As a rule of thumb, I would normally fuse the main panel 125% of the FLA. But I was shown that the NEC has a calculation specifically for constraining how high the rating can go. Article 409.21(C) roughly say that the over-current protection device shall not be greater than the sum of the largest branch circuit protective device, plus 125% fla of all heating elements, plus all FLA of all other motors and apparatus that could be on at the same time.
So lets say:
Largest branch - 50A (One big motor, actual motor FLA 40A)
Other motor FLA - 19A
SO that would mean the total FLA is 59A (40+19), Which I would want to fuse at 59*1.25=73.75->80A (next fuse size up)
Following 409.21, my maximum fuse size can be 69A (50+19) which means I will have to go down to a 60A fuse, too close to my FLA. Am I interpreting this code right?
Oh and by the way, I cant remember the code off hand but the disconnect rating needs to be 115% the FLA so 59*1.15= 67.85->100A which can take a minimum of a 70A fuse, and fuse adapters are frowned upon.
So lets say:
Largest branch - 50A (One big motor, actual motor FLA 40A)
Other motor FLA - 19A
SO that would mean the total FLA is 59A (40+19), Which I would want to fuse at 59*1.25=73.75->80A (next fuse size up)
Following 409.21, my maximum fuse size can be 69A (50+19) which means I will have to go down to a 60A fuse, too close to my FLA. Am I interpreting this code right?
Oh and by the way, I cant remember the code off hand but the disconnect rating needs to be 115% the FLA so 59*1.15= 67.85->100A which can take a minimum of a 70A fuse, and fuse adapters are frowned upon.
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