ggunn
PE (Electrical), NABCEP certified
- Location
- Austin, TX, USA
- Occupation
- Consulting Electrical Engineer - Photovoltaic Systems
Every once in a while I have a disagreement with a plan reviewer about PV ampacities where they want to include continuous usage derate in the conditions of use calculation. It's been a long time since I have had to do it, so here is what my response will be for (18) #10 90 degree conductors in a single conduit to a SolarEdge inverter. Note that SolarEdge optimizers are current limited to 18.0A, so the 1.25 factor for excess insolation does not apply. Also note that I realize that the technically correct calculation for continuous use multiplies Imax by 1.25 rather than the ampacity by 0.8, but both get you to the same result and this way is easier for most people to understand.
Conditions of use derate calculation, which ensures that the 90 degree conductor insulation is not compromised:
(90 degree ampacity)(derate for ambient conditions)(derate for conduit fill) = Ampacity 1
Continuous use derate calculation, which ensures that the 75 degree rated terminals are not compromised:
(75 degree ampacity)(derate for continuous use) = Ampacity 2
For a 90 degree conductor, both Ampacity 1 and Ampacity 2 must be greater than the maximum current in the conductor (Imax).
In this case:
Imax = 18.0A
90 degree ampacity of #10 = 40A
75 degree ampacity of #10 = 35A
Derate for ambient conditions = 0.91
Derate for conduit fill = 0.5
Derate for continuous use= 0.8
Conditions of use derate calculation:
(40A)(0.91)(0.5) = Ampacity 1 =18.2A
Continuous use derate calculation:
(35A)(0.8) = Ampacity 2 =28.0A
Ampacity 1 and Ampacity 2 are both greater than 18.0A.
Conditions of use derate calculation, which ensures that the 90 degree conductor insulation is not compromised:
(90 degree ampacity)(derate for ambient conditions)(derate for conduit fill) = Ampacity 1
Continuous use derate calculation, which ensures that the 75 degree rated terminals are not compromised:
(75 degree ampacity)(derate for continuous use) = Ampacity 2
For a 90 degree conductor, both Ampacity 1 and Ampacity 2 must be greater than the maximum current in the conductor (Imax).
In this case:
Imax = 18.0A
90 degree ampacity of #10 = 40A
75 degree ampacity of #10 = 35A
Derate for ambient conditions = 0.91
Derate for conduit fill = 0.5
Derate for continuous use= 0.8
Conditions of use derate calculation:
(40A)(0.91)(0.5) = Ampacity 1 =18.2A
Continuous use derate calculation:
(35A)(0.8) = Ampacity 2 =28.0A
Ampacity 1 and Ampacity 2 are both greater than 18.0A.