmps 7.5, 1.5 & 1.5 HP and respective FLA (full load amps)as per NEC code are 11,3 and 3 Amps.

The customer would like to know the load requirement from my side so that he can provide the 480V service accordingly.

My question is; should I just add up the 3 FLAs and submit that to customer?? or I should also provide him the instantaneous current information or the circuit breakers amp. rating I am using in my Motor Center.

Thanks.

do you just have a motor, or are there other devices as well?

As there is no standard other than the NEC, start there. As a rule, take all the motor FLA, add the devices, then add 10-15% for possible upgrades.

yeah i do have a load center for my valves and control panel ...but thats a 120V side and customer is providing sepearte service on my 120 side ..that helps to get rid of transformer....

125% Of Largest Motor Then 100% Of All Other Motors Thats The Code Rule Article 430

Connected Load

If the customer is installing the Service, they are responsible for meeting code requirments. They need to know everything you are going to connect to the service in order to calculate conductor ampacity. This includes plugs, heating, lights etc. They will have to size the disconnect and fuses as per your load requirments. If you supply them with a print with motor sizes and all connected loads they will be able to figure thing out for you. Hope this helps.. have a good weekend..

Baxter,

It may not be quite that simple to arive at the SMALLEST allowable circuit size. Which section of Article 430? Article 430-110(a) says that " the disconnecting means for motor circuits rated 600 volts, nominal, or lesss, shall have an ampere rating of at least 115 percent of the full-load current rating of the motor."

In this case there are 3 motors, so 430-110(c) would apply. The combination leads are added together and multiplied by 1.15 to get 19.55 Amps. The next largest disconnect switch would be 30 Amps with 20 Amps fuses. Then you have to look at starting current. Will the 7.5 HP motor start up on 20 Amp fuses? Probably not, depending on if it is an air compressor (definitely not), or a fan (maybe).

In which case, we are allowed by Article 430-52 to go up to 225% of 11 Amps, or 25 Amps for our main branch circuit protection. Unless the motors are part of a factory assembly, or a group installation, then each one will also need individual short-circuit protection. They all still need individual overload protection.