Mike Dolt Enterprises
Mike Dolt Enterprises
User: rjspark, Location: Branchburg, NJ
Asked on: Tuesday, April 5, 2011 at 7:52 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
The wiring configuration you describe does indeed have some potential issues
Oversized Breaker:
The 40A breaker in the disconnect enclosure is oversized for the #8 AWG conductors feeding it. According to the National Electrical Code (NEC), #8 AWG copper conductors are rated for a maximum of 40A. However, the 40A breaker could allow higher currents to flow, which could overheat the conductors.
Phase Imbalance
The single-phase heaters are connected to different phases, which can create a phase imbalance in the system. This can lead to uneven distribution of current and potential overheating of the heaters.
Overloading of Contactors
The 120V contactors are rated for a maximum of 40A, which is the same as the breaker in the disconnect enclosure. However, each contactor is controlling four 3000W heaters, resulting in a total load of 12,000W. This exceeds the rating of the contactors and could lead to premature failure.
Recommended Modifications
To address these issues, you should consider the following modifications: Install a smaller breaker (30A) in the disconnect enclosure to match the rating of the #8 AWG conductors. Use a two-pole contactor rated for at least 60A to handle the load of four 3000W heaters. Connect the single-phase heaters to the same phase to avoid phase imbalance.
Additional Considerations: Use amp probes to verify the actual current draw of the heaters and ensure that it is within the ratings of the components. Check the thermostat and its compatibility with the contactors and heating system. Ensure that the electrical connections are tight and secure to prevent overheating. Consult with a qualified electrician for any wiring or component modifications. Flag for review