NEC 690.8(A)(3)

[Engineer#4827502]

Hello All,

First post, but I've been lurking for a while. I have a question about NEC 690.8(A)(3). The 'inverter continuous output rating' is typically listed in the specs/cutsheet as a max AC output current. Typically this is under a low voltage condition, high PV overload, and possibly non-unity power factor. Every inverter manufacturer I've talked to has said that this rating can be continuous however rare the conditions may be.

My question is, this maximum value is what should be used for conductor/equipment sizing, correct? Not the nominal AC output current (which is typically what you would calculate at rated output power at nominal voltage).

I've seen both interpretations by engineers, electricians, and AHJs so there may be a range of opinions on this one.... Thanks for reading!

 

[Dennis Alwon]

690.8(A) states the max. circuit current shall be calculated based on (A)(1) thru (A)(5). 690.8(A)(3) states the maximum current shall be the inverter continuous output current rating. IMO that is the current possible so the conductor must be sized for the max.

 

[Engineer#4827502]

Thanks, yeah I would agree with that and it seems pretty straightforward but I have had many arguments with people on the conductor/OCPD sizing to that end.

 

[ggunn]

Thanks, yeah I would agree with that and it seems pretty straightforward but I have had many arguments with people on the conductor/OCPD sizing to that end.

I don't see how. The NEC is pretty explicit on the matter. Inverter max current X 1.25, the next OCPD up from that.

 

[ryan_mayfield]

My question is, this maximum value is what should be used for conductor/equipment sizing, correct? Not the nominal AC output current (which is typically what you would calculate at rated output power at nominal voltage).

I believe the value you should use is the continuous current value per the manufacturer. For grid-direct systems, this is straight-forward as the maximum current often equals the continuous. For battery based inverter though, they can have short durations of higher current to help with surge loads. For example, OutBack's 8kW Radian inverter lists the continuous current at 33.3Aac yet it can have a 100ms RMS maximum current (their language) of 70.7Aac. I would contend the proper OCP and conductor sizing would be based on the 33.3A continuous current value, per 690.8(A)(3).

 

[Smart $]

Another thing to watch for is manufacturer's that publish both an inverter output rating and a different, higher maximum output current. You use the inverter output rating.

 

[jaggedben]

I believe the value you should use is the continuous current value per the manufacturer. For grid-direct systems, this is straight-forward as the maximum current often equals the continuous. For battery based inverter though, they can have short durations of higher current to help with surge loads. For example, OutBack's 8kW Radian inverter lists the continuous current at 33.3Aac yet it can have a 100ms RMS maximum current (their language) of 70.7Aac. I would contend the proper OCP and conductor sizing would be based on the 33.3A continuous current value, per 690.8(A)(3).

Best answer. :thumbsup: Anything listed as a 'surge' or 'peak' output is not continuous.

I would just add that the 'nominal output current' for utility interactive inverters is pretty irrelevant because a) inverters tend to raise the line voltage above nominal at high output, and b) the inverter is going to clip at the max output current, regardless of the voltage.