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Electric Heat Strip kW to Amps Calculator

Find the current and heat output of an electric heat kit — including how much less heat a 240 V heater gives on a 208 V supply.

Method reviewed October 26, 2026. Version 1.0.0. How it works

Your inputs

From the heater kit nameplate, at its rated voltage.

A heater rated at 240 V connected to a 208 V supply puts out less heat.

Result

Current
41.7A
Output at the actual voltage
10kW
Heat output
34,121BTU/h

41.7 A and 34,121 BTU/h (10 kW) at 240 V.

  • For wire and breaker sizing use the minimum circuit ampacity and maximum overcurrent protection printed on the equipment nameplate, and the electrical code that applies — not this current.
Share of rated output
100%
Show the working
  1. A resistance heater’s power varies with voltage squared (P = V² ÷ R)
    10 × (240 ÷ 240)² = 10 kW
  2. Current = W ÷ V
    10,000 ÷ 240 = 41.67 A
  3. Heat = W × 3.412
    10,000 × 3.412 = 34,121 BTU/h

Amps from kilowatts

For a single-phase resistance heater, current = watts ÷ volts: a 10 kW heater at 240 V draws about 41.7 A. For a balanced three-phase heater, current = watts ÷ (1.732 × line-to-line volts). Every watt becomes heat: 1 kW = 3,412 BTU/h.

Why 208 V gives less heat

A heating element is a fixed resistance, so its power follows P = V² ÷ R. Run a 240 V element on 208 V and it delivers (208 ÷ 240)² = 75 % of its rating: a 10 kW strip gives about 7.5 kW (25,600 BTU/h) and draws about 36 A.

That matters when sizing backup heat for a heat pump in a building with a 208 V supply.

Worked example

These are the example values the calculator opens with, and the result the engine calculates for them.

Inputs

Rated heater output
10 kW
Rated voltage
240 V
Actual supply voltage
240 V
Supply
Single-phase

Result

Current
41.7 A
Output at the actual voltage
10 kW
Heat output
34,121 BTU/h
Share of rated output
100 %

41.7 A and 34,121 BTU/h (10 kW) at 240 V.

Assumptions

  • The heating element is a constant resistance, so output scales with the square of the voltage.
  • Three-phase heaters are balanced.

Limitations

  • Element resistance changes slightly with temperature; nameplate values govern.
  • Does not size wire or overcurrent protection.

Frequently asked questions

How many amps is a 5 kW heat strip?

About 20.8 A at 240 V, or about 18 A (with 3.75 kW output) if the 240 V strip runs on 208 V.

Can I use this to size the breaker?

No. Use the minimum circuit ampacity and maximum overcurrent protection on the equipment nameplate and the electrical code that applies.

Does the blower motor add to the current?

Yes. This tool gives the heater only; the air handler’s nameplate lists the total.

Sources

  1. University Physics Volume 2, §9.4 Ohm’s Law — OpenStax, Rice University (CC BY 4.0). V = IR; electrical power P = IV.
  2. NIST Guide to the SI (SP 811), Appendix B.9 — conversion factors — National Institute of Standards and Technology. 1 ton of refrigeration = 12,000 BTU/h; 1 BTU/h = 0.2930711 W; 1 atm = 101.325 kPa.

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