Example: A Typical HVAC Nameplate
From this nameplate: wire must be sized for ≥ 19.7A MCA (use #12 AWG copper, 25A rated at 75°C — satisfies MCA). Breaker must be ≤ 35A MOCP. The 40A breaker matches both: 19.7A ≤ 40A ≤ 35A... wait — 40A exceeds MOCP of 35A. Correct answer is a 35A breaker. See the MOCP vs MCA section below.
Nameplate Field Glossary
The minimum ampacity the supply conductors must have. Set by the manufacturer per NEC 440.32. Wire size must be at least this value. Not the operating current — already includes required safety margins (125% of compressor current + fan current).
The maximum breaker or fuse size the manufacturer certifies the equipment for. The breaker must not exceed this. Exceeding MOCP voids the equipment listing and is a NEC violation. Check NEC 440.22 for HVAC equipment.
Current drawn at rated full load. For motor branch circuits, conductors must be at least 125% of FLA per NEC 430.22. Not the same as MCA — MCA may be higher to account for starting current nuances.
Inrush current when motor starts from standstill. Typically 6–8× FLA. The breaker must tolerate this momentary inrush without tripping — which is why OCPD ratings for motors can be much higher than FLA (up to 250% per NEC 430.52).
HVAC-specific equivalent of FLA, accounting for the compressor operating at design conditions. Used by MCA calculation. Sometimes labeled "RLA" instead of "FLA" on refrigeration/HVAC equipment.
Voltage (120V, 208V, 240V, 277V, 480V), number of phases (1PH = single-phase, 3PH = three-phase), and frequency (60Hz in North America). Equipment must be connected to a supply matching these specs exactly.
Motor output in horsepower. NEC Table 430.248 (1-phase) and 430.250 (3-phase) provide the FLA values for a motor of a given HP and voltage — used when sizing conductors if the actual FLA is unknown.
Output capacity of a heating or cooling appliance. 1 kW = 1000W = 3412 BTU/h. Used to calculate expected current: I = W / V (single-phase, resistive) or I = kW × 1000 / (V × 1.732) for three-phase. Useful for estimating load if FLA isn't listed.
Ratio of real power to apparent power, expressed as a decimal (0.85) or percent (85%). Current draw = kW / (V × PF) for single-phase. Poor power factor increases current demand without delivering more work. Most motors run 0.80–0.95 PF at full load.
How to Use MCA and MOCP for Wire and Breaker Sizing
- Wire size: Look up MCA in NEC Table 310.15(B)(16) at 75°C. Select the first AWG size whose ampacity meets or exceeds MCA. Use the wire size calculator for long runs where voltage drop may require a larger conductor.
- Breaker size: Use the nameplate MOCP as the maximum. Per NEC 440.22, the breaker must not exceed MOCP. For HVAC/motor circuits, the breaker can be larger than the running current (to handle LRA) up to the MOCP limit.
- Range check: The breaker must also be large enough to protect the conductor — at minimum equal to or larger than the conductor's ampacity divided by 1.25 for continuous loads. In practice, MOCP usually controls the upper bound.
HVAC Sizing Example (Full Walkthrough)
Nameplate Values for Common Appliances
| Appliance | Typical Nameplate | Wire | Breaker |
|---|---|---|---|
| Electric dryer | 240V, 24A, 5760W | #10 AWG Cu | 30A |
| Electric range | 240V, 40–50A, 12–14kW | #6 AWG Cu | 50A |
| Central A/C (3-ton) | MCA 17A, MOCP 35A | #12 AWG Cu | ≤35A |
| Water heater (4500W) | 240V, 18.75A | #10 AWG Cu | 30A |
| EV charger (EVSE, 48A) | 240V, 48A continuous | #6 AWG Cu | 60A (NEC 625.42) |
Frequently Asked Questions
MCA (Minimum Circuit Ampacity) is the minimum conductor ampacity required per NEC 440.32. Wire size your supply conductors to at least MCA — it already includes the manufacturer-required safety margin.
MOCP (Maximum Overcurrent Protection) is the largest breaker or fuse the manufacturer certifies. Your breaker must not exceed this value per NEC 440.22, regardless of the calculated result.
MCA is the minimum for conductors; MOCP is the maximum for the breaker. Together they define the valid range: wire sized for ≥ MCA, breaker sized ≤ MOCP.
Full Load Amps — the current the motor draws at its rated load. Used per NEC 430 to size motor branch circuit conductors at 125% of FLA.
Use MOCP as the maximum breaker size. For HVAC, the breaker can be as large as MOCP to handle starting inrush. For simple resistive loads (heaters, ranges), size at 125% of rated amps for continuous loads per NEC 210.20, up to next standard NEC 240.6 size.