Heat Pump Amperage Requirements and How to Size Circuits

Heat pump amperage requirements determine the electrical circuit size, breaker rating, and wire gauge needed for safe operation. This article explains running vs starting amperage, NEC guidelines, calculation methods, and practical examples for residential systems to help homeowners and technicians estimate proper electrical sizing. Accurate amperage sizing improves safety, efficiency, and equipment longevity.

Heat Pump Capacity	Typical Running Amps (RLA)	Typical Minimum Breaker
1.5 Ton	6–10 A	15 A
2.5 Ton	8–14 A	15–20 A
3.0 Ton	10–18 A	20 A
4.0 Ton	15–25 A	25–30 A
5.0 Ton	18–35 A	35–45 A

How Heat Pumps Draw Current

Heat pumps use a compressor, fan motors, controls, and sometimes electric resistance backup elements; the compressor is the primary current draw. Two distinct amperage values matter: running amperage and starting (inrush) amperage. Running amperage is the steady-state current when the compressor operates normally. Starting amperage is a short-duration surge when the compressor motor starts.

Running Amps Versus Locked-Rotor And Starting Amps

Running amps, often listed on the unit’s nameplate as RLA or LRA (full-load amps vs locked-rotor amps), are used for continuous circuit sizing. Locked-rotor amps (LRA) can be many times higher than running amps and influence starting equipment like soft starters and contactors. Design must account for sustained run current for breaker selection and transient starting current for contactors and starter devices.

Reading The Nameplate And Manufacturer Specs

The heat pump nameplate includes RLA, LRA, and maximum fuse size. Always use the manufacturer’s RLA for calculations. If the nameplate is missing, consult the installation manual or manufacturer’s website. Do not rely on approximate tables alone when the nameplate is available.

NEC Requirements And Code Considerations

The National Electrical Code (NEC) sets minimums for branch-circuit protection and conductor ampacity. NEC typically requires branch circuits to be sized to at least 125% of the continuous load for continuous equipment. Local codes can add requirements, so installers must verify jurisdictional rules.

Calculating Circuit Breaker Size And Wire Gauge

To size a circuit: use the heat pump’s RLA, add 25% for continuous operation, then select a breaker and conductor with ampacity above that value. For example, if the RLA is 16 A, 16 A × 1.25 = 20 A, so a 25 A breaker with 10 AWG copper is common. Always follow NEC ampacity tables and derating factors for multiple conductors.

Typical Wire And Breaker Guide

Approx Running Amps	Recommended Breaker	Typical Copper Wire
Up To 15 A	15–20 A	14–12 AWG
15–25 A	20–30 A	12–10 AWG
25–40 A	30–45 A	10–8 AWG
40–60 A	45–70 A	6–4 AWG

Examples By Heat Pump Size And Type

Residential heat pumps vary by tonnage, compressor type, and efficiency. Below are practical examples using typical RLA ranges.

Mini-Split/Small Ductless Units

Many single-zone mini-splits have low running amps, often 5–12 A, with small inrush currents. These commonly use 15–20 A breakers and 14–12 AWG wiring.

3-Ton Standard Split Heat Pump

A 3-ton unit with RLA ~14–18 A typically needs a 20–30 A breaker and 10–8 AWG wire. The outdoor unit nameplate might list a maximum fuse size of 35 A; follow that and the 125% NEC rule for continuous loads.

5-Ton Heat Pump

Larger 5-ton units can have RLA values of 18–35 A and LRAs exceeding 200 A. These systems often require 35–60 A breakers, with 8–4 AWG conductors and sometimes a separate fused disconnect or two-pole breaker.

Factors That Increase Amperage Draw

Several operational and environmental factors raise heat pump amperage: low outdoor temperatures, dirty coils, restricted airflow, low refrigerant charge, failing capacitors, or mechanical wear. Regular maintenance reduces unexpected high current and prolongs compressor life.

Cold Climate Operation

In cold weather, the compressor works harder and defrost cycles may engage frequently, temporarily increasing current. Systems with electric resistance backup elements can draw significant additional amperage when the auxiliary heat runs.

Start/Stop Frequency

Frequent cycling increases cumulative inrush stress on contactors and could cause nuisance trips if the breaker is marginally sized. Proper thermostat and system configuration reduce unnecessary starts.

Starting Devices And Soft-Starters

Because locked-rotor amperage can be high, many installations use soft-starters, variable-speed compressors, or reclosing contactors to limit inrush. Variable-speed compressors significantly lower starting current and reduce feeder and breaker stress.

Sizing For Replacement Versus New Installations

For replacement installs, confirm existing circuit capacity against the new unit’s nameplate. Upgrading to a higher-efficiency or larger-capacity heat pump may require upsizing the breaker and conductors. Never install a new heat pump on undersized existing wiring.

Practical Troubleshooting For High Amperage

If the heat pump draws higher-than-expected amps, check the following: air filter and coil cleanliness, condenser airflow, refrigerant pressures, capacitor condition, contactor wear, and motor bearings. Use a clamp meter to record running and starting amps. Document readings and compare to nameplate values.

When To Call A Technician

Persistent high running amps, frequent breaker trips, or abnormal noises require professional diagnosis. A qualified HVAC electrician or technician can perform amp tests, measure voltage drop, and inspect refrigerant and mechanical systems.

Voltage, Phase, And Service Considerations

Residential heat pumps are commonly 240 V single-phase in the U.S., though larger commercial systems may be three-phase. Voltage imbalances or low supply voltage increase current draw. Measure line voltage at the unit under load to check for proper supply voltage and drop.

Energy Efficiency, SEER Ratings, And Amperage

Higher SEER or HSPF rated heat pumps typically use more sophisticated compressors and variable-speed technology that can reduce average amperage and improve efficiency. Efficiency improvements can lower operating amps and energy costs over time.

Cost And Safety Implications Of Undersizing

Undersized circuitry causes nuisance trips, potential equipment damage, and safety hazards from overheated conductors. Oversized breakers that exceed conductor ampacity also create fire risk. Appropriate sizing balances safety, code compliance, and reliability.

NEC Compliance Checklist For Installers

• Verify RLA on the nameplate and use it as the primary sizing metric.
• Apply 125% multiplier for continuous loads per NEC.
• Use the maximum overcurrent protection value from the nameplate as a limit.
• Account for voltage drop on long runs—keep under 3% where possible.
• Install an outdoor fused disconnect or two-pole breaker as required.
• Document wire gauge, breaker rating, and measured amps in the service record.

Frequently Asked Questions

How Much Current Does A Heat Pump Use To Start?

Starting current varies with compressor type. Single-stage piston compressors can have LRAs several times the RLA, sometimes 6–12×. Modern scroll and variable-speed compressors have much lower starting spikes. Consult the LRA on the nameplate for expected peaks.

Can A Heat Pump Share A Circuit With Other Loads?

NEC typically requires dedicated branch circuits for heat pump compressors. Shared circuits risk nuisance tripping and violate code in many jurisdictions. Install a dedicated circuit sized for the heat pump’s continuous load.

How To Reduce Heat Pump Amperage Draw?

Regular maintenance—clean filters and coils, ensure proper airflow, correct refrigerant charge, and replace failing capacitors—reduces amperage. Upgrading to variable-speed technology also lowers average draw.

Is It Safe To Use The Maximum Fuse Size On The Nameplate?

The nameplate’s maximum fuse value is the manufacturer’s recommended limit. Use it as a guideline, not a substitute for NEC continuous-load sizing rules. Breaker sizing must also consider conductor ampacity and local code.

Recommended Measurement And Documentation Practices

Measure running and starting amps with a certified clamp meter and record voltage, ambient temperature, and operating mode. Maintain service logs comparing measured values against nameplate data. Consistent documentation helps diagnose trends and prevent failures.

Resources And Further Reading

Refer to the latest NEC handbook, manufacturer installation manuals, and HVAC training materials for in-depth guidance. Local utility programs and HVAC associations often provide wiring and efficiency recommendations specific to regional climates.

Key Takeaway: Use the heat pump nameplate RLA as the foundation for sizing, apply NEC 125% rule for continuous loads, account for starting currents with appropriate starters or contactors, and ensure conductors and breakers match required ampacity and local code for safe, reliable operation.