winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
The derate calculation is also based on the conductors heating up to their rated temperature. If you derate based on 90C, then you are implying that the conductors will heat to 90C when used at the calculated ampacity.
A #12 has a 90C ampacity of 30A, and there is a 50% derate for 20 conductors.
If you have 20 #12 conductors together in a single conduit, each carrying 15A, then you expect the conductors to heat up to 90C.
Again, I am ignoring the conservative and approximate nature of NEC ampacity calculations (real world vs theory). This is just running the standard calculations backwards.
The standard calculation says 'given this maximum temperature rating and these conditions of use, what is the maximum allowed current. Running the calculation backwards we say 'given these conditions of use and this current level, what is the maximum conductor temperature. '
Jon
A #12 has a 90C ampacity of 30A, and there is a 50% derate for 20 conductors.
If you have 20 #12 conductors together in a single conduit, each carrying 15A, then you expect the conductors to heat up to 90C.
Again, I am ignoring the conservative and approximate nature of NEC ampacity calculations (real world vs theory). This is just running the standard calculations backwards.
The standard calculation says 'given this maximum temperature rating and these conditions of use, what is the maximum allowed current. Running the calculation backwards we say 'given these conditions of use and this current level, what is the maximum conductor temperature. '
Jon