NEC 220 and 690.8: 0.5A rounding for solar PV output circuits (NEC 2014)

[IPS]

Hi,

After browsing this forum and the internet, it is unclear if the rounding rule of 220.5(B) can apply to PV calcs of section 690.8. However, here is my take on it:


1) NEC 220: applies to branch circuits, Feeder and Service calcs


2) NEC 220.5(B) allows us to round down for decimals <0.5A


3) NEC 220.3 asks us to check Table 220.3 for 'additions' or 'modifications' to article 220.


4) Table 220.3 tells us to refer to 690.8 (for 'additions' or 'modifications' of article 220)


5) NEC 690.8 says nothing about rounding up or down. All calcs have to be 'not less than 125%...'


CONCLUSION: 690.8 does not modify the rounding rules of 220.5(B) but adds safety factors to it. For example, if my total current is 20.125A I should be able to use a 20A breaker.


Does this sound OK to you guys? What is your experience on this matter?

Thanks!

 

[Adamjamma]

hmm... I have been taught different but will wait for experts to chime in.. was taught breakers and wire size both require up to.. if it is exceeded then next size up... but, I could totally be wrong on this as I am just learning the real code instead of what I was told...

 

[jaggedben]

...


CONCLUSION: 690.8 does not modify the rounding rules of 220.5(B) but adds safety factors to it. For example, if my total current is 20.125A I should be able to use a 20A breaker.


...

What are you using a breaker for that you're calculating according to 690.8? (Certainly not a branch circuit, feeder, or service.) Most likely you should be calculating according to 705, if it's inverter output calculations. (Sorry if I'm making unwarranted assumptions.)

But anyway, the issue is the same, so I'll offer my opinion.

I think you're basically correct. If the result of your 690.8 required calculation is 20.125A, you could use a 20A overcurrent device. If its 20.625, you gotta go to 25A.

 

[ggunn]

Well, OK, but remember that the breaker size thus determined is the minimum breaker size so that you don't have nuisance trips from the PV output. You can have a breaker as big as you want as long as the conductors to the PV system are sized accordingly to be protected by the breaker from fault current from the service. That's the only reason you need OCP at all; in a properly designed PV system the conductors cannot be damaged by the inverter(s).