ggunn
PE (Electrical), NABCEP certified
- Location
- Austin, TX, USA
- Occupation
- Consulting Electrical Engineer - Photovoltaic Systems
I keep getting confused about this. Say I have some rooftop DC wiring that I need to size and the heat load is significant - hot roof in Texas, wiring very close to it - with 90 degree wire and 75 degree terminals.
Calculate necessary ampacity two ways, one at continuous use, one at conditions of use, choose the higher.
1.25 X 1.25 X Isc for continuous use
1.25 X temp derate (with adder) reciprocal X Isc for conditions of use (fewer than 4 CCC's per conduit - no derate for that).
OK so far, but what confuses me is when to use the 75 degree column and when to use the 90 degree column. I remember a Holt video that says to use 90 degrees for one calculation and 75 degrees for the other, but which and why? If I understood the why I wouldn't have so much trouble the remembering the which.
I could play it safe and use the 75 degree column throughout (pretending that I am using 75 degree wire), but the difference in the temp derate between 75 and 90 degree conductors is significant and there is a lot of wire involved.
Calculate necessary ampacity two ways, one at continuous use, one at conditions of use, choose the higher.
1.25 X 1.25 X Isc for continuous use
1.25 X temp derate (with adder) reciprocal X Isc for conditions of use (fewer than 4 CCC's per conduit - no derate for that).
OK so far, but what confuses me is when to use the 75 degree column and when to use the 90 degree column. I remember a Holt video that says to use 90 degrees for one calculation and 75 degrees for the other, but which and why? If I understood the why I wouldn't have so much trouble the remembering the which.
I could play it safe and use the 75 degree column throughout (pretending that I am using 75 degree wire), but the difference in the temp derate between 75 and 90 degree conductors is significant and there is a lot of wire involved.