circuit breaker
I purchased a Comfort Zone Heater. Model CZ230ER. It is a 7,500 watt 240V unit.
This was installed with 10/2 wire to a brand new 30 amp breaker. Once the heater has been running for about a minute it trips the breaker. I have verified there are no shorts.
I do have it about 60 feet from the panel and my wire conduit was only 25 feet long. The connections are made using wire nuts mounted in a metal box.
Any thoughts?
Since it's a Ground Fault Circuit Interrupter (GFCI) breaker, it's very likely that you have a ground fault.
GFCIs work by detecting an imbalance in current between the ungrounded (hot), and grounded (neutral) conductors. This is why you only see the trip when the circuit is under load. If there is 0 current flow, the current is balanced.
I'd start by inspecting the receptacle. Turn off the breaker, then open up the box where the receptacle is installed. Look for damage to the receptacle itself, or water/moisture in the box.
For outdoor installations you should always have a weatherproof box, and a cover that is weatherproof even when the receptacle is in use.
I'm not sure where you live but US code calls for a minimum of #8 if you're using copper or #6 if you're using aluminum. There would be no advantage in using a larger wire size in this case because at 30ft you will have negligible losses.
I don't know what your voltage is or how many phases you have but for an example of what your losses may be, I will assume a worst case scenario - 100V single phase. If you used the #6 aluminum wire, you would have a maximum of 1.72% voltage drop (loss). If you used the copper #8, you would have a maximum voltage drop of 1.66%. And just remember, when you are calculating losses (voltage drop), the higher the voltage, the better because higher voltages have less voltage drop.
Note: I use the Southwire Voltage Drop Calculator for most of my voltage drop calculations. http://www.southwire.com/support/voltage-drop-calculator.htm
Best Answer
As noted by isherwood, you are pulling too much power. 7,500 W /240 V = 31.25 A on a 30 A circuit. Circuit breakers are typically designed to handle a somewhat higher than rated current for a short time - e.g., motor startup, a really high current (e.g., hundreds of Amps on a 30 A circuit) for an extremely short time, and a moderate overage for a moderate amount of time. This actually matches well with another rule: continuous loads should not exceed 80% of the rated circuit. In the case of 30 A, that means your continuous load should be no more than 24 A = 7,200 W.
So this is actually normal operation.
You need either a slightly smaller heater or a larger breaker with matching appropriately sized wiring.