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Calcuating Amps

Too many Christmas lights plugged into an outlet or extension cord will cause the lights to fail. The number of amps consumed must be calculated to determine how many lights can be plugged end-to-end, into an extension cord, and into an electrical outlet.

Christmas lights consume a certain amount of power measured in Watts. Each product that carries electricity through it has an Amp rating. The power flowing through each product cannot exceed its rating. A simple equation can be used to calculate the number of Amps on a line.

Amperage Equation

Total Watts = Number of lights * Watts per light
Total Amps = Total Watts / 120 Volts

Alternately, if the load is provided in Amps:

Total Amps = Number of lights * Amps


The Amps consumed by a stringer with 125 C9 incandescent lights on a line: 

125 bulbs * 7 Watts = 875 Watts 
875 Watts / 120 Volts = 7.29 Amps

A tree with 20 incandescent mini lights:

20 strands * 0.2 Amps = 4 Amps

This stringer with 120 C9 incandescent bulbs pulls 7 Amps, for which the light cord, zip-line, and connectors are rated. Exceeding the 8 Amp rating on these products will cause failure.

Keep in mind the wattage of all the lights that are being used to make sure they are not overloading any extension cords or the breaker.

Amp Cheat Sheet

Amp ratings for selected products:

  • Cube Tap: 10 Amps
  • SPT-1 Connectors: 7 Amps
  • Lampholder Adapter: 6 Amps
  • 40' Extension Cord: 13 Amps
  • Grounded Triple Tap: 13 Amps
  • Photo-electric Timer: 15 Amps
  • SPT-1 Cord and Lamp Wire: 7 Amps

Watt or Amp consumption for selected products:

  • LED Minis: 2.4 Watts
  • Wreaths and Garland: vary
  • Incandescent Minis: 0.2 Amps
  • C9 Incandescent Bulb: 7 Watts
  • C7 Incandescent Bulb: 5 Watts
  • C9 LED Bulb: 0.4 Watts