You are upgrading a central air unit from a 30-amp circuit to one rated at MOCP 40 A. The unit nameplate shows MCA 29.7 A. Which action is permitted when upgrading the protection?

You are upgrading a central air unit from a 30-amp circuit to one rated at MOCP 40 A. The unit nameplate shows MCA 29.7 A. Which action is permitted when upgrading the protection?

• A. Install a 50-amp breaker and leave fuses unchanged
• B. Install a 40-amp breaker and leave fuses unchanged
• C. Install a 40-amp fuse in the disconnect only
• D. Install a 60-amp breaker and 60-amp fuses

Answer: (A)

Understand how MOCP and MCA work on equipment nameplates. MCA is the minimum circuit ampacity—the conductors feeding the unit must be able to carry at least this much current. MOCP is the maximum overcurrent protection rating the equipment can safely tolerate. When upgrading protection for a piece of equipment, you must not exceed the MOCP rating shown on the nameplate. For this unit, the MOCP is 40 A, so the protective device on the branch circuit must be 40 A or smaller. The MCA is 29.7 A, and 125% of MCA is about 37.1 A, which points to a 40 A as the standard-sized protection for safe, allowed operation. Therefore, the correct approach is to install a 40 A protective device and keep the existing disconnect fuses unchanged. Using a larger protection—such as 50 A or 60 A—would exceed the equipment’s listed MOCP and could compromise protection for the unit.

Understand how MOCP and MCA work on equipment nameplates. MCA is the minimum circuit ampacity—the conductors feeding the unit must be able to carry at least this much current. MOCP is the maximum overcurrent protection rating the equipment can safely tolerate. When upgrading protection for a piece of equipment, you must not exceed the MOCP rating shown on the nameplate.

For this unit, the MOCP is 40 A, so the protective device on the branch circuit must be 40 A or smaller. The MCA is 29.7 A, and 125% of MCA is about 37.1 A, which points to a 40 A as the standard-sized protection for safe, allowed operation. Therefore, the correct approach is to install a 40 A protective device and keep the existing disconnect fuses unchanged. Using a larger protection—such as 50 A or 60 A—would exceed the equipment’s listed MOCP and could compromise protection for the unit.