Extension reel capacity check
Compare continuous holiday light watts to an extension-cord reel’s amp/watt rating and see remaining headroom.
amps = watts ÷ volts · ok if amps ≤ reel_amps × continuous_derate · headroom_W = limit_W − load_W
When to use
This tool helps when you want a extension reel capacity check planning figure with the assumptions written down. You will enter total light / load watts on the reel, voltage (v), reel amp rating, and other watts already on same reel.
Compare continuous holiday light watts to an extension-cord reel’s amp/watt rating and see remaining headroom. Keep units consistent (feet vs inches, watts vs amps), then use the formula box to see how each term contributes.
Stop and ask a licensed electrician when loads, hardwired outlets, or roof work go beyond temporary plug-in holiday displays.
From the reel label (often 10–15 A depending on AWG).
Coiled reels heat up — many planners stay under 80% and fully uncoil outdoors.
Worked example
Example inputs from the form defaults — Total light / load watts on the reel = 600; Voltage (V) = 120; Reel amp rating = 10; Other watts already on same reel = 0 — are plausible homeowner figures, not a survey of your roofline.
Compute with: amps = watts ÷ volts · ok if amps ≤ reel_amps × continuous_derate · headroom_W = limit_W − load_W. Show intermediates before the final count or cost.
When the calculator and scratch math disagree, fix units first; the extension reel capacity check model is only as good as consistent inputs.
Limits / assumptions
Units: keep every length in one system (feet or inches end-to-end) and every electrical quantity in the units the labels name. Mixing without converting invents a wrong answer.
Where the formula fails: odd roof geometry, wind on inflatables, worn sockets, and conditions not listed on the page are outside the model. Zero or nonsense inputs should error — do not force a number.
Not a substitute: the extension reel capacity check result is a planning aid. It is not an NEC determination, not a licensed electrician’s opinion, and not a manufacturer max-connect chart.
Not electrical advice. Fully uncoil outdoor reels when loaded, use GFCI protection, and obey the lowest rating among reel, cord, plugs, and breaker.
FAQ
Why derate?
Heat builds when cord stays on the drum under load; uncoiling and 80% planning reduce risk.
AWG matters?
Heavier gauge (lower AWG) usually allows higher reel ratings — trust the stamped label.
Shared with gauge picker?
Use the gauge picker when choosing cord size; this page checks an existing reel’s capacity.
15A breaker?
Circuit limit may be tighter than the reel — check outlet load / GFI headroom too.