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Monday, March 16, 2015
Question:
Thank you for hosting this forum. It adds to our sense of community and it gives us another opportunity to know what situations others in the field are dealing with.
Questions:
A customer was remodeling their kitchen and wanted to move the electric range to the other side of the kitchen. The existing 240V 3-wire circuit was run in 8-2 SE cable. The frame of the old range was grounded by connection to the neutral. We ran a new 4-wire circuit (not in SE cable) from the panel to a new receptacle for a new range, but I wonder if the code would allow an existing 3-wire circuit to be extended.
Secondly, the new range is rated at 11.3kW total load. If my calculations are right, this would be work out to 47A @ full load. The manufacturer's instructions state that the circuit for this range should be protected with a 40A or 50 A circuit breaker. Just for grins and giggles, the rangetop elements and the oven were all set to "high" and we checked the amp draw. It was drawing just over 45 amps. For this reason, we advised the homeowners not to use all of the rangetop elements and the oven at the same time. Even though a 40A or 50A OCPD doesn't seem right for the full load amp draw, would this be a case for following the manufacturer's instructions per NEC 110.3(B)?
Steve M.
A
Answer:
Hey Steve thanks for your questions and you're welcome. A grounded neutral conductor is allowed to be used as an effective ground fault current path, in place of an equipment grounding conductor under certain conditions, as stated in 250.140 Exception. This is only permitted for existing branch-circuit installations and I think that if you extend a branch circuit that portion did not previously exist.
The demand load for a single range is 8kW from Column C in Table 220.55. For household ranges 8 3/4 kW or more the minimum branch circuit rating is 40 amperes as stated in 210.19(A)(3). I agree with your math and the current measurement approximately confirmed the value to be 47 amperes. But that would probably only occur if the range was used as a space heater when the furnace failed. The oven elements typically cycle on and off to maintain the temperature setting and the cooktop elements would not all likely be on the highest setting, even during holidays or family gatherings, unless we wanted everything burnt.