Sizing conductors based on NEC?

[julian padilla]

What is the minimum size Type XHHW conductor that's permitted to terminate on a transformer protected on the primary side with a 50 A breaker if the circuit breaker and equipment terminals are listed for 75°C conductor sizing? Answer: 8 AWG copper wire.

▫ Comment_1: Seems to me that no math is required since conductors should not be sized at 125% of the fuse size; instead, the fuse size should be sized at 125% of the continuous load on the circuit, which means the conductor should be sized to handle that calculated load, not the other way around.
▫ Comment_1: In the past, I would have used the 125% rule and calculated 50 x 1.25 = 62.5 A, and therefore oversized the wire to 6 AWG copper wire.

Any insite per the NEC would minimum requirement for this conductor is appreciated.

Julian

 

[julian padilla]

Similarly, What is the minimum size SUPPLY SIDE bonding jumper required for a 208/120-volt panel fed from a transformer with a primary voltage of 480 volts, a primary overcurrent protection device rated at 200 amperes, and the primary and secondary conductor length is 21 feet?
I solved as follows, but again did not use the 125% rule:
▫ Step_1: Size the ungrounded conductor per Table 310.16, 75 deg C column, slide down to find 200 A, which lists 3/0 ungrounded conductor
▫ Step_2: Size the SUPPLY SIDE bonding jumper per Table 250.102(C)(1) which lists 4 AWG.

Again, it seems no math is required, only proper use of the NEC tables.

Comments?

Julian