Tripping breaker on gearbox motor.

Thread Starter


Joined Jun 22, 2007
New installation on a motor that runs a gear box. It is a 480 v about 2 hp motor. I suspect the inertia needed to overcome the gearbox is tripping the breaker. How can I confirm my theory on why it is tripping? How do I engineer the correct motor for the gearbox? The breaker is set at 1.4 amps to match FLC on the motor.


Joined Nov 29, 2006
Breaker is set too low?

1 Hp=746 watts 2hp =1492 watts

At 480 v with a perfect efficiency no loss motor and power factor of 1

P=ExI or 1492/480=3.1Amps
with Eff and PF P=ExIxEFFxPF or I=P/(ExEFFxPF)
since eff and pf are always less than 1, a smaller number in the denominator with give a larger result. ie more current

Is the motor 3 phase?
What is on the nameplate?
Any capacitors?
what is your location, continent wise.
FLC =the current in amps required to run a motor at it’s full capacity.

1.4 amps at 480 =672 watts= 0.9HP. Starting current with be larger.

What is the rating of your gearbox and what is it driving?
Have you disconnected your gearbox output and measured the current draw?


Joined Nov 21, 2006
The motor will draw 4-6 times its fla for a short time during startup.
You will have to find a way to time delay the tripping of your breaker or go with a different method of protection.
A google search for electric motor protection should yeild some good info.