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PV Cable Sizing, Part Four: How to Reap Savings with the NEC Round-Up Rule



The NEC “round-up” rule of 240.4(B) can yield significant cost savings during the solar energy design and installation process. It enables you to round up to the next standard size overcurrent device higher than the ampacity rating of the conductors that are to be protected. However, there remains a lack of information on how to apply this round up rule to each of the 28 standard circuit breaker sizes up to the 800 Amp limit for which this rule applies. Below we explain the round up rule in greater detail and provide guidance on how this rule can be applied to standard circuit breaker sizes.


NEC Section 240.4(B) “Round Up” Rule


When designing and installing solar energy systems, you may encounter the scenario in which the ampacity of a conductor does not align with a standard overcurrent device rating from NEC Table 240.6(A). If this happens when you are selecting an overcurrent device rated 800 amps or less to protect conductors, NEC Section 240.4(B) applies. This code section states that it is permitted, unless prohibited elsewhere in the NEC, to select the next standard size overcurrent device higher than the ampacity rating of the conductors being protected.

For example, let’s say you are installing a type THWN, 6 AWG copper branch circuit conductor between utilization equipment and an overcurrent device, both of which have 75°C terminals. According to NEC Table 310.16, this conductor has an ampacity of 65 amps in the 75°C column, but Table 240.6(A) does not show 65 amps as a standard-size overcurrent device. In this scenario, Section 240.4(B) would allow you to round up to the next higher standard size shown in the table, which would be a 70 amp overcurrent device to protect the conductor that is rated 65 amps.


Guidance on Applying the Rule to Standard Circuit Breaker Sizes


The NEC “round up” rule of 240.4(B) can be a really large cost saver, particularly with average-length cable runs where voltage drop considerations will not be the ruling factor. There are times when a client or authority having jurisdiction (AHJ) will hold a strict design stance that if the calculated NEC cable ampacity requirement is just one ampere more than a conductor rating in an NEC table, then you need to jump up to the next larger conductor size. Articles with good examples and a description of the three necessary criteria required to implement this rule have been published by others.1


What does not appear to be readily available is an illustration of how this rule can be applied to each of the 28 standard circuit breaker sizes up to the 800 Amp limit for which this rule applies. The table below is a non-exhaustive attempt to look at some common situations, assuming that no other derating factors, such as those for temperature or raceway fill, apply. Coming straight out of situations where a single ampere’s difference required a conductor size increase, the table below is quite enlightening. There are a small handful of very helpful 5 Amp and 10 Amp bonuses, and even a couple of 40 Amp bonuses. It is like lifting the curtain on the wizard and finding out that there is even more conservatism in those ampacity tables than you ever imagined.


If you have any questions about PV cable sizing, voltage drop, or otherwise, get in touch with one of our design and engineering experts today.





Citations:

  1. Sizing Conductors- Part XIX, Charles Miller, Electrical Contractor Magazine, Dec 15, 2012


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