Wire and breaker size

JeremyJones · Jul 20, 2021

I'm in tech school and would appreciate if someone checked my work. Thank you
We have a 480/277 panel with a 3-pole 45amp breaker feeding a vfd. The vfd main breaker is 50amps 460 volts. I figured #10 cu thhn with a #10 cu for the ground. My question is how can #10 wire be on a 45amp breaker We? Isn't #10 undersized for that big of a breaker? Appreciate it.

Megawatt · Jul 20, 2021

JeremyJones said:
I'm in tech school and would appreciate if someone checked my work. Thank you
We have a 480/277 panel with a 3-pole 45amp breaker feeding a vfd. The vfd main breaker is 50amps 460 volts. I figured #10 cu thhn with a #10 cu for the ground. My question is how can #10 wire be on a 45amp breaker We? Isn't #10 undersized for that big of a breaker? Appreciate it.

On motors it’s common to see a breaker that appears to be more amperage than the actual wire size. You have what is called your calculated load which is your breaker and then you have a connected load which is your motor. The breaker is sized so on the start up of your motor, which is when you are going to pull the most amperage and then you have overloads which will not let the motor pull more amperage than the name plate on the motor states. We have many people add up all the amperage on all the breakers which will exceed the main breakers in the panel’s. Just remember that you have a calculated load and an actual connected load. Good luck