To figure the load on an extension cord, add the amperage of the equipment that will run at the same time, then compare that total with the cord’s marked rating and the limits of the tools, connectors, and supply circuit. If a tool lists watts instead of amps, estimate its current by dividing watts by its rated voltage. Cord length, condition, motor startup, and the work environment also affect whether a cord is suitable.
How do you calculate amps from watts?
Read the tool or appliance nameplate first. Use its amp rating when one is provided. If it lists watts but not amps, estimate current with this formula:
Amps = watts ÷ volts
Use the equipment’s rated voltage—not an assumed voltage. For example, a hypothetical 1,200-watt device rated at 120 volts draws an estimated 10 amps while operating (1,200 ÷ 120). This is a basic estimate; equipment with motors can draw more current at startup or when working under load.
How do you add loads on one extension cord?
Add the current for the devices that will operate simultaneously. Do not add equipment that will not be used at the same time, but account for motor startup and operating conditions when deciding whether the cord is suitable.
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For example, if the 1,200-watt, 120-volt device above runs alongside a second device drawing 3 amps, the estimated combined running load is 13 amps (10 + 3). That arithmetic alone does not confirm that a cord or circuit can safely supply the load.
How many amps can an extension cord handle?
Check the cord itself for its marked amp or watt rating, wire gauge, length, and use or service markings. Choose a cord rated for at least the intended load and suitable for the tool and location. A gauge marking such as “12/3” identifies wire construction but does not, by itself, establish a complete amp rating for every cord, length, or use.
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There is no single universal gauge-and-length chart that can determine suitability for every cord. Compare the actual cord markings and the equipment requirements rather than relying on gauge alone.
Does extension cord length affect the load?
A longer cord can increase voltage drop, so use the shortest practical cord for the job. A government extension-cord guide gives a 100-foot 12/3 cord with a 1.8-kilowatt load as an example in warning that damage or pinching can still cause overheating; that example is not a general capacity chart.
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Inspect cords before use. Do not use cords that are damaged or pinched, and avoid tightly coiling a cord near capacity, since damage and reduced cooling can contribute to overheating or fire.
What other limits apply to the circuit?
The cord is only one part of the electrical path. The equipment, cord, plug and receptacle, and supplying circuit each have limits; the applicable safe load cannot exceed the lowest one. A cord with a high marked rating does not increase the capacity of the outlet or circuit.
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As a workplace-specific example, OSHA’s general-industry rule in 29 CFR 1910.304 sets a maximum cord-and-plug-connected load of 12 amps for a 15-amp receptacle on a 15- or 20-amp circuit, and 16 amps for a 20-amp receptacle on a 20-amp circuit. These are regulatory values for those stated configurations, not universal homeowner rules. See OSHA 1910.304.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How do you match the cord to a construction job?
Along with the marked load rating, check the cord’s length, service or use rating, connector configuration, grounding, and physical condition. Match its indoor or outdoor designation to the location, and use grounded three-wire construction where the tool or application requires it. For workplace use, OSHA construction requirements for extension-cord sets used with portable tools and appliances specify three-wire cords rated for hard or extra-hard use; these requirements apply within the scope of that standard, not as general consumer code instructions. See OSHA 1926.405 and its guidance on flexible cords.
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CPSC’s household extension-cord guidance covers construction and safety features including wire size, strain relief, polarity and continuity, and jacketed cords for outdoor cords. Its page, dated November 18, 2015, references UL 817; that guidance does not establish that any particular product listing is certified or compliant. See CPSC Extension Cords Business Guidance.
Quick check before plugging in
- Read each tool’s nameplate and use its amp rating, or estimate amps as watts divided by rated volts.
- Add the loads that will run at the same time, considering motor startup and loaded operation.
- Check the cord’s own amp or watt rating, gauge, length, and use markings.
- Confirm that the equipment, cord, connectors, and supply circuit can all handle the intended load.
- Use the shortest practical, undamaged cord suited to the tool and work environment; remove damaged cords from service.
The U.S. government’s extension-cord guide and NIOSH electrical safety training material explain the load-calculation approach and cord-safety considerations. For unusual or high-current installations, follow equipment instructions and applicable local requirements, or consult a qualified electrician.
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