Are Christmas lights in series or parallel?

December 7, 2018

Short answer:

Traditional incandescent Christmas tree lights and modern LED light strings that are wired with a 2 or 3-harness pre-wired configuration are wired in series. Traditional C7 and C9 bulbs and cords that can be purchased separately (where, for example,  the C7 bulbs have a screw-in base the same size as a nightlight) and the cord is heavy duty 18 AWG wire  – these are wired in parallel.

See this photo.

Long Answer:

What does how Christmas lights are wired mean to me?

Light wired in series

  • all the bulbs are wired in a single complete circuit with content current
  • If a bulb is pulled out of it’s socket, the string goes out
  • Since all the bulbs are rated for similar current, incandescent and LED series strings can be run in the same circuit
  • the string cannot be shortened or lengthened so extra lights just need to be rolled and tucked away
  • sockets can’t be skipped

If a light wired in series goes out:

  1. check the fuse, then don’t run so many in series before plugging back into power
  2. check to make sure the bulbs are all properly seated, present and accounted for
  3. check the wiring harness for damage, if the wires are damaged, harvest a few bulbs to act as replacements in other identical sets and discard the light string – it’s toast
  4. replace bulbs as soon as they go out since the same amount of current is now running through all the bulbs and will burn them out faster (a good thing to do every season, in fact, this is the topic of an upcoming blog post).

Lights wired in parallel

  • All the bulbs are wired at constant voltage
  • If a bulb is removed, the circuit isn’t broken
  • LED and incandescent bulbs can be installed in the same circuit if that is the desired look
  • the wire can be cut and properly terminated to fit a specific length
  • sockets can be skipped and wrapped with a couple turns of electrical tape

If lights wired in parallel go out:

  1. check the fuse – then touch base with an electrician to make sure the wattage of your bulbs is not exceeding the rated current for the cord and that your cord/total circuit is not longer than 250 feet for 18 AWG wire
  2. check the wiring harness for damage, squirrels and attic heat during storage can take a toll. If a section is damaged, repair the string by splicing in a repair using gilbert plugs or have an electrician take care of that
  3. visually examine the insulation of the wiring, if it is uniformly degraded and the cord has seen better days, pull the bulbs and start with a new cord


Aside from throwing dinner parties that feature at least two kinds of cheese dip, Shellie's passions include travel, Mid-Century Modern furnishings and finding the perfect street taco. Has been known to snort laugh champagne.

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