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Monday, May 8, 2017
Question:
Re: CQD answer published Monday, May 1, 2017 -Small Conductors and terminations
Folks, The retention of the 60° C rating is entirely due to the design of overcurrent protective devices and other electrical equipment relying on the mass of conductor for heat dissipation. If you place too small a conductor on a breaker or fuse due to using the wrong temperature rating of the insulation the device will be operating at a higher temperature than designed and you will have nuisance tripping. Unfortunately this is never explained either by the code or instructors. A circuit breaker or fuse is typically sized based on the temperature rating of the insulation, not the conductor size.
Per NFPA 70 110.14(C)(1)(a) & (b), all electrical equipment rated 100 amperes or less or marked for conductors #14 AWG through #1 AWG are designed for 60° C, unless otherwise marked by the manufacturer, while designs for over 100 amperes or marked for conductors larger than # 1 AWG are intended for 75° C, unless otherwise marked by the manufacturer. Currently there is no default for the 90° C conductor insulation range.
Cordially, Bill Nolte
A
Answer:
Hey Bill thanks for your comment. If the only reason for the small conductor rule is terminal temperature limitations it seems the small conductor rule should match the values in 110.14(C)(1)(a) -100 amperes or 1 AWG, not stop at 10 AWG. Notice also that the small conductor rule does not apply for conductors covered by 240.4(E) and 240.4(G) as stated in 240.4(D). You are correct that if equipment has a 60 degree C limitation then the conductor ampacity cannot exceed the value in the 60 Degree C column of Table 310.15()B)(16) as stated in the first sentence of 110.14(C). This is explained by some instructors. But even if the equipment has a 75 degree C temperature rating the small conductor overcurrent protection values in 240.4(D) cannot be exceeded unless allowed by 240.4(E) and 240.4(G).