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Can I Use # 4 NMB To Feed A 100 Amp Subpanel?

March 20, 2006

Q: What is the ampacity of # 4 AWG copper NMB cable when used as the main feeder to supply a branch circuit panelboard in a single family home? The service conductors are three # 4 AWG copper conductors with Type THWN insulation in 1 inch rigid metal conduit protected by a 100A main circuit breaker.

A: Type NMB cable has conductors that have an insulation rating of 90 C. However, 334.80 limits the ampacity to the values shown in Table 310-16 for 60 C insulation, but if the conductors must be derated, the 90 C ampacity is permitted to be used provided the final derated ampacity does not exceed the ampacity for 60 C conductors. To further confuse the issue, this sentence appears in 320/15(B)(6): “The feeder conductors to a dwelling unit shall not be required to be larger than their service-entrance conductors.” Based on these various NEC references, the electrical inspector should accept the 4 AWG copper NMB cable as a 100A feeder. Because this is an area that is not very clear, you should check with the authority having jurisdiction before making this installation.

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