FlasherZ
Member
- Location
- Southern IL (near STL), USA
Hi all,
An interesting question has popped up over time for me. While this is primarily in context of EV charging loads, it has a general application elsewhere as well.
In reading various threads on other forums and hearing of various electricians and inspectors taking different positions, I thought I might bring this interesting question here for you all to noodle on, as well.
The basic question is this: while article 240 permits the "next size up" on a breaker, does this entitle you to allow a larger load to be carried on a conductor in line with that breaker? A reading of 310.19(A)(1)(a) suggests that you would not be permitted to carry a larger load than that listed in table 310.15(B)(16). However, I've heard several cases where licensed electricians would indeed do this.
Some case studies for your answers:
I think we would all agree that if an appliance had a nameplate minimum circuit ampacity of 55A, we could use a #6 NM-B cable (rated at 55A) with a 60A breaker via the next-size-up rule in art. 240. If I presented a similar appliance that had a nameplate minimum circuit ampacity of 60A, could you still use a #6 NM-B cable with that?
My interpretation and what I've been taught: because of 334.80 (requiring the 60 degC column for Romex/NM-B), the ampacity of #6 NM-B cable would be 55A (310.15(B)(16)). 310.19(A)(1)(a) says that conductors must not be sized smaller than the non-continuous load plus 125% of the continuous load; there is no exception for "next size up" on conductors. For 60A, the 55A ampacity of #6 NM-B is insufficient.
[FONT=verdana, geneva, lucida, lucida grande, arial, helvetica, sans-serif]A second case study:
Electric vehicle charging loads are to be considered "continuous loads" (625.40/625.41) for purposes of branch circuit sizing. A particular brand of electric vehicle has a piece of electric vehicle supply equipment (EVSE) that offers a charging load of 80A. It is my interpretation that 2/3 NM-B is insufficient for this application, because 625.40/625.41 requires the ampacity to be at 125% of the offered continuous load - requiring a 100A circuit. Because a 100A circuit rating is required, 310.19(A)(1)(a) would seem to rule out 2/3 NM-B because of its 95A ampacity (60 degC column per 334.80, 95A rating per 310.15(B)(16)), and regardless of whether you would use a 100A breaker (per 240.4) or not.
Some electricians have reported that they either have, or would, spec 2/3 NM-B cable for this purpose, and that it has, or would likely pass inspection; their reasoning is that 240.4 permits them to upsize the breaker to the next size and therefore it means you may offer the larger load on the conductor. Another alternative explanation is that the inspector would approve, because the offered continuous load (80A) wouldn't ever come near the actual rating of the conductor (95A), despite the 125% requirement.
[/FONT]http://www.mikeholt.com/nec-conductor-sizing-and-protection.php seems to suggest my interpretation, but I wanted to gather thoughts. The alternative interpretation would change much of what I've been taught about sizing circuits - I was always taught you size conductors for the load, then protect it with the appropriate breaker.
Thoughts?
An interesting question has popped up over time for me. While this is primarily in context of EV charging loads, it has a general application elsewhere as well.
In reading various threads on other forums and hearing of various electricians and inspectors taking different positions, I thought I might bring this interesting question here for you all to noodle on, as well.
The basic question is this: while article 240 permits the "next size up" on a breaker, does this entitle you to allow a larger load to be carried on a conductor in line with that breaker? A reading of 310.19(A)(1)(a) suggests that you would not be permitted to carry a larger load than that listed in table 310.15(B)(16). However, I've heard several cases where licensed electricians would indeed do this.
Some case studies for your answers:
I think we would all agree that if an appliance had a nameplate minimum circuit ampacity of 55A, we could use a #6 NM-B cable (rated at 55A) with a 60A breaker via the next-size-up rule in art. 240. If I presented a similar appliance that had a nameplate minimum circuit ampacity of 60A, could you still use a #6 NM-B cable with that?
My interpretation and what I've been taught: because of 334.80 (requiring the 60 degC column for Romex/NM-B), the ampacity of #6 NM-B cable would be 55A (310.15(B)(16)). 310.19(A)(1)(a) says that conductors must not be sized smaller than the non-continuous load plus 125% of the continuous load; there is no exception for "next size up" on conductors. For 60A, the 55A ampacity of #6 NM-B is insufficient.
[FONT=verdana, geneva, lucida, lucida grande, arial, helvetica, sans-serif]A second case study:
Electric vehicle charging loads are to be considered "continuous loads" (625.40/625.41) for purposes of branch circuit sizing. A particular brand of electric vehicle has a piece of electric vehicle supply equipment (EVSE) that offers a charging load of 80A. It is my interpretation that 2/3 NM-B is insufficient for this application, because 625.40/625.41 requires the ampacity to be at 125% of the offered continuous load - requiring a 100A circuit. Because a 100A circuit rating is required, 310.19(A)(1)(a) would seem to rule out 2/3 NM-B because of its 95A ampacity (60 degC column per 334.80, 95A rating per 310.15(B)(16)), and regardless of whether you would use a 100A breaker (per 240.4) or not.
Some electricians have reported that they either have, or would, spec 2/3 NM-B cable for this purpose, and that it has, or would likely pass inspection; their reasoning is that 240.4 permits them to upsize the breaker to the next size and therefore it means you may offer the larger load on the conductor. Another alternative explanation is that the inspector would approve, because the offered continuous load (80A) wouldn't ever come near the actual rating of the conductor (95A), despite the 125% requirement.
[/FONT]http://www.mikeholt.com/nec-conductor-sizing-and-protection.php seems to suggest my interpretation, but I wanted to gather thoughts. The alternative interpretation would change much of what I've been taught about sizing circuits - I was always taught you size conductors for the load, then protect it with the appropriate breaker.
Thoughts?