120% rule...again

[SteveO NE]

Hi guys,

I imagine if I search hard enough I could find this mentioned on the forum but something came up that is a subtle nuance in the wording of 2017 705.12(B)(2)(3)(b) that did make me pause for a moment and wonder what everyone else's thoughts are:

"...the sum of 125% of the power source(s) output circuit current and the rating of the overcurrent device protecting the busbar shall not exceed 120%..."

I've interpreted that as a fail if either condition is not met -> an example of how you could pass the breaker test but fail the 125% would be using a 100% rated breaker.

It was recently posed to me that because overcurrent device is not pluralized, the intention is 125% of the PV source circuit (or other onsite parallel power source) plus the rating of the main breaker. Has anyone else ever thought this? Example of why this would make a difference:

200kW of PV
400A main 480V, 3PH

400A OCPD---[600A Panel]---301A of PV = 701A (less than 720A from 120% rule)

However, it would be installed as;

400A OCPD---[600A Panel]---350A OCPD (301A of PV next size up) = 750A (120% fail)

Thoughts?

Regards,
Steve

 

[SteveO NE]

Sorry, it won't let me edit and I meant to point out that (c) more specifically refers to the "overcurrent device protecting the busbar" as the main breaker only. That could lead one to believe that in (b) they do only mean main OCPD.

 

[wwhitney]

The language clearly calls for the first computation, as it refers to 125% of the power source output current, not to the size of the breaker protecting the power source output. The proposed layout with a 350A OCPD complies with the "120%" rule.

Cheers, Wayne

 

[Carultch]

Hi guys,

I imagine if I search hard enough I could find this mentioned on the forum but something came up that is a subtle nuance in the wording of 2017 705.12(B)(2)(3)(b) that did make me pause for a moment and wonder what everyone else's thoughts are:

"...the sum of 125% of the power source(s) output circuit current and the rating of the overcurrent device protecting the busbar shall not exceed 120%..."

I've interpreted that as a fail if either condition is not met -> an example of how you could pass the breaker test but fail the 125% would be using a 100% rated breaker.

It was recently posed to me that because overcurrent device is not pluralized, the intention is 125% of the PV source circuit (or other onsite parallel power source) plus the rating of the main breaker. Has anyone else ever thought this? Example of why this would make a difference:

200kW of PV
400A main 480V, 3PH

400A OCPD---[600A Panel]---301A of PV = 701A (less than 720A from 120% rule)

However, it would be installed as;

400A OCPD---[600A Panel]---350A OCPD (301A of PV next size up) = 750A (120% fail)

Thoughts?

Regards,
Steve

It is 125% of the total inverter current that matters. In otherwords, it is the calculation that sizes an ordinary overcurrent protection device, before you round up to select your actual OCPD. It used to be the interconnection breaker in NEC2011 and prior, but this was change in 2014 to eliminate rounding errors, and subpanel interconnections from being a show stopper.

 

[SteveO NE]

Thanks for the replies. It is rare for me that it matters as most of my projects have been utility scale or interconnecting in primary switchgear since I've been out of 2014, so I've not put much thought to that change in wording.

Regards,
Steve

 

[Carultch]

Thanks for the replies. It is rare for me that it matters as most of my projects have been utility scale or interconnecting in primary switchgear since I've been out of 2014, so I've not put much thought to that change in wording.

Regards,
Steve

No problem.

I'd like to add another point to your example of a 200 kW system. That fraction of an ampere as the "straw that broke the camel's back" will not necessarily occur in practice. It is common for inverter manufacturers to program the firmware so their current limit is 80% of a standard OCPD rating, when the theoretical amps from KW are this close to that value. Thus, you can expect that 200kW would likely end up being 240A on the dot, which would mean 300A for the breaker rating.