Code Forum

Nov 1, 1999 12:00 PM, By Fred Hartwell, Hartwell Electrical Services, Inc.

Yes, it’s true. Sec. 240-3(d) (the former Table 310-16 obelisk note) doesn’t apply to pure motor circuit conductors sized in accordance with Sec. 430-22(a).

The drawing you see to the right (available in print version) appears in the December 1998 issue of EC&M, on page 40, as well as in my NEC Illustrated Changes books. It shows a 2-hp motor fed from a 30A circuit breaker and, as covered in the associated text, No. 14 THWN wires. The 1998 article states the wire was sized per Sec. 430-22(a).

The submitter believes the No. 14 wire is incorrect, arguing Sec. 430-22(a) says the conductor ampacity needs to be a minimum of 125% of the motor rating, as determined by Sec. 430-6(a)(1). This section requires the conductors to be selected in accordance with Sec. 310-15(b) or 310-15(c).

Sec. 310-15(b) requires you to use ampacity tables for conductors rated 0 to 2000V (which include the 600V-rated conductors used). Tables 310-16 and 310-17 have an asterisk notation that refers to Sec. 240-3. This section requires conductors to be protected by an overcurrent device, which limits the current to the ampacity of the conductor. Subparagraph (d) to Sec. 240-3 requires the overcurrent device for small conductors to be limited to 15A for No. 14, No. 20A for No. 12, and 30A for No. 10.

The submitter concludes that following this, the minimum conductor size would be No. 10, not No. 14—based on the 30A circuit breaker. He adds a No. 10 conductor meets requirements of Sec. 430-22(a) because it exceeds the minimum of 125% of motor full load current.

The response. The submitter is entirely incorrect. The former obelisk note to the ampacity tables, now Sec. 240-3(d), has never applied if other sections of the Code make alternate provisions. In its new location (Sec. 240-3), this concept is clear in the parent language, which refers to Sec. 240-3(a) through (g). Included therein is Sec. 240-3(g), which in part includes motors. One of the reasons the Usability Committee recommended moving the obelisk note into Art. 240 was to make this point more obvious.

The overcurrent protection for this circuit is divided. The circuit breaker in this instance provides short-circuit protection only. The running overload protection for the motor provides the overload protection. The reason Sec. 430-22(a) uses the 125% parameter is to correlate with Sec. 430-32(a)(1), which also uses 125%. With the motor and its conductors thus fully protected against overload, the only remaining issue is short-circuit and ground-fault protection, and the ratios in Table 430-152 do just that.

This question comes up often. I’d hoped the relocation into Sec. 240-3 would solve the problem, but perhaps not. Be assured, however, there has never been any intention to apply the former obelisk notes to motor circuits, nor is there in the ’99 NEC, now that the rules appear in Sec. 240-3(d).