Heat Pump Wire Size Guide: Choosing the Right Cable for HVAC Systems

The correct wire size for a heat pump affects safety, performance, and code compliance. This guide explains how to determine the right conductor size, common gauge recommendations, voltage drop considerations, and NEC rules so homeowners and technicians can choose properly sized wiring for residential heat pump installations.

Heat Pump Circuit Amps	Typical Minimum Copper Wire	Typical Breaker Size
10–15 Amps	14 or 12 AWG	15–20 A
15–20 Amps	12 AWG	20 A
20–30 Amps	10 AWG	30 A
30–40 Amps	8 AWG	40 A
40–60 Amps	6 AWG	50–60 A

How Heat Pumps Use Electrical Power

Heat pumps use electricity to run a compressor, outdoor fan motor, and reversing valves; typical residential units draw from 10 to 60 amps depending on capacity and compressor type. The outdoor unit requires a dedicated circuit sized for the maximum continuous and noncontinuous loads.

Why Wire Size Matters

Correct conductor sizing prevents overheating, reduces voltage drop, ensures reliable operation, and meets code. Undersized wire can trip breakers, damage compressors, and create a fire hazard. Oversized wire increases cost but improves durability and reduces voltage drop.

Key Factors For Determining Heat Pump Wire Size

Wire size selection depends on several variables: the unit’s nameplate full-load amps (FLA), compressor locked-rotor amps (LRA) for short-time sizing, supply voltage (120/240/208/480), circuit type (single-phase or three-phase), and the length of the run.

Full-Load Amps (FLA)

Always start with the manufacturer’s nameplate FLA. The NEC requires conductor ampacity to be at or above the unit’s continuous and noncontinuous load requirements, with adjustments for continuous operation where applicable.

Locked-Rotor Amps (LRA) And Short-Time Ratings

LRA is important for sizing short-time delay fuses or time-delay breakers that protect the compressor during startup. LRA does not determine continuous conductor sizing but influences overcurrent protection selection.

NEC Rules And Sizing Basics

The National Electrical Code (NEC) provides rules for HVAC equipment: conductors must have ampacity not less than 125% of the continuous load and not less than 100% of noncontinuous loads. Many heat pumps are considered continuous loads when they run for three hours or more.

Typical NEC steps: use nameplate FLA, multiply continuous load by 125% for conductor ampacity, select a breaker or fuse per NEC tables and manufacturer requirements.

Common Wire Sizes For Typical Residential Heat Pumps

Practical wire gauge recommendations vary by unit size and local code. Below are common pairings for copper conductors used in single-phase residential installations.

Heat Pump Circuit Amps	Common Copper Wire (THHN)	Notes
Up To 15 A	14 or 12 AWG	12 AWG preferred for short runs; 14 AWG limited by breaker size
15–20 A	12 AWG	Used for small mini-splits and low-capacity units
20–30 A	10 AWG	Typical for 1.5–3 ton units depending on nameplate
30–40 A	8 AWG	Common for 3–4 ton units
40–60 A	6 AWG	Used for 4–5+ ton and high-startup models

Always consult the unit’s nameplate and installation manual. Manufacturer instructions can require larger conductors or specific overcurrent protective devices.

Voltage Drop And Long Runs

Voltage drop becomes significant on long conductor runs and can reduce motor performance or cause nuisance trips. For motors, NEC recommends limiting voltage drop to around 3% for branch circuits and 5% total for feeder plus branch circuits.

Calculate voltage drop using: Voltage Drop = 2 × Length × Current × Resistance per foot (for single-phase). Use wire resistance tables to determine conductor resistance at operating temperature.

Approx. Voltage Drop At 25A	100 Feet	200 Feet
10 AWG Copper	~2.0% (at 240V)	~4.0%
8 AWG Copper	~1.3%	~2.6%

If voltage drop exceeds acceptable limits, increase conductor size to maintain proper voltage and motor starting capability. In many cases, one larger AWG size can correct a long-run voltage drop problem.

Overcurrent Protection: Breakers And Fuses

Overcurrent protection must match both NEC rules and manufacturer requirements. Use time-delay (slow-blow) fuses or HACR-rated breakers where specified. Breaker sizing often uses the greater of 125% of continuous load or the manufacturer’s recommended maximum-fuse size for conductors.

Example: A unit with a 32 A FLA might require conductors sized for 40 A (125% rule) and an overcurrent device sized per the nameplate or NEC tables, commonly 40–45 A.

Grounding And Equipment Bonding

All heat pumps require equipment grounding conductors sized per NEC Table 250.122 based on the rating of overcurrent protection. Grounding conductors should be continuous from the outdoor unit to the service ground and bonded to the unit cabinet.

Use green or bare conductors for equipment grounding and ensure connections are tight and corrosion-resistant, especially in outdoor locations.

Conduit, Cable Type, And Insulation

Choose cable types and conduit based on installation environment. THHN/THWN conductors in conduit are common for outdoor units. Use jacketed SE or NM cable only where allowed and protected from moisture and sunlight.

Consider conductor insulation temperature ratings; 90°C conductors permit higher ampacity but overcurrent protection and terminations must be rated to support higher temperatures.

Control And Thermostat Wiring

Control wiring for thermostats and communication (e.g., 24VAC) uses low-voltage cable, typically 18–22 AWG. For heat pump systems with multiple zones or controls, run dedicated control conductors and follow manufacturer wiring diagrams to avoid interference.

For powered accessories (crankcase heaters, reversing valves with heavy loads), check if separate circuits or relays are required to avoid overloading control transformers.

Mini-Splits And Multi-Zone Systems

Mini-split systems often have smaller power requirements and can use 12 or 10 AWG for indoor/outdoor connections, but large multi-zone condensers may require 6 AWG or larger feeders. Verify each indoor/outdoor pairing’s wiring instructions.

Some mini-splits use power and control integrated in a factory cable; always use the supplied wiring harness or approved equivalents.

Common Installation Mistakes To Avoid

• Undersizing conductors by ignoring manufacturer nameplate FLA and NEC ampacity rules.
• Failing to compensate for voltage drop on long runs, leading to poor compressor starting.
• Using incorrect overcurrent protection type (e.g., fast-blow fuses where time-delay is required).
• Poor grounding or loose equipment bond connections causing safety and nuisance-trip issues.
• Running outdoor conductors without proper conduit or UV-rated cable protection.

Permits, Inspections, And Code Compliance

Most jurisdictions require permits and inspections for new heat pump electrical work. Local amendments to the NEC may impose additional requirements, so obtain permits and have work inspected for safety and insurance compliance.

Hiring a licensed HVAC electrician or contractor ensures compliance with NEC, UL listings, and manufacturer warranty conditions.

Troubleshooting Wire-Related Problems

Symptoms of wiring issues include frequent breaker trips, poor heating/cooling performance, humming or hard starting compressors, or burned connector smells. Use a clamp meter and voltmeter to verify supply voltage, current draw, and continuity before replacing components.

If voltage at the unit is more than a few percent below service voltage under load, check connections, tighten lugs, and consider upsizing the conductor to mitigate voltage drop.

When To Upgrade Existing Wiring

Consider upgrading when replacing a unit with a larger capacity, when long runs cause voltage drop, or when older aluminum or undersized wiring is present. Upgrades often require new breakers, conduit, and grounding improvements.

For aluminum conductors, follow special termination practices and consider replacing with copper for long-term reliability, per NEC and manufacturer guidance.

Useful Resources And References

Consult these resources for authoritative guidance: the National Electrical Code (NEC), manufacturer’s installation manuals, local building codes, and HVAC industry reference guides. Professional electricians can interpret NEC tables and adapt them to specific installations.

Online resources: NFPA (NEC), AHRI, and major manufacturers’ installation manuals are recommended for load calculations and wiring diagrams.

Frequently Asked Questions

How To Determine The Right Wire Size For A Specific Heat Pump?

Check the unit nameplate for FLA and maximum fuse/breaker sizes, apply NEC 125% rule for continuous loads, calculate voltage drop for the run length, then select the smallest AWG meeting ampacity and voltage drop limits.

Can I Use Aluminum Wire For A Heat Pump?

Aluminum is acceptable when installed per NEC requirements and with anti-oxidant compounds and correct lugs, but copper is preferred for better conductivity and smaller conductor sizes.

Is It Necessary To Upsize Wire For Long Runs?

Yes. Long runs can produce voltage drops that impair motor starting. Upsize conductor(s) to keep voltage drop under target limits, often 3% on branch circuits.

Should A Heat Pump Have A Dedicated Circuit?

Yes. Heat pumps require a dedicated circuit with appropriately sized conductors and overcurrent protection per manufacturer instructions and NEC rules.

Installation Checklist

• Read the heat pump nameplate and installation manual.
• Calculate conductor ampacity using FLA and NEC 125% rule.
• Account for voltage drop on long runs and increase wire size if needed.
• Select proper overcurrent device type and rating per manufacturer and NEC.
• Install correct grounding and bond conductor sized per NEC Table 250.122.
• Use appropriate cable type, conduit, and weatherproof disconnects for outdoor equipment.
• Obtain permits and schedule inspections to ensure code compliance.