Electrical and Safety
This domain tests electrical fundamentals applied to HVAC equipment nameplates and the safety practices that surround service work.
Core rules
- Ohm's law and power relationships: V = I x R, P = V x I, with a power factor for alternating current, and three-phase power using the square root of three.
- Minimum circuit ampacity (MCA) uses 125 percent of the largest motor load plus all other loads. Maximum overcurrent protection (MOP) uses 225 percent of the largest motor load plus other loads, then the next standard breaker size not exceeding that value.
- Conductors are sized to at least the MCA; the breaker is chosen within the MOP range. They are not the same number.
- Disconnects must be within sight of the equipment they serve. Grounding and bonding are separate functions; neither replaces the other.
- Lockout/tagout, arc-flash awareness and correct PPE apply before any panel work. Refrigerant work adds EPA 608 certification and A2L charge-limit rules.
Worked example
A condensing unit nameplate lists a compressor rated load amps of 22 A and a fan full load amps of 2.5 A. MCA = (1.25 x 22) + 2.5 = 30.0 A. MOP = (2.25 x 22) + 2.5 = 52.0 A, so a 50 A breaker is acceptable while a 60 A breaker is not. Conductors must carry at least 30 A; a 10 AWG copper conductor at 75C, rated 35 A, satisfies the MCA if derating allows.
Traps that cost marks
- Installing a breaker equal to the MCA instead of within the MOP range.
- Upsizing a breaker to stop nuisance trips without finding the actual cause.
- Ignoring ambient temperature and conduit-fill derating of conductors.
- Working live without lockout, or bypassing a safety control.
- Using the wrong refrigerant safety class procedures on A2L equipment.