Voltage Drop & OCPD Sizing

[Bullwinkle]

I am starting a job where someone else has abruptly left off.

A 480-Volt Sub-Panel supplies power to two Refrigeration Units. That Sub-Panel is located 1560 feet from the Main Panel and is supplied by 4/0 Aluminum Feeders. According to the calculation V_d = (√3(21.2) x L x I)/CM, the maximum current carrying capacity of these Feeders is 53 ampere if no more than a 3 percent voltage drop is allowed. The Feeders are presently protected with a 125-Ampere OCPD located at the Main Panel.

Should the 125-Ampere OCPD be replaced with a 60-Ampere OCPD because that is the maximum current (53 Amperes rounded up to the next ?Standard Size?) that the Feeders can carry and remain within 3 percent voltage recommended by the NEC?

 

[Smart $]

I am starting a job where someone else has abruptly left off.

A 480-Volt Sub-Panel supplies power to two Refrigeration Units. That Sub-Panel is located 1560 feet from the Main Panel and is supplied by 4/0 Aluminum Feeders. According to the calculation V_d = (√3(21.2) x L x I)/CM, the maximum current carrying capacity of these Feeders is 53 ampere if no more than a 3 percent voltage drop is allowed. The Feeders are presently protected with a 125-Ampere OCPD located at the Main Panel.

Should the 125-Ampere OCPD be replaced with a 60-Ampere OCPD because that is the maximum current (53 Amperes rounded up to the next “Standard Size”) that the Feeders can carry and remain within 3 percent voltage recommended by the NEC?

It's not required. Additionally, refrigeration units may need the higher COPD to avoid nuisance tripping on startup.

 

[jim dungar]

Should the 125-Ampere OCPD be replaced with a 60-Ampere OCPD because that is the maximum current (53 Amperes rounded up to the next ?Standard Size?) that the Feeders can carry and remain within 3 percent voltage recommended by the NEC?

Current is a function of the load, not the 'circuit' OCPD.

If you want to maintain the 3% voltage drop, you either increase your conductor size or decrease the load.
All the NEC cares, is that the OCPD protects the conductor.

 

[Smart $]

... All the NEC cares, is that the OCPD protects the conductor.

Motor conductors are a bit different than most other load types. For the most part, they don't have an OCPD per se. They have a Ground Fault, Short Circuit protective device [fuse(s) or breaker], while the conductors are protected from overcurrent by the motor overload protection scheme.