Extension Cord Amp Rating & Safety Check
Check if your extension cord is rated for the load.
Enter AWG gauge, cord length, and device wattage to verify amp capacity and calculate voltage drop.
Extension cords are the most misused electrical product in the house. Run a heavy load through an undersized one and the resistance turns into heat inside the jacket, which is a fire risk. It is much worse for a cord still wound on its reel, where the turns warm each other and nothing can shed heat.
Gauge and amp ratings
Cord gauge uses AWG, where lower numbers mean thicker wire. The capacity also depends on how many current-carrying conductors the cord has, which is the part most tables leave out:
| AWG | 2-wire cord | 3-wire grounded cord | Typical use |
|---|---|---|---|
| 18 | 10 A | 7 A | Lamps, chargers, small electronics |
| 16 | 13 A | 10 A | Household electronics, light tools |
| 14 | 18 A | 15 A | Power tools, appliances |
| 12 | 25 A | 20 A | High-draw tools on long runs |
| 10 | 30 A | 25 A | Heavy duty and site work |
Those come from NEC Table 400.5, columns A and B. Almost every orange cord in a garage is the 3-wire kind, so the right-hand column is the one that applies, and it is meaningfully stricter. A 16 AWG grounded cord is a 10 amp cord, not a 13 amp one, whatever the packaging on a similar-looking 2-wire cord says.
Manufacturers commonly advise stepping up one gauge past 50 feet on heavy loads. That is sensible practice rather than a code rule, and the voltage drop figure below is the reason for it.
Voltage drop
Voltage drop = current × 2 × length (ft) × resistance per foot
Resistance per 1,000 ft of stranded copper, from NEC Chapter 9 Table 8: AWG 18 = 7.95 Ω, 16 = 4.99, 14 = 3.14, 12 = 1.98, 10 = 1.24.
The 2 is there because current travels out along one conductor and back along the other, so a 50-foot cord is 100 feet of wire. Aim to keep the drop under 3%; past 5% motors strain, tools lose power and lights dim visibly.
The mistake that starts fires
A space heater, shop vacuum or compressor on a light 16 or 18 AWG cord. Those draw 10 to 15 amps continuously, which is at or past what a thin grounded cord can take, and continuously is the operative word. A drill pulling 12 amps for ten seconds is fine; a heater pulling 12 amps for four hours is not.
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