To convert a standard 12,000 BTU/hr load to amps on a 120V circuit (assuming a resistive space heater with a Power Factor of 1.0), the draw is 29.3 amps. However, if that 12,000 BTU rating belongs to a modern window air conditioner with an Energy Efficiency Ratio (EER) of 10, the electrical input is only 1,200 watts, pulling 10 amps at 120V. The universal formula to convert BTU/hr to amps is:

Amps = (BTU/hr ÷ 3.412) ÷ (Volts × Power Factor × Efficiency)

Substituting the values for a 12,000 BTU/hr resistive heater at 120V:

Amps = (12,000 ÷ 3.412) ÷ (120 × 1.0 × 1.0) = 3,516.85W ÷ 120V = 29.3A

The Core Assumptions: Why Raw Math Fails in the Field

British Thermal Units (BTU) measure thermal energy, while Amperes measure electrical current. You cannot convert one to the other without bridging the gap with time (BTU per hour) and electrical physics. The exact amp draw is fixed by three assumptions:

  • Voltage & Phase: Doubling the voltage (120V to 240V) halves the amperage for the same wattage. Three-phase power introduces the square root of 3 (1.732) into the denominator, drastically dropping the current per leg.
  • Power Factor (PF): Resistive loads (like baseboard heaters or toaster ovens) have a PF of 1.0. Inductive loads (like AC compressor motors) have a PF between 0.80 and 0.90, meaning they draw more current to achieve the same real power (Watts).
  • Efficiency (EER/COP): This is where most DIYers get burned. An air conditioner doesn't create cold; it moves heat. A 12,000 BTU/hr mini-split might only consume 1,000 electrical watts to move 12,000 BTUs of thermal energy, yielding a Coefficient of Performance (COP) of roughly 3.5. According to the U.S. Department of Energy, modern SEER2 ratings dictate how efficiently this transfer happens.
When the Conversion is Meaningless: If you are sizing wire for a heat pump and you do not know the EER/COP, or if you lack the Power Factor for a large 3-phase blower motor, a raw mathematical conversion will give you a dangerously wrong answer. Always defer to the manufacturer's nameplate Minimum Circuit Ampacity (MCA) for motorized HVAC equipment.

Master BTU/hr to Amps Sizing Chart

The table below demonstrates how the amp draw shifts across common voltages and load types. The 120V and 230V resistive columns assume pure heating elements (PF 1.0). The motor columns assume a standard compressor load (PF 0.85). The NIST Guide to the SI confirms the exact conversion factor of 1 Watt = 3.41214 BTU/hr used in these calculations.

BTU/hr Rating Thermal Watts (100% Eff.) Amps @ 120V (1φ, PF 1.0) Amps @ 230V (1φ, PF 1.0) Amps @ 230V (Motor, PF 0.85) Amps @ 480V (3φ, PF 0.85)
5,000 1,465 W 12.2 A 6.4 A 7.5 A 2.1 A
8,000 2,344 W 19.5 A 10.2 A 12.0 A 3.3 A
10,000 2,930 W 24.4 A 12.7 A 15.0 A 4.2 A
12,000 3,517 W 29.3 A 15.3 A 18.0 A 5.0 A
15,000 4,396 W 36.6 A 19.1 A 22.5 A 6.2 A
18,000 5,275 W 43.9 A 22.9 A 27.0 A 7.5 A
24,000 7,033 W 58.6 A 30.6 A 36.0 A 10.0 A
36,000 10,550 W 87.9 A 45.9 A 54.0 A 15.0 A

Note: For the 3-phase 480V column, the formula used is Amps = Watts ÷ (√3 × Volts × PF). This is why large commercial rooftop units run on 480V 3-phase—the current per leg is remarkably low, allowing for smaller gauge THHN wire in conduit.

Neighboring Values & NEC Breaker Sizing

When sizing a branch circuit, you rarely hit an exact round number. Below is a ±20% spread around the common 12,000 BTU/hr baseline, calculated for a 240V resistive heater (PF 1.0). We have also included the required breaker size based on NEC Article 210.20, which mandates that continuous loads (those running for 3 hours or more) must be derated to 125% of the circuit's capacity.

BTU/hr Load (±20%) Thermal Watts Actual Amps @ 240V NEC Min Breaker (125% Rule) Recommended Wire (Copper 75°C)
9,600 2,813 W 11.7 A 15 A 14 AWG
10,800 3,165 W 13.2 A 20 A 12 AWG
12,000 3,517 W 14.6 A 20 A 12 AWG
13,200 3,868 W 16.1 A 25 A 10 AWG
14,400 4,220 W 17.6 A 25 A 10 AWG
Pro-Tip: A 12,000 BTU/hr baseboard heater pulls 14.6A. While a 15A breaker technically exceeds 14.6A, the 125% continuous load rule requires 14.6A × 1.25 = 18.25A. Therefore, you must step up to a 20A breaker and 12 AWG wire.

Edge Cases: Inverter Compressors and Heat Pumps

If you are wiring a modern ductless mini-split, throw the raw BTU-to-Amps math out the window. The EPA Energy Star program notes that inverter-driven compressors modulate their speed. A 12,000 BTU/hr mini-split might have a nameplate Maximum Overcurrent Protection (MOP) of 20A, but its actual running amp draw at partial load might only be 2.5 amps.

Why does my nameplate show lower amps than the math?

Because the manufacturer is utilizing a high COP (Coefficient of Performance). In heating mode, a heat pump might output 12,000 BTU/hr while drawing only 900 watts (3.75 amps at 240V). However, if the outdoor temperature drops below the unit's threshold, the internal PTC resistance strip heaters will engage. This is why you must always size your wire and breaker to the nameplate MCA (Minimum Circuit Ampacity), which accounts for the worst-case scenario of the compressor and backup electric heat running simultaneously.

Can I use this for 3-phase commercial RTUs?

Yes, but you must account for the square root of 3 (1.732). A 36,000 BTU/hr (3-ton) commercial rooftop unit on a 480V 3-phase supply draws roughly 15 amps per leg at a 0.85 Power Factor. If you mistakenly use single-phase math (Watts ÷ Volts), you will calculate 8.7 amps and severely undersize your disconnect switch and conduit fill.