Mike Dolt Enterprises
Mike Dolt Enterprises
User: rjspark, Location: Branchburg, NJ
Asked on: Tuesday, April 5, 2011 at 7:54 pm EDT
Answered on: Tuesday, May 14, 2024 at 3:49 am EDT
Question: I am presently working at a location that has existing electric heating. They are single phase 480V 3000W heaters. They have them fed from a 3 phase 20A breaker with 3- #8 awg feeding a 40A breaker mounted in a separate disconnect enclosure. Then they are wired through a 120V contactor, which is controlled by a 120V thermostat. Then there are 4 of the single phase 3000W heaters wired to the 3 phases. Now is it me or am I missing something I am going to bring my amp probe with me tomorrow to see exactly what these are drawing. I would appreciate any enlightenment to what I may or may not be missing. Thanks
Possible Discrepancies
Breaker Overload
The 20A breaker is rated for 3 phase service, but the total load connected to it is 3000W x 4 = 12000W, which is 25A (12000W / 480V / 3ph). This could potentially overload the breaker.
Contactor Rating
The 120V contactor should be rated for at least 25A (the total load current). If it is not, it could overheat and fail.
Wiring Configuration
The use of #8 AWG wires from the 20A breaker to the 40A breaker is not recommended. #8 AWG is rated for 40A, but the breakers are not. This could result in the wires overheating.
Recommended Actions
Verify Amperage Draw
Use an amp probe to measure the actual current drawn by the heaters. This will confirm if the breaker and wiring are adequately sized.
Check Contactor Rating
Confirm that the contactor is rated for at least 25A. If not, replace with a higher-rated contactor.
Adjust Wiring
Replace the #8 AWG wires with #6 AWG wires (rated for 55A) from the 20A breaker to the disconnect enclosure.
Additional Considerations
Grounding
Ensure that the heaters are properly grounded to prevent electrical shock.
Overcurrent Protection
Make sure that there is overcurrent protection (fuses or circuit breakers) on all conductors in the circuit.*
Voltage Balance: It is important to ensure that the voltage balance is within acceptable limits on all three phases. Unbalanced voltage can cause overheating of equipment. Flag for review