Breaker sizing for 480V single phase loads

[TESEngineering]

How do I calculate the loads on each leg of a 480V three phase system that has 17, 1000W, 480V single phase lights on it.

 

[dpc]

Watts are watts, no matter how many phases you have. If you have 17 1000W lights, you will be using 17 kW (but you really need to add in ballasts losses). If you balance this as best you can, there will be about 22 amps in each phase -- (17,000 + ballast losses) / (480*1.732). In reality there will be slight imbalance since the 17 fixtures cannot be equally divided by 3.

I was going to ask if these were 480V or 277V, but it really doesn't matter, as long as they are balanced between the phases.

 

[lyledunn]

In order to choose a breaker you would also need further information. The characteristics of the load need to be assessed in order to select an appropriate mcb type. Generally, for inductive lighting loads a type c mcb to BSEn60898 would be chosen. Consideration also needs to be given to the earth loop impedance of the circuit in order to ensure shock protection (indirect contact) measures are appropriate. The prospective fault current of the breaker also needs to be matched with the pfc at the point of installation unless upstream devices are affording attenuation of let through energy.
I note your voltage is not UK standard and thus other than UK regulations apply. Anyway, I hope info is of some use. Regards,

Lyledunn

 

[dpc]

Assuming a continuous load of 22 amps, in the U.S, you will need a 30 A breaker, minimum. It is rated to carry 80% of 30 A or 24 A on a continuous basis.

Wire size is #10 with #10 Gnd.

 

[busbar]

One note: For other than solidly-grounded wye systems, remember that it is imperative to install non-slash-rated overcurrent devices.

Molded-case breakers with 480Y/277-volt ratings, or 300-volt fuses must only be served by a wye solidly-grounded transformer secondary.

 

[lyledunn]

DPC

good point. Here in the UK many forget to derate for loads of long duration, say over 1 hr. Further derating may be required if the breaker is installed in the horizontal position at the top of a stack of loaded breakers. I have noted in the past the seemingly unexplained operation of a non-loaded breaker being entirely due to the thermal stacking of loaded breakers below it. Regards,

Lyledunn