clarification for load side PV connections.

Status
Not open for further replies.
mwm1752

mwm1752

Senior Member
Location
Aspen, Colo
705.12(B)(2) states :
Bus or Conductor Ampere Rating. One hundred twenty-five percent of the power source output circuit current shall be used in ampacity calculations for the following: Simple enough definition of power source output circuit current - So- if you have a 50 Amp power source output circuit current the 50 x 125% = 62.5 amps
The latter sub sections of 705.12(B)(2) use a consistent statement for feeder/taps/busbar limits:
the sum of 125 percent of the power source(s) output circuit current
Doe s this mean you need to recalculate the defined power source output circuit current 62.5 amps as 62.5 x 125% = 78.1 amps for use in feeder/taps/busbar limit calculations
 
It would not make any sense to apply the 125% factor twice. The power source output current refers always to the current rating at the device, irrespective of breaker size or upstream wiring and panels, not the current coming from that direction at an upstream panel.
At a panel upstream of a feeder which serves PV device(s), it does not refer to 125% of the feeder rating.
 
mwm1752 said:
705.12(B)(2) states :
Bus or Conductor Ampere Rating. One hundred twenty-five percent of the power source output circuit current shall be used in ampacity calculations for the following: Simple enough definition of power source output circuit current - So- if you have a 50 Amp power source output circuit current the 50 x 125% = 62.5 amps
The latter sub sections of 705.12(B)(2) use a consistent statement for feeder/taps/busbar limits:
the sum of 125 percent of the power source(s) output circuit current
Doe s this mean you need to recalculate the defined power source output circuit current 62.5 amps as 62.5 x 125% = 78.1 amps for use in feeder/taps/busbar limit calculations
Click to expand...

They re-wrote this language in 2014, so that rounding up to the breaker you need to use, isn't a show stopper. It used to specify the breaker rating, but now no longer requires you to use the breaker rating directly. Essentially, they are telling you to use the output of the calculation for sizing the breaker, before you round up to the next standard size. Add up the total inverter currents, and multiply by 125%.

In your example, assuming 50A is the total of the inverter output currents, that means you'd need a 70A breaker (because 63A is not an NEC standard rating). Instead of using 70A in the calculation, like you would've had to do in 2011, you now get to use 62.5A in your calculation. You could even interconnect on a 100A branch breaker, and still use 62.5A in your calculation for Article 705.12(B)(2).

An example of where this makes a difference:
Given a 600A panel with a 600A main. It is desired to connect 96A of inverter output current, which implies a 125A breaker.

In 2011, the 120% rule calculation would be:
(600A + 125A) <= 120%*600A, which fails.

In 2014 and onward, it would be:
(600A + 125%*96A) <= 120%*600A, which passes.
 
GoldDigger said:
It would not make any sense to apply the 125% factor twice. The power source output current refers always to the current rating at the device, irrespective of breaker size or upstream wiring and panels, not the current coming from that direction at an upstream panel.
At a panel upstream of a feeder which serves PV device(s), it does not refer to 125% of the feeder rating.
Click to expand...
I am not in disagreement with your statement as I find it not sensible.- The query was about why the NFPA defining in 705.12(B)(2) "calculations for power source output circuit current" then, using language they defined with requirement of additional 125% for ratings of feeder/taps/busbars.
 
We don't have a defined term for (125% of the inverter output current). That's why 125% has to be mentioned, to make it clear that the continuous load factor is included. Or 156% of the short circuit current in the context of DC directly from the modules, due to the NEC compounding two of the 125% factors for irradiance enhancement and continuous duty.
 
Status
Not open for further replies.
Top