ColdSparks
Member
- Location
- Michigan
- Occupation
- Compliance Engineer
I have something of a theoretical question that came up as a tangent from an actual problem but I cant find a satisfactory answer. The idea came up that if you had all small VFDs (1-3hp 480v) in a group and you had 75C terminations could you load a 12ga THHN wire to the full 25a allowed per 310.16?
When I first looked into it I thought the answer was simple, 12ga is good for 20a maximum, that is just one of those things you know as an electrician. When I went looking deeper it was not that simple. I started at the beginning, Table 310.16 references 240.4 for conductors under 10ga but doesn’t set a maximum ampacity itself. Looking at 240.4, it only restricts the OCPD to 20a for 12ga wire in 240.4(D)(5) unless modified by the articles in Table 240.4(G) , it does not set a maximum loading for the wire itself.
Table 420.4(G) lists Article 430 parts 2-7 as modifying 240.4. 430.122(A) states that the VFDs must be calculated at 120% load but does not give a maximum ampacity nor does anything else that I can find in Article 430 impose a restriction on 12ga wire. 430.131 covers overcurrent protection of VFDs and points to 430.53 which allows the OCPD to be calculated as per 430.52 as an exception to 240.4. This allows for a OCPD to be sized larger than 240.4 allows.
I can find lots of discussions on increasing the OCPD above 20a per 430.52 but almost nothing on what this does to the rated ampacity of the wire itself. As I understand it, it is permitted to load a 12ga wire to the full 25a allowed in the 75deg column of Table 310.16. This is supported by 430.53 and 430.52 allowing higher breaker sizes than 240.4. This is something of a circular argument, increasing the motor load past 20a allows the breaker to be increased past 20a, so it seems wrong but I can’t find an obvious hole in it.
When I first looked into it I thought the answer was simple, 12ga is good for 20a maximum, that is just one of those things you know as an electrician. When I went looking deeper it was not that simple. I started at the beginning, Table 310.16 references 240.4 for conductors under 10ga but doesn’t set a maximum ampacity itself. Looking at 240.4, it only restricts the OCPD to 20a for 12ga wire in 240.4(D)(5) unless modified by the articles in Table 240.4(G) , it does not set a maximum loading for the wire itself.
Table 420.4(G) lists Article 430 parts 2-7 as modifying 240.4. 430.122(A) states that the VFDs must be calculated at 120% load but does not give a maximum ampacity nor does anything else that I can find in Article 430 impose a restriction on 12ga wire. 430.131 covers overcurrent protection of VFDs and points to 430.53 which allows the OCPD to be calculated as per 430.52 as an exception to 240.4. This allows for a OCPD to be sized larger than 240.4 allows.
I can find lots of discussions on increasing the OCPD above 20a per 430.52 but almost nothing on what this does to the rated ampacity of the wire itself. As I understand it, it is permitted to load a 12ga wire to the full 25a allowed in the 75deg column of Table 310.16. This is supported by 430.53 and 430.52 allowing higher breaker sizes than 240.4. This is something of a circular argument, increasing the motor load past 20a allows the breaker to be increased past 20a, so it seems wrong but I can’t find an obvious hole in it.
