wwhitney
Senior Member
- Location
- Berkeley, CA
- Occupation
- Retired
215.2(A)(1)(a) calls for the conductors supplying a continuous load to have an ampacity increased beyond the actual load current by 25% of the continuous load current. [And 210.19(A)(1)(a) is similar.] Why is this requirement part of the NEC?
The limitation is entirely due to the behavior of OCPD; apparently for continuous loads regular OCPD may trip at below the nameplate rating, and so to avoid this nuisance (that could turn into a safety hazard if a user repeatedly resets a tripping breaker), the OCPD should be upsized by 25% of the continuous load current.
However, that is already covered in 215.3. And 240.4 already requires conductors to be protected in accordance with their ampacities. So the requirement of 215.2(A)(1)(a) is generally a logical consequence of 215.3 and 240.4, and at best redundant. But 215.2(A)(1)(a) prevents the use of 240.4(B) [next size up rule] to comply with 215.3.
Real world example: an EVSE with a 48A output current (e.g. Tesla Wall Connector with the proper internal setting) is a continuous load, so the OCPD protecting it should be 60A. #6 NM cable has an ampacity of 55A because it is limited to the 60C rating. 240.4(B) allows protecting this with a 60A breaker [when 55A ampacity is otherwise sufficient per other sections of the NEC.]
So what is the hazard of supplying a 48A continuous load with cable with an ampacity of 55A that is judged sufficiently well protected by a 60A breaker when the load is non-continuous? I don't see any.
Thus I would preliminarily propose that 215.2(A)(1)(a) should be deleted and replaced with a fine print note to 215.3 and 240.4. [With an according change to Article 210.] I don't see any other consequences, but perhaps I am missing the logical interplay of some other rules.
Comments?
Cheers, Wayne
The limitation is entirely due to the behavior of OCPD; apparently for continuous loads regular OCPD may trip at below the nameplate rating, and so to avoid this nuisance (that could turn into a safety hazard if a user repeatedly resets a tripping breaker), the OCPD should be upsized by 25% of the continuous load current.
However, that is already covered in 215.3. And 240.4 already requires conductors to be protected in accordance with their ampacities. So the requirement of 215.2(A)(1)(a) is generally a logical consequence of 215.3 and 240.4, and at best redundant. But 215.2(A)(1)(a) prevents the use of 240.4(B) [next size up rule] to comply with 215.3.
Real world example: an EVSE with a 48A output current (e.g. Tesla Wall Connector with the proper internal setting) is a continuous load, so the OCPD protecting it should be 60A. #6 NM cable has an ampacity of 55A because it is limited to the 60C rating. 240.4(B) allows protecting this with a 60A breaker [when 55A ampacity is otherwise sufficient per other sections of the NEC.]
So what is the hazard of supplying a 48A continuous load with cable with an ampacity of 55A that is judged sufficiently well protected by a 60A breaker when the load is non-continuous? I don't see any.
Thus I would preliminarily propose that 215.2(A)(1)(a) should be deleted and replaced with a fine print note to 215.3 and 240.4. [With an according change to Article 210.] I don't see any other consequences, but perhaps I am missing the logical interplay of some other rules.
Comments?
Cheers, Wayne
