HVAC Contactor Selection Guide: Amp Ratings, Pole Count & Coil Voltage
April 27, 2026 · Shopify API
· CONTACTOR, ELECTRICAL, REFERENCE
What a Contactor Does
The contactor is an electrically operated switch that connects line voltage (208-240VAC or 460VAC) to the compressor and condenser fan motor in the outdoor unit. When the thermostat calls for cooling, the 24VAC control board energizes the contactor coil, which creates a magnetic field that pulls the contacts closed, completing the high-voltage circuit. The coil draws only 0.5-1A at 24V; the contacts carry up to 80A of compressor load.
Contactor Selection Parameters
1. Pole Count (1-Pole vs 2-Pole)
Type
Poles Switched
Application
Safety Consideration
1-Pole
Switches L1 only
Some older residential units (pre-2002), heat pumps
L2 remains live even when contactor is open - still energizes compressor windings and fan motor partially
2-Pole
Switches L1 and L2
All modern residential, commercial
Both legs of 240V disconnected when open - safer for service work
Best practice: Always replace a 1-pole contactor with a 2-pole when servicing modern equipment. Most equipment manufacturers specify 2-pole in current service literature.
2. Coil Voltage
The coil voltage must match the control voltage of the system. Virtually all residential and light commercial HVAC uses 24VAC coils. Commercial systems may use 120VAC or 240VAC coils. Check the control board output voltage if unsure - never assume.
24VAC: Standard residential and light commercial
120VAC: Some commercial rooftop units, older equipment
240VAC: Some larger commercial systems
3. FLA / LRA Ratings
FLA (Full Load Amps): The current drawn by the compressor under normal operating conditions. Find on the compressor nameplate or equipment data plate.
LRA (Locked Rotor Amps): The surge current when the compressor starts (motor is briefly stalled). Can be 5-7° FLA. The contactor must handle LRA without welding its contacts.
Select a contactor with a continuous current rating at or above the system FLA. Ensure the LRA rating is not exceeded by the compressor's locked rotor current.
Contactor Sizing by Equipment Tonnage
Equipment Tons
Typical FLA (230V)
Typical LRA
Recommended Contactor
1.5 ton
9-11A
55-65A
20-25A, 2-pole, 24V coil
2 ton
11-14A
65-75A
25-30A, 2-pole, 24V coil
2.5 ton
13-16A
70-85A
30A, 2-pole, 24V coil
3 ton
15-19A
80-95A
30-35A, 2-pole, 24V coil
3.5 ton
18-22A
90-105A
35-40A, 2-pole, 24V coil
4 ton
20-25A
100-120A
40A, 2-pole, 24V coil
5 ton
24-30A
115-140A
40-50A, 2-pole, 24V coil
Inspecting a Contactor for Failure
Visual Signs of a Failed Contactor
Pitted or burned contact faces: The silver alloy contact surfaces erode with each arc on make/break. Deep pitting = replace now.
Welded contacts: Contacts stick closed - compressor runs continuously even when thermostat is satisfied. Unit freezes up.
Burned or cracked housing: Sign of severe arcing or overcurrent event. Replace immediately.
Insect nesting in coil: Ants and other insects nest inside contactors, blocking the plunger from seating fully. Clean and inspect.
Electrical Testing
Coil test: With power OFF and coil leads disconnected, measure coil resistance. A healthy 24VAC coil reads 8-20?. OL (open) = failed coil. Near 0? = shorted coil.
Contact continuity test (open state): With coil de-energized and power OFF, measure resistance across each contact set (L1 to T1, L2 to T2). Should read OL (open). Any continuity = welded contacts.
Contact continuity test (closed state): Manually press the contactor plunger in (power OFF). Resistance across each contact set should be below 0.1?. High resistance = corroded or pitted contacts.
Voltage drop test (live - advanced): With contactor energized and running, measure voltage across each contact set (L1 to T1 while energized). Should be less than 1V. More than 2V drop across a contact = high resistance from pitting - replace.
Coil Resistance Reference Table
Coil Voltage Rating
Normal Resistance Range
24 VAC
8 - 20 ?
120 VAC
200 - 600 ?
240 VAC
800 - 1,800 ?
Common OEM Contactor Part Numbers
Brand
Part Number
Specs
Carrier / Bryant
HN51KC024
30A, 2-pole, 24VAC coil
Carrier / Bryant
HN51KC040
40A, 2-pole, 24VAC coil
Goodman / Amana
CONT2P030024VS
30A, 2-pole, 24VAC coil
Honeywell / Mars
42-25100-01
30A, 2-pole, 24VAC coil (universal)
Rheem / Ruud
42-25101-03
40A, 2-pole, 24VAC coil
Pro Tips
Always discharge the capacitor before working near the contactor - the capacitor can hold lethal voltage even with power disconnected
Replace the contactor and capacitor together as a matched set - if one has failed, the other is typically near end of life
Photograph wiring before disconnecting - L1/L2 are line side, T1/T2 are load side; swapping them does not matter electrically but can cause confusion
For systems with a hard start kit, the start relay connects to the S (start) terminal on the compressor - separate from the contactor circuit
Ants inside a contactor are the #1 cause of nuisance service calls in the southern US - install an ant guard or check during every PM visit