690.8 Ampacity Calculation

[yesterlectric]

I am wanting confirmation on how to apply the requirements of 690.8 to ampacity.

690.8(A)(1) PHOTOVOLTAIC SOURCE CIRCUIT CURRENTS: the maximum current shall be the sum of the parallel module rated short circuit currents multiplied by 125 percent.

690.8(B)(2)(a) 125 percent of the maximum currents calculated in 690.8(A) without any additional correction factors for conditional use

The question arises when we have source circuits connected to single strings of modules (no parallel sets). Do we still calculate source circuit ampacity by saying (1) parallel module string * 125% of open circuit current?

If so, do we still use the 125% factor twice for PV output circuits coming from combiners to central inverters?

 

[Carultch]

I am wanting confirmation on how to apply the requirements of 690.8 to ampacity.

690.8(A)(1) PHOTOVOLTAIC SOURCE CIRCUIT CURRENTS: the maximum current shall be the sum of the parallel module rated short circuit currents multiplied by 125 percent.

690.8(B)(2)(a) 125 percent of the maximum currents calculated in 690.8(A) without any additional correction factors for conditional use

The question arises when we have source circuits connected to single strings of modules (no parallel sets). Do we still calculate source circuit ampacity by saying (1) parallel module string * 125% of open circuit current?

If so, do we still use the 125% factor twice for PV output circuits coming from combiners to central inverters?

Open circuit current is not the term your are looking for, as this value would be zero by definition. Short circuit current (Isc) is the correct term for the value that you use in your ampacity calculations.

And consistently yes, it would be treated as "1 parallel strings", when you are talking about the ampacity requirements of individual source circuits.

Using the 125% factor twice always applies to any part of the DC side that is not current-controlled. Such as individual PV modules, individual source circuits, and combined output circuits. Devices that are current-controlled are DC-to-DC converters (optimizers) and inverters, where the 125% factor only applies once.

The first 125% factor is called the enhancement factor, and accounts for the possibility of there being "more than one sun" of irradiance. "One sun" = 1000 Watts/meter^2, which is taken as an industry standard test condition, and generally does correspond to full direct sunlight at the surface of Earth. But there are reasons why background light can add to this, which is why we put a safety factor on it. This is the factor that applies to current directly to a module or assembly of modules, and the factor that doesn't apply to current-controlled device outputs as discussed above.

The second 125% factor is the continuous load factor, due to it operating 3 hours or more, and treated as other examples of continuous loads.

 

[ggunn]

I am wanting confirmation on how to apply the requirements of 690.8 to ampacity.

690.8(A)(1) PHOTOVOLTAIC SOURCE CIRCUIT CURRENTS: the maximum current shall be the sum of the parallel module rated short circuit currents multiplied by 125 percent.

690.8(B)(2)(a) 125 percent of the maximum currents calculated in 690.8(A) without any additional correction factors for conditional use

The question arises when we have source circuits connected to single strings of modules (no parallel sets). Do we still calculate source circuit ampacity by saying (1) parallel module string * 125% of open circuit current?

If so, do we still use the 125% factor twice for PV output circuits coming from combiners to central inverters?

To avoid confusion I multiply Isc by 1.25 to get maximum DC current and then treat it like any other Imax, i.e., rather than multiply it by 1.25 again I start with conductor ampacity and derate it appropriately for conditions of use and continuous use. Both derated ampacities must exceed Imax, the same as for AC circuits.

 

[pv_n00b]

This still provides a lot of confusion for people.

690.8(A)(1) multiplies the STC Isc value by 1.25 to take into account that the modules may be exposed to solar irradiance in excess of the 1000W/m^2 that STC Isc is based on. This happens because the Isc is not fixed, it depends on how much sun is shining on the module and the NEC uses a belt and suspenders approach to oversizing. Once you get the new value, I tend to call it Imax, you use that value in all the other calculations just like you would any maximum current. One string current is treated the same as multiple string currents in parallel.

690.8(B)(2) directs you to use Imax with the conditions of use adjustments to check the conductor size. This is in contrast to 690.8(B)(1) which uses Imax multiplied by 1.25, because it is a continuous current, and that value is used to check the conductor size without any conditions of use adjustments.

Generally 690.8(B)(2) drives the bus on conductor sizing unless you are in a location with a cool high temperature and low number of conductors in a conduit. Then 690.8(B)(1) will probably be the driver.

In any case never double dip and use the continuous current correction and the conditions of use adjustment at the same time. It will give you a conservatively sized larger conductor that you don't need. But in the end once you correct for voltage drop most conductors end up being oversized based on ampacity rating anyway.