Limiting Heat Lamp Inrush Current Protects Relay Contacts

limiting heat lamp inrush current

Limiting Heat Lamp Inrush Current

Incandescent heat lamps create a large inrush current when first powered on. Limiting heat lamp inrush current prevents relay damage and extends lamp life.

Heat lamps can be used to warm enclosed areas such as livestock pens and chicken coops. Banks of incandescent heat lamps are mounted overhead. As the number of heat lamps increases, limiting heat lamp inrush current becomes more important. Over current can damage relay contacts in control circuits.

A customer of ours installed heat lamps in a commercial chicken coop. He used four 250W heat lamps on 20A circuit and found that pitting and sticking of the 20A relay contacts had occurred. This prompted him to call us for advice regarding the inrush current problem.

A cold incandescent lamp has a low resistance but as the tungsten filament heats the resistance increases dramatically. After an incandescent heat lamp is powered on the filament takes only one tenth of a second to reach over 6,000 degrees Fahrenheit. During this time the resistance increases to 15 times that of the cold filament.

A single 5″ R40 reflective heat lamp which is rated at 250 watts and 120 VAC has a hot resistance of 58 ohms and a steady state current of just over 2 amps. However each lamp will produce an inrush current of around 31 amps for one tenth of a second when power is turned on.

The customer’s four lamps created a combined inrush current of 125 amps. He needed a way to limit inrush current to 20 amps for each branch to prevent damage to the relay contacts. We calculated the requirements for limiting inrush current of each lamp to 5 amps. Limiting inrush current saved the relay contacts and warmed the chicken coop.


Selecting the Correct Inrush Current Limiter for Heat Lamps

Heat Lamp Inrush Application Given:

  • Limiting inrush current is necessary 4 lamps on single 120VAC 20A circuit
  • Inrush current for an incandescent lamp is about 15x the steady-state current
  • Maximum allowable inrush current for this application is 5A for each lamp
  • Duration of the inrush current of incandescent lamps is 100 milliseconds
  • Lamp resistance during operation = input voltage squared / watt rating of lamp
  • Steady state current = input voltage / resistance of lamp during operation
  • Peak AC line voltage = 1.414 x VAC RMS

Inrush Calculation for Single Heat Lamp:

  • Resistance of heat lamp during operation = 120VAC² / 250W = 57.6Ω
  • Steady state current of singel heat lamp = 120VAC / 57.6Ω = 2.08A
  • Inrush current for each heat lamp based on our assumption = 15 x 2.08A = 31.2A
  • Minimum cold resistance of inrush current limiter = peak voltage / maximum allowable inrush current = (115 VAC x 1.414) / 5.0A = 34.0Ω
  • Energy in joules that inrush current limiter must handle = voltage x current x duration of the inrush = 120VAC x 31.2A x 0.100S = 374.4J

Heat Lamp Inrush Current limiter Solution:

  • Minimum resistance of current limiter: 34Ω
  • Steady-state current: 3A
  • Energy dissipation: 375J

Select an Ametherm Inrush Current Limiter:

  • Browse the Ametherm Full Line of Inrush Current Limiters web page for the part that matches the resistance, current, and joules ratings as shown in the step above
  • Note: Multiple inrush current limiters of the same part number can be used so that the resistance or joules ratings when added will attain the minimum values required in the Heat Lamp Inrush Current Limiter Solution section above
  • Install inrush current limiter(s) in-series between the main power and heat lamp power lead


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