Next size up: conductor size, too?

Status
Not open for further replies.
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?
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
You can upsize the breaker from 800 amps and lower but there are some restrictions. One is that the ampacity of the conductor must be sized to cover the calculated load. Hence if you have a calculate load of 68 amps and a wire rated for 65 amps then that would not be compliant. If the load were 65 amps then you could use the next size breaker and go to 70 amps
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician

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.

In this installation if the ampacity of the conductor is 55 amps and you could not use the 55 amp conductor where the minimum circuit ampacity is 60 amps.
 
Thank you, that matches my interpretation. Do any of you change your mind when you consider an EV (continuous) charging load at 80A on an NM-B (2/3) cable? I read 625.40/625.41 requiring a 100A circuit rating and therefore 2/3 NM-B (95A) would not be sufficient, but do you give any leeway to the it when you consider that the load would never be more than 80A (although it's continuous)?

Personally, my interpretation is that this would not be legal - but apparently there are a handful of electricians who would spec 2/3 NM-B cable, based on some installations that are being reported.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
Thank you, that matches my interpretation. Do any of you change your mind when you consider an EV (continuous) charging load at 80A on an NM-B (2/3) cable? I read 625.40/625.41 requiring a 100A circuit rating and therefore 2/3 NM-B (95A) would not be sufficient, but do you give any leeway to the it when you consider that the load would never be more than 80A (although it's continuous)?

Personally, my interpretation is that this would not be legal - but apparently there are a handful of electricians who would spec 2/3 NM-B cable, based on some installations that are being reported.

I have to agree with you. It have an ampacity of the calculated load which in this case is 100 amps. No good IMO-- I assume you are talking copper
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
The round up rule does not change the ampacity of the conductor...it only permits the use of an oversized OCPD.
 

jglavin427

Member
Location
Denver, CO
Thank you, that matches my interpretation. Do any of you change your mind when you consider an EV (continuous) charging load at 80A on an NM-B (2/3) cable? I read 625.40/625.41 requiring a 100A circuit rating and therefore 2/3 NM-B (95A) would not be sufficient, but do you give any leeway to the it when you consider that the load would never be more than 80A (although it's continuous)?

Personally, my interpretation is that this would not be legal - but apparently there are a handful of electricians who would spec 2/3 NM-B cable, based on some installations that are being reported.
I tend to think that your interpretation is correct. However those who may disagree might point out that 625.40 only specifies the overcurrent protection and 625.41 seems to only be talking about the rating of the equipment, and not necessarily the conductors. If you make the assumption that article 625 does not specify conductor ampacity, you can then argue that the installation of a 2/3 NM-B with a 100A breaker serving an 80A continuous load is a compliant installation.
 
I tend to think that your interpretation is correct. However those who may disagree might point out that 625.40 only specifies the overcurrent protection and 625.41 seems to only be talking about the rating of the equipment, and not necessarily the conductors. If you make the assumption that article 625 does not specify conductor ampacity, you can then argue that the installation of a 2/3 NM-B with a 100A breaker serving an 80A continuous load is a compliant installation.

Wouldn't you still fall afoul of 210.19(A)(1)(a), which says the conductors shall have an allowable ampacity not less than the noncontinuous load plus 125 percent of the continuous load?

(Note in the original post I twice referred to 310.19, that's supposed to be 210.19. Fat fingers or bad memory, either way...)
 
Status
Not open for further replies.
Top