We have to agree that the provisions on the code are there to guide us all in making safe installations. The code specifically says you should follow the 80% of rated capacity rule, meaning, breakers should be designed to be loaded at no more than 80% of its rating (unless specified as 100%-rated).
Here's the catch:
You will terminate your cable to a breaker. It follows that, since no breaker will be installed with a rating above the cable capacity rating, the cable rating will have to be 125% of the load current you have expected. That is because you have implemented an 80% limit on the load applicable to the breaker rating, you factor-in 125% on the cable sizing calcs (1.25 = 1/0.80). The cable rating should match the breaker capacity, or cable rating never below the breaker capacity.
Example:
FLA = 280 amps load; breaker rating = 1.25*280 = 350 amps; and your cable size = 500MCM(75 deg)
Reasons:
- Using cables sized 500 MCM (380 amps @75 deg. temp) is above the 350 amps circuit breaker rating, i.e. code compliant - that is, your breaker protects your cables before the cables get damaged.
- With an load of 350 amps (125% of expected 280 amps load) on your cable, you will be loading the cables 350/380 = 92% which is safe.
- You are not imposing a load limitation from your source; if it is a transformer, you are allowed to overload transformers for a specified period of time.
This topic has been an exam question, and you have to understand everything written in the code. If you haven't seen a code provision saying 125%, try looking for the "80% rule".
I hope this puts this question to rest. Crystal!