ElectricalToolbox

Can You Plug a Power Strip Into an Extension Cord for Halloween?

No. A power strip is made to plug directly into a wall outlet, not into the end of an extension cord. The safety listing on the strip says so, and there is a physical reason behind it: a strip on a cord puts two long runs of wire in series, which piles up voltage drop and creates one more hot, overloaded connection out in the weather. If you need a strip out in the yard, run one properly sized outdoor cord to a single outdoor-rated strip plugged into it as its only load, or better, feed each device with its own cord. The worked example below shows how much voltage a long cord eats before the strip even sees it.

Check the run before you trust it. Put your cord length, gauge, and the load into the Extension Cord Voltage Drop Calculator and it will tell you the voltage actually reaching the far end, which is the number a power strip hides from you.

Why the listing says not to

A power strip is a listed product called a relocatable power tap. Under its safety standard (UL 1363), it is meant to be connected to a permanently installed branch-circuit receptacle, in other words a real wall outlet, and not to an extension cord or to another power strip. OSHA treats a strip fed by an extension cord as the same hazard as chaining two strips. The National Electrical Code (NEC) backs the idea up from the other direction: flexible cords are for temporary, portable use and may not be used as a substitute for a building's permanent wiring (NEC 400.8), and equipment has to be used the way it is listed (NEC 110.3(B)).

The practical danger is heat at a connection nobody is watching. Every plug-and-socket joint has a little resistance, and resistance under load makes heat. A strip on the end of a cord adds a joint, usually the one lying in the wet grass, and loads it with everything plugged into the strip. It also strips away the coordination that keeps a circuit safe: the breaker in the panel is now protecting a long, thin cord it was never sized for.

The hidden cost: voltage drop across the cord

Even setting the listing aside, a long cord feeding a loaded strip runs into voltage drop. Wire has resistance, and the longer and thinner the cord, the more voltage it loses on the way out. A strip makes this worse by inviting you to hang several things off one long run. The standard way to figure it, and the method the calculator uses, is:

Voltage drop = 2 × K × I × L ÷ CM

K = 12.9 (copper), CM = 2,580 circular mils for 16 AWG (NEC Chapter 9, Table 8)

The 2 accounts for the current going out and coming back, I is the load in amps, and L is the one-way cord length in feet.

Worked example: a strip at the end of a 100-foot cord

Say you run a 100-foot, 16-gauge outdoor extension cord out to a power strip and load the strip to 10 amps of decorations, on a 120-volt circuit:

Voltage drop = 2 × 12.9 × 10 × 100 ÷ 2,580

Voltage drop = 25,800 ÷ 2,580 = 10.0 volts

Percent drop = 10.0 ÷ 120 = 8.3%

Voltage at the strip: 120 - 10 = 110 volts

At 8.3 percent, that is well past the 3 percent a long run should stay under, and every decoration on the strip is running on 110 volts instead of 120. Motors in inflatables and fog machines run hotter and pull more current on low voltage, and incandescent strings dim. Step the same run up to a 12-gauge cord and the drop falls to under 4 volts. But the cord gauge is only half the problem. The strip is still the wrong tool for the end of a long cord: it is unlisted for the job and it invites you to overload that thin run. Use the calculator to size a single cord to a single load, and skip the strip.

The safe way to reach the yard

  • Run one cord to one device where you can. A single outdoor-rated cord sized to the load, going straight to the inflatable or the light string, is the cleanest setup.
  • If you must fan out, use an outdoor-rated strip or a listed outdoor power block, plugged into the wall, not the cord. Then run the cord from the wall side, not stacked behind a strip.
  • Size the cord to the run, not just the plug. Longer runs need a heavier gauge to hold voltage. Check it in the voltage drop calculator before you commit the layout.
  • Keep every connection up off wet ground and covered. The joint is the weak point, and a strip adds one more of them.
  • Protect it all with GFCI, and call a licensed electrician for a permanent outdoor circuit if the display keeps outrunning the outlets you have.

Standards and NEC reference

NEC 2020. Power strips are relocatable power taps listed to UL 1363, which intends them to plug directly into a permanently installed receptacle, not an extension cord or another tap. Equipment must be used per its listing (NEC 110.3(B)), and flexible cords may not substitute for permanent wiring (NEC 400.8). The voltage-drop math uses the K-factor method with copper resistance and the 16 AWG conductor area from NEC Chapter 9, Table 8; the 3 percent figure is a common field guideline for a cord run, not an NEC requirement. OSHA cites a power strip fed by an extension cord as a hazard. Load values are typical product figures; your own labels govern.

Results are for reference only. Verify against the applicable adopted edition of the NEC and consult a licensed electrician for code compliance.

For the whole season

The Holiday Electrical Safety Handbook

This guide answers one question. The Handbook covers Halloween through New Year's: how many amps a circuit can really carry (a 15 amp breaker means 12 amps, a 20 means 16), extension cords, GFCI outlets, outdoor displays in the weather, the Thanksgiving kitchen, tree lights, space heaters, and what to do when something goes wrong. Every figure is pre-converted to amps, so you add instead of calculate.

Read on Kindle Paperback on Amazon Free circuit map and load worksheet

Get the next seasonal safety check by email

Optional. Short emails before each holiday, written for homeowners.

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