A 15-Amp Saw on a 100-Foot Cord Is Off the Chart Printed in Its Own Manual

Part of the guide: Tools and DIY Buying Mistakes: What Fails and How to Check Before You Buy

A cord is the cheapest part of a job and the only part rated in two directions at once. It has to carry the tool’s current, and it has to carry that current over its own length, because the voltage reaching the motor falls as wire gets longer and thinner. Makita states the consequence in one line in its own manual: an undersized cord will cause a drop in line voltage resulting in loss of power and overheating.

The numbers that prevent it are printed on three surfaces: the tool’s nameplate, the cord’s jacket and the cord’s packaging tag. The arithmetic is one multiplication, and the chart that settles the question came in the box with the tool.

Start at the nameplate, not the cord rack

Corded tools are rated in amps, on the nameplate and in the published specifications. DeWalt lists its DWE575SB 7-1/4 inch circular saw as a 15 amp motor at 120 volts. That is the figure the cord has to serve, and multiplying gives the load in watts: 15 amps at 120 volts is 1,800 watts.

Makita’s manual says to use a cord heavy enough to carry the current the product will draw, sized against the nameplate ampere rating, and supplies the tie-breaker: if in doubt, use the next heavier gage, because the smaller the gage number, the heavier the cord. Appliances are usually labeled in watts instead, so the arithmetic runs backwards. A 1,500 watt heater on a 120 volt outlet draws 12.5 amps, which lands it in the same chart band as a full-size circular saw.

The cord is sized to the tool’s nameplate amps and the length of the run, not to the outlet it plugs into.

The gauge chart printed in your tool manual

Makita prints Table 1, Minimum gage for cord, in the instruction manual for its 5007N circular saw, with ampere bands down the side and total cord length across the top. At 120 volts it reads as follows.

Read the bottom row twice. A 15 amp saw, router or compressor sits in the 12 to 16 amp band, and Makita’s answer at 100 and 150 feet is not a heavier gauge but Not Recommended: the chart stops rather than continuing. At 50 feet that tool wants 12 AWG and at 25 feet 14 AWG, so the 16 AWG cord bundled with something else is one or two sizes short at any length. Joining two cords does not solve a long run either, because the lengths add and the tool moves a column to the right.

Tool nameplate amps	25 ft	50 ft	100 ft	150 ft
More than 0 to 6	18 AWG	16 AWG	16 AWG	14 AWG
More than 6 to 10	18 AWG	16 AWG	14 AWG	12 AWG
More than 10 to 12	16 AWG	16 AWG	14 AWG	12 AWG
More than 12 to 16	14 AWG	12 AWG	Not recommended	Not recommended

A 15 amp tool is outside Makita’s chart at 100 feet. If the work is that far from an outlet, the answer is a shorter run, not a longer cord.

What the markings on the jacket mean

CPSC says extension cords should be at least 16 AWG, unless they are 18 AWG with fuse protection, that AWG refers to the size of the wires in the cord, and that the wire size is imprinted on the cord’s surface. Capacity is published separately: CPSC says you can check a cord’s capacity, or rating, by looking at the tag on the cord or its packaging, and warns against powering appliances beyond that capacity.

Two further markings decide where a cord may be used. CPSC says to only use an extension cord outdoors if it is marked for outdoor use, and to check that cords have been listed by a recognized national testing laboratory such as Underwriters Laboratories (UL), Intertek (ETL) or CSA-International. Its Safety at Home sheet is blunter: if a cord does not say it is listed by a Nationally Recognized Testing Laboratory, do not use it, because unlisted or counterfeit cords can cause fires or electrical shocks.

OSHA adds the jobsite version. Its construction standard requires extension cord sets used with portable electric tools and appliances to be of three-wire type and designed for hard or extra-hard usage, and the note to that paragraph names qualifying types including S, ST, SO and STO for hard service and SJ, SJO, SJT and SJTO for junior hard service.

Three markings carry the answer: the imprinted AWG, the rating on the tag, and the testing laboratory listing. CPSC says not to use a cord missing the last one.

Space heaters are not a cord job at all

CPSC’s instruction here is unconditional: always plug electric space heaters directly into a wall outlet and never into an extension cord or power strip. In the same 2023 release, CPSC said portable heaters, including electric space heaters, were involved in an average of 1,700 fires per year, with an average of 70 deaths and 160 injuries annually from 2017 to 2019, and advised keeping them at least three feet from anything that can burn.

The difference from the saw is duration rather than current. A 1,500 watt heater draws 12.5 amps and holds it for hours, where a circular saw pulls its 15 amps in bursts of seconds between cuts; in our judgment that continuous load is why heaters are singled out while power tools get a gauge chart. CPSC also gives the symptom: if any part of the extension cord is hot while in use, it is a warning sign that it may be overloaded. Removing the cord does not remove the load, which is the same arithmetic that catches people switching a heater through a smart plug; see our analysis of smart plugs and space heater loads.

No gauge makes an extension cord the right answer for a space heater. CPSC says wall outlet only, and the hours-long draw is the reason.

Temporary means temporary

CPSC says extension cords should only be used temporarily, should not be run under carpets, through doorways or under furniture, and should be unplugged when not in use, because a cord is energized while plugged in and can overheat if shorted. It also says never to alter a cord to change its length or tape up damaged insulation.

OSHA states the same limit in regulatory language: flexible cords and cables may not be used as a substitute for the fixed wiring of a structure, nor run through holes in walls, ceilings or floors, through doorways or windows, attached to building surfaces, or concealed behind walls, ceilings or floors. Its construction rule adds that flexible cords and cables shall be protected from damage, with sharp corners and projections avoided. Makita’s manual covers wet work separately: if operating a power tool in a damp location is unavoidable, use a ground fault circuit interrupter (GFCI) protected supply, which guards against shock and does nothing about voltage drop.

Once a cord is stapled to a wall, run through a doorway or left plugged in permanently, the fix is an added outlet rather than a longer cord.

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Questions people actually ask

What gauge extension cord does a 15 amp tool need?

A 15 amp tool falls in the 12 to 16 amp band of Makita’s published table, which lists 14 AWG at 25 feet and 12 AWG at 50 feet, and marks 100 and 150 feet as Not Recommended. Check the chart in your own tool’s manual, since it is printed there for that tool.

Where do I find the amps for my tool?

On the nameplate and in the maker’s published specifications. DeWalt lists its DWE575SB circular saw as a 15 amp motor at 120 volts. Multiply amps by volts for the load in watts, which is how heaters and other appliances are usually labeled.

Can I run a space heater on an extension cord?

CPSC says to always plug electric space heaters directly into a wall outlet and never into an extension cord or power strip. In its 2023 release CPSC estimated portable heaters were involved in an average of 1,700 fires per year, with 70 deaths and 160 injuries annually from 2017 to 2019.

What do the letters on an extension cord jacket mean?

They describe service grade. OSHA requires cords used with portable tools to be three-wire and designed for hard or extra-hard usage, naming types including S, ST, SO and STO for hard service and SJ, SJO, SJT and SJTO for junior hard service. The gauge is imprinted on the same jacket.