🖌 Conductor Sizing

Wire Size Calculator

Size copper conductors for HVAC equipment from minimum circuit ampacity, run length, maximum voltage drop, and insulation type. Covers single and three-phase circuits.

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⚡ Circuit Configuration

📋 Circuit Requirements

Minimum Circuit Ampacity (MCA) i

Circuit Voltage

One-Way Run Length

Maximum Voltage Drop i

🌡 Insulation Temperature Rating

🖌 Wire Size Results

AWG

🖌 Recommended Wire Size

AWG

Enter values and calculate

Sizing Breakdown

Full Calculation Table

Value	Basis	Result

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How to Use the Wire Size Calculator

Select Phase Configuration

Three-phase circuits draw less current than single-phase for the same power, which changes both the ampacity and voltage drop calculation.

Enter MCA, Voltage, Run Length, and Voltage Drop Limit

Use the MCA / MOCP calculator first if you don't already have an MCA value. Measure the one-way run length, not the round trip.

Select Insulation Temperature Rating

75°C (THWN) is the standard choice for most HVAC equipment circuits. Only use 90°C if every termination in the circuit is also rated for it.

Calculate

The calculator checks both ampacity and voltage drop, then recommends whichever wire size satisfies both requirements.

Sizing Conductors for HVAC Circuits

Picking the right wire size for an HVAC circuit means satisfying two separate, unrelated requirements at the same time: the conductor has to carry the circuit's ampacity without overheating, and it has to keep voltage drop within an acceptable limit over the actual run length. A wire that passes one check can still fail the other, which is why this calculator checks both before giving a final recommendation.

The Ampacity Check

Every AWG wire size has a maximum continuous current rating, called ampacity, that depends on the conductor's insulation temperature rating. CEC Table 2 and the equivalent NEC Table 310.16 list these values for 60°C, 75°C, and 90°C insulation. The conductor's ampacity at the chosen temperature rating has to be at or above the circuit's MCA, which is the minimum starting point before voltage drop is even considered.

Why Voltage Drop Can Force a Bigger Wire

Voltage drop happens because every length of wire has electrical resistance, and that resistance causes a small voltage loss proportional to both the current and the distance. A short 3-metre circuit might never come close to the voltage drop limit even at the smallest code-compliant wire size, but the same circuit run 60 metres can lose enough voltage to require stepping up one or two wire sizes beyond what ampacity alone would require. CEC Rule 8-102 recommends keeping voltage drop to 3% on a branch circuit and 5% total including any feeder, since excessive voltage drop causes motors to run hotter and less efficiently, and can shorten compressor life.

Single-Phase vs. Three-Phase Voltage Drop

Single-phase voltage drop calculations use a factor of 2 to account for both the outgoing and returning conductor, while three-phase calculations use a factor of the square root of 3 (about 1.732) instead, reflecting how current splits across three conductors rather than two. This means a three-phase circuit generally experiences less voltage drop than an equivalent single-phase circuit carrying the same current over the same distance.

Choosing an Insulation Temperature Rating

THHN and THWN-2 conductors are dual-rated for 90°C in dry locations and 75°C in wet locations, which makes the 75°C ampacity column the practical standard for most outdoor HVAC equipment circuits, since most breakers and equipment terminals are only rated for 75°C. Using the full 90°C ampacity column to select a smaller wire is only valid when every termination point in the circuit, including the breaker terminal and the equipment's own terminal block, is also rated for 90°C, which isn't the case for most standard equipment.

Next Steps

Once you've confirmed wire size, check the conduit fill calculator to make sure the conductor fits the conduit, and the breaker size calculator to confirm the matching overcurrent protection.

Frequently Asked Questions

Ampacity and voltage drop are two separate checks, and either one can drive the final wire size. A short run might only need the ampacity table's minimum size, but a long run carrying the same current can need a noticeably larger conductor just to keep voltage drop within the allowed limit, since resistance and voltage drop both increase with conductor length.

THHN and THWN-2 conductors are dual-rated for 90 degrees C in dry locations and 75 degrees C in wet locations, making the 75 degrees C ampacity column the standard choice for most outdoor HVAC equipment circuits where terminations are rated for 75 degrees C, which covers the vast majority of breakers and disconnects. Only use the 90 degrees C column if every termination point in the circuit, including the breaker and equipment terminals, is also rated for 90 degrees C.

Yes. CEC's small conductor rule caps the overcurrent protection for 14, 12, and 10 AWG conductors at 15A, 20A, and 30A respectively for general use circuits, even though the ampacity table shows a higher value at 75°C or 90°C insulation. This calculator's ampacity check uses the full table value, so always confirm the small conductor rule doesn't apply to your specific circuit before finalizing the breaker size.

Enter the one-way distance from the panel to the equipment. The voltage drop formula already accounts for the return path internally through its multiplier (2 for single-phase, the square root of 3 for three-phase), so entering the round-trip distance would double-count that factor and produce an inflated result.