What Gauge Extension Cord Does a 15 Amp Tool Need?

Cord charts say a 12 gauge cord runs a 15 A saw to 100 ft. The saw makers' manuals put a dash in that cell, or print "Not Recommended". Both are arithmetic on different budgets: Milwaukee's manual allows 5 volts at 150 percent of the tool's rated current, and the cord charts allow 5 percent of 120 V at the rated current. Below, four makers' tables are set side by side, and the drop for every gauge and length is worked out from the NIST copper wire tables.

What four tool makers print for a 12 to 16 A tool

Corded tools sold in the US carry an extension cord table in the manual. DeWalt, Makita and Bosch print almost the same table, in ampere bands of 0 to 6, 6 to 10, 10 to 12 and 12 to 16. Milwaukee uses narrower bands and adds a 75 ft column. A 15 A circular saw, miter saw or table saw falls in the top band of every one of them.

All four agree on 25 and 50 ft. Nobody lists anything at 100 ft. Only Milwaukee fills the gap between, and it asks for 10 gauge there, not 12.

For lighter tools, Makita and DeWalt give a 10 to 12 A tool 14 AWG at 100 ft, and Milwaukee gives a 7.1 to 12.0 A tool 10 AWG at 100 ft. Every manual reads the table by the nameplate amps on the tool's rating label. The Makita 5007MG is listed at 15 A on Makita's product page, so its own manual tells its owner to stop at 50 ft.

The two budgets, side by side

Milwaukee's table carries a footnote: "Based on limiting the line voltage drop to five volts at 150% of the rated amperes." That is a complete method. DeWalt, Makita and Bosch print no basis, so only Milwaukee's can be checked line by line.

The cord chart answer has a basis too. WC Safety's chart, which is typical, uses "a 5% voltage-drop budget at 120 volts" at the tool's rated current, and concludes that "at 100 feet, 12 AWG is the thinnest wire that still delivers a full 15-amp saw load within the voltage-drop budget."

Put both on the same footing, a tool drawing its nameplate current:

The cord chart allows 1.8 times as much drop. Voltage drop scales with length, so a chart cord can be 1.8 times longer for the same gauge. That one factor is the whole disagreement. Neither side has its arithmetic wrong. They start from different budgets, and only one of them is the tool maker's.

The voltage drop for every gauge and length

Voltage drop in a cord is current times the resistance of both conductors, out and back, so the wire length is twice the cord length. NBS Handbook 100, the NIST copper wire tables, gives the resistance of annealed copper at 20 C (68 F) in ohms per 1,000 ft:

Worked for a 100 ft 12 AWG cord at 15 A: the wire is 200 ft long, 200 / 1000 x 1.59 = 0.318 ohm, and 0.318 x 15 = 4.8 V. At Milwaukee's 150 percent, 22.5 A, the same cord drops 0.318 x 22.5 = 7.2 V.

Here is the full grid for a 15 A tool. Each cell is the drop at nameplate current, then at 150 percent.

Read the right-hand number in each cell against Milwaukee's 5 V, and the left-hand number against the chart's 6 V. Turned into maximum lengths for a 15 A tool:

On the chart basis, 12 AWG reaches 126 ft, so "12 gauge to 100 ft" passes. On Milwaukee's, it stops at 70 ft, which is why its 75 ft column jumps to 10 gauge.

Why the 100 ft cell is blank

On paper, 10 AWG reaches 111 ft for a 15.0 A tool on Milwaukee's basis. So why does Milwaukee print a dash at 100 ft? Its band runs to 16.0 A, not 15. At 16 A, 150 percent is 24 A, and 100 ft of 10 AWG drops 0.19976 x 24 = 4.8 V on the 20 C solid-wire figure. Add NIST's 2 to 5 percent for stranding and it lands between 4.9 and 5.0 V. At the 50 C resistance, the drop is 5.4 V. The cell sits on the line, and Milwaukee leaves it empty. The manual does not say why, but the margin is visibly gone.

For DeWalt, Makita and Bosch, voltage drop alone does not explain the "Not Recommended". At 16 A, 100 ft of 12 AWG drops 5.1 V, 4.2 percent of 120 V, which a 5 percent rule would pass. Those makers publish no basis, so the reason cannot be checked. The instruction itself is clear.

Note too that Milwaukee's table is stricter than its own footnote at the short end. At 25 ft, a 16 AWG cord drops only 4.8 V at 24 A, inside the 5 V budget, yet Milwaukee lists 14 AWG for the 12.1 to 16.0 A band. Something besides voltage drop sets that cell, and the manual does not name it. Follow the table, not just the footnote.

Where the extra drop goes

A volt lost in the cord is a volt the motor does not get. Makita's manual puts it plainly: "An undersized cord will cause a drop in line voltage resulting in loss of power and overheating." Milwaukee's says that inadequately sized wire "causes a serious drop in voltage, resulting in loss of power and possible tool damage."

The lost power also turns into heat in the cord. A 100 ft 12 AWG cord at 15 A dissipates 15 x 15 x 0.318 = 72 W along its 200 ft of conductor. A 10 AWG cord of the same length dissipates 15 x 15 x 0.19976 = 45 W. Spread out on the floor, that heat leaves. Left coiled, it cannot leave as easily, so uncoil a long cord before it carries a saw.

The cord is also not the whole circuit. The receptacle is fed through building wire that has its own drop. The NEC requires at least one 20 A branch circuit for garage receptacles, as covered in garage shop layout. As an illustration, a receptacle 50 ft of 12 AWG from the panel already loses 0.159 x 15 = 2.4 V at 15 A before the cord begins. A cord chart that spends the whole 5 percent in the cord leaves nothing for that. Milwaukee's tighter budget leaves room for it.

Two tools, one cord, and chained cords

Two rules from the Milwaukee manual cover most jobsite mistakes:

1. Add the nameplate amps. "If you are using one extension cord for more than one tool, add the nameplate amperes and use the sum to determine the required minimum wire size." A 15 A saw and a 6 A vacuum on one cord are a 21 A load. That is above Milwaukee's highest band, 16.1 to 20.0 A, so no length of any gauge in the table covers it. Give each tool its own cord, and remember both cords still share the circuit.
2. Size every cord for the total length. "When using more than one extension cord to make up the total length, be sure each cord contains at least the minimum wire size required." A 25 ft 14 gauge cord plugged into a 50 ft 12 gauge cord is a 75 ft run. For a 15 A tool that needs 10 gauge in both pieces, not just the long one.

On a construction site, OSHA adds requirements on the cord itself. Under 29 CFR 1926.405(a)(2)(ii)(J), extension cord sets used with portable electric tools "shall be of three-wire type and shall be designed for hard or extra-hard usage." OSHA's note names the hard service types S, ST, SO and STO and the junior hard service types SJ, SJO, SJT and SJTO. Under 1926.405(g)(2), those cords must be durably marked with type, size and number of conductors, and used only "in continuous lengths without splice or tap." A repaired cord with a splice in the middle fails that rule however heavy its gauge. Milwaukee also asks that outdoor cords carry the "W-A" suffix ("W" in Canada).

On the jacket, 12/3 means three 12 AWG conductors: hot, neutral and ground. AWG runs backwards, smaller number for thicker wire, and has nothing to do with the gauge on a tool cabinet, which comes from a different table, as the steel gauge guide shows.

The decision, by length

For a tool rated 12 to 16 A at 120 V, with the cord as the only drop you control:

When the work really is 100 ft from power, the better answers are:

• Bring the receptacle closer. Plan a receptacle where the cutting happens, the same logic that puts one where a mobile bench parks. On that page, the bench is fed by one short 12 AWG cord.
• Run 10 AWG and accept the margin. For a tool rated exactly 15.0 A, 100 ft of 10 AWG drops 4.5 V at 22.5 A on the 20 C figure, inside Milwaukee's 5 V. That is arithmetic, not a listing. No manual here prints it, and the margin disappears as the cord warms or the tool's rating rises toward 16 A.
• Go cordless for the far cut. A battery saw has no cord drop at all. If the crew already runs a platform, compare packs on watt hours rather than the volts on the label, as worked out in whether 20V MAX is the same as 18V.

Heavy cords are heavy to store as well. A 100 ft 10 gauge cord is a bulky coil, and it belongs on a hook or reel off the floor. That is the kind of item track wall handles best in the wall storage comparison. Laid across the shop floor, a cord that thick is also one of the half-inch class obstacles that stall small casters, as the roller cabinet caster guide works out.

Where this answer stops applying

• 240 V tools. The DeWalt and Makita tables double every length at 240 V. Nothing above applies to 240 V tools directly.
• Tools rated above 16 A. Milwaukee's last band, 16.1 to 20.0 A, stops at 50 ft with 10 gauge. DeWalt, Makita and Bosch have no band above 16 A.
• Air compressors, heaters and other continuous loads. The tables are in power tool manuals. Equipment with its own manual and its own cord instructions follows those instead.
• Reels and cord sets with lighter leads. A reel or splitter with a thinner lead or pigtail anywhere in the chain is sized by its thinnest part, per Milwaukee's rule that every cord in a run meets the minimum.
• The grid is copper at 20 C. Real stranded cords run 2 to 5 percent higher by NIST's note, and warm copper higher again. Every drop above is the best case.

The thirty-second check

1. Read the amps on the tool's rating label, and add them up if two tools share the cord.
2. Measure the run from receptacle to tool, including every cord in the chain.
3. At 12 to 16 A: 14 gauge to 25 ft, 12 gauge to 50 ft, 10 gauge to 75 ft.
4. Past 75 ft, shorten the run before you buy a heavier cord.
5. If your tool's manual prints a table, it overrides this page and every cord chart.