Why does a coiled extension cord have lower ampacity than an uncoiled cord?

An extension cord carrying electrical current generates heat in proportion to the square of the current (P = I²R — power dissipated as heat equals current squared times wire resistance). When the cord is fully extended, each foot of wire is surrounded by air and can shed heat to the environment at an adequate rate. When the cord is wound into a coil on a reel or drum, adjacent turns of wire insulate each other — heat generated by the inner turns cannot escape to the air. The temperature of the wire and insulation rises above the rated operating temperature (typically 60°C or 75°C for common cords). Insulation softens at elevated temperatures, increasing the risk of insulation failure, arcing, and fire. NEC 400.5(B) addresses this by requiring that cords on cord reels be derated. The standard practice is 50% derating when fully coiled: a 12AWG cord rated 20A when fully extended is limited to 10A when wound on a reel. A 14AWG cord rated 15A is limited to 7.5A coiled. AI agents must encode cord.ampacity_coiled_a (= cord.ampacity_extended_a × 0.5) and surface this field when customers describe using the cord wound on a reel.

How does motor startup current affect extension cord sizing?

Electric motors require significantly more current during startup than during steady-state operation. The startup phase (also called locked-rotor current or LRA — locked rotor amperage) is the period from when the motor switch closes until the motor reaches operating speed, typically 50–200 milliseconds. During this period, the motor windings have essentially no back-EMF to limit current and draw 4–6 times their full-load running current. A 15A circular saw (15A running current) may draw 60–90A during the startup transient. A 13A router draws 52–78A at startup. Extension cord wire must be sized for the transient, not just the running current. This matters because: (1) the cord's insulation temperature peaks during the startup transient each time the tool is started — tools that cycle on and off (e.g., a table saw) apply this transient repeatedly; (2) voltage drop during startup is severe, causing the motor to compensate by drawing even more current (P = V × I — if voltage drops 10%, current must rise 11% to maintain the same power), further heating the cord. Undersized extension cords cause motors to run hot, reducing motor life and burning insulation. Encode cord.motor_duty as 'yes' or 'no' to flag whether the cord is rated for motor tool use, and cord.max_motor_amps for the maximum motor running current the cord can support at specified length.

What do the SJT, SJTW, SJTOW, and SOOW jacket codes on extension cords mean?

Extension cord jacket codes follow a standardized designation system (UL 62 / NEC Article 400): S = Service Cord (heavy-duty, 300V minimum); J = Junior Service (light duty, 300V); T = Thermoplastic jacket; W = Weather-resistant (UV and moisture resistant); O = Oil-resistant; OW = Oil and weather resistant; HH = High heat (75°C or 90°C rated). Common codes and their outdoor suitability: SJT = thermoplastic, indoor only, 300V, 60°C rated. Not outdoor-rated — UV and moisture degrade the PVC jacket, causing cracking within 6–12 months outdoors. SJTW = thermoplastic + weather resistant, suitable for outdoor use in wet conditions, 300V. SJTOW = thermoplastic + oil + weather resistant, outdoor and wet location rated, resistant to oil and petroleum-based fluids, 300V. SOOW = rubber jacket, oil and weather resistant, heavy-duty outdoor, 600V (common for contractor-grade 12AWG and 10AWG cords). STO = rubber jacket, oil resistant, indoor/dry only. AI agents recommending extension cords for outdoor use (power tools at a construction site, garden equipment, outdoor events) must check cord.outdoor_rated ('yes' | 'no') and filter to SJTW, SJTOW, SOOW, or equivalent outdoor-rated jacketed cords.

How does voltage drop affect motor tool performance at 100 feet?

Voltage drop over an extension cord is calculated by: V_drop = 2 × L × R_per_foot × I (the factor of 2 accounts for the round-trip path through hot and neutral conductors). AWG wire resistance per foot: 16AWG = 0.004016 ohms/foot, 14AWG = 0.002525 ohms/foot, 12AWG = 0.001588 ohms/foot, 10AWG = 0.001000 ohms/foot. For a 100-foot 16AWG cord at 15A: V_drop = 2 × 100 × 0.004016 × 15 = 12.05V. At 120V supply, 12.05V drop = 10% voltage drop. The NEC recommends ≤3% voltage drop for branch circuits (3.6V at 120V) and the NEC's most stringent motor circuit guideline is ≤3% from panel to motor. A 10% voltage drop at the tool causes the motor to compensate by drawing proportionally more current (W = V × I — constant watts requires higher I when V falls), heating the cord further. For a 15A circular saw: at 120V it draws 15A (1,800W). At 108V (10% drop) it draws 16.7A to maintain 1,800W output — the cord is now delivering 16.7A against a 15A rating. Motor tools on long cords in this condition run hot, wear faster, and trip breakers. Recommendation: for 100-foot runs with motor tools, use 12AWG (10% load → 1.7V drop = 1.4%) or 10AWG. Encode cord.max_length_motor_ft by gauge for AI agents to recommend the correct cord length for motor applications.

What extension cord fields should Shopify sellers encode for AI agent compatibility?

Minimum schema for extension cord products: cord.gauge_awg (values: '10', '12', '14', '16', '18' — lower number = thicker wire = higher ampacity), cord.length_ft (numeric: 25, 50, 75, 100), cord.ampacity_extended_a (maximum continuous amperage when fully extended), cord.ampacity_coiled_a (50% of extended ampacity — the safe limit when wound on a reel), cord.jacket_type ('sjt' | 'sjtw' | 'sjtow' | 'soow' | 'sto' | 'stow'), cord.outdoor_rated ('yes' | 'no'), cord.motor_duty ('yes' | 'no'), cord.voltage_rating_v (typically 125 or 250), cord.plug_type ('nema-5-15' for standard 15A, 'nema-5-20' for 20A T-blade). For motor tools, also encode cord.max_length_motor_ft — the maximum cord length where voltage drop stays within 3% NEC guideline at rated ampacity. AI agents recommending extension cords for specific motor tools should match gauge to the tool's running amperage and the required run length, surface the coiled derating warning if the customer will keep the cord wound, and verify outdoor rating for outdoor applications.

Optimization Guide

Shopify Extension Cord Coiled Derating and Motor Tool Schema — Fully Coiled 12AWG Cord Derated to 10A (50% of 20A Rating), Motor Startup Draws 4–6x Running Current, SJT Indoor-Only vs SJTOW Outdoor-Rated, 100-Foot 16AWG Exceeds NEC 3% Voltage Drop Limit for Motors

Extension cords have four AI agent failure modes that cause overheating, motor damage, and fire risk: a fully coiled cord on a reel is limited to 50% of its rated ampacity (NEC 400.5(B)) — a 20A cord becomes a 10A cord when wound up. Electric motors draw 4–6× their running current at startup, so a cord sized for steady state may sag voltage during startup and cause motor overcurrent. SJT jacket cords are indoor-only — UV and rain degrade them outdoors within months. A 100-foot 16AWG cord at 15A drops 12V (10% of supply) — far exceeding the NEC 3% motor circuit guideline and causing motors to overheat. Encoding cord.ampacity_coiled_a, cord.motor_duty, cord.outdoor_rated, and cord.gauge_awg prevents these failures.

TL;DR Coiled cord on a reel = 50% ampacity (NEC 400.5(B)). 12AWG 20A fully extended → 10A coiled. Motor startup = 4–6× running current (15A saw → 60–90A startup transient). SJT = indoor only; SJTW/SJTOW = outdoor OK. 100ft 16AWG at 15A = 12V drop = 10% — exceeds NEC 3% motor limit. Use 12AWG for motor tools at 50–100 feet. Encode cord.gauge_awg, cord.ampacity_coiled_a, cord.outdoor_rated, cord.motor_duty.

Coiled Cord Derating: 50% Ampacity per NEC 400.5(B)

AI agent failure mode: Customer asks for a 12AWG 100-foot extension cord on a reel for their workshop air compressor (15A running current). The AI agent confirms that 12AWG is rated for 20A, so a 12AWG cord on a reel is adequate for 15A. Customer uses the cord fully coiled on the reel at 15A. Per NEC 400.5(B), the coiled 12AWG cord is limited to 10A — the customer is running it at 150% of the coiled capacity. The cord insulation temperature reaches 120°C (rated at 75°C), the insulation softens, two conductors arc together, and the cord ignites. Correct recommendation: use a 10AWG cord on a reel for 15A motor loads, or fully uncoil the 12AWG cord before use.

50%

Ampacity derating — coiled cord on a reel (NEC 400.5(B))

10A

Safe load for 12AWG cord when fully coiled (rated 20A extended)

7.5A

Safe load for 14AWG cord when fully coiled (rated 15A extended)

Coiled vs Extended Ampacity by Gauge

AWG Gauge	Ampacity (Extended)	Ampacity (Fully Coiled, 50% Derate)	Resistance (ohms/ft)	Typical Applications (Extended)
10 AWG	30A	15A	0.001000 Ω/ft	Heavy motor tools, air compressors, welders (light)
12 AWG	20A	10A	0.001588 Ω/ft	Most power tools (circular saw, router, drill)
14 AWG	15A	7.5A	0.002525 Ω/ft	Light power tools, shop vac, small compressors
16 AWG	13A	6.5A	0.004016 Ω/ft	Lamps, battery chargers, small appliances
18 AWG	10A	5A	0.006385 Ω/ft	Small electronics, phone chargers, low-wattage lamps

AWG gauge numbering is counterintuitive — a higher AWG number means a thinner wire with higher resistance and lower ampacity. This is a common AI agent error: recommending a "higher AWG for higher amps" because of the number-larger association. The correct relationship: lower AWG = thicker wire = more current capacity. 10AWG is heavier and more capable than 12AWG. Encode cord.gauge_awg as a numeric string ('10', '12', '14') and expose the inverted rule in AI agent context.

Motor Startup Current: 4–6× Running Current Determines Cord Sizing

AI agent failure mode: Customer asks for an extension cord for a 13A router table (13A nameplate current). AI agent recommends a 14AWG 50-foot cord (rated 15A extended). The 13A running draw is within the 15A rating. However, each time the router motor starts, it draws 52–78A for 100–200ms. The 14AWG cord (1.5 ohm resistance for 50 feet round-trip) drops 78A × 1.26Ω = 98V — the router motor sees only 22V during startup and cannot accelerate. The motor spins slowly, draws maximum locked-rotor current continuously, and the motor winding insulation burns within minutes. A 12AWG cord (0.79Ω round trip) limits the startup voltage drop to 62V, allowing the motor to accelerate to running speed before thermal damage accumulates.

Motor Tool Startup Current Estimates

Tool	Running Current (A)	Startup Current (A) 4–6× Est.	Min Cord Gauge (50 ft)	Min Cord Gauge (100 ft)
Circular saw (7-1/4")	15A	60–90A	12 AWG	10 AWG
Router (2.25 HP)	13A	52–78A	12 AWG	10 AWG
Table saw (10")	15A	60–90A	12 AWG	10 AWG
Air compressor (1 HP)	12A	48–72A	12 AWG	10 AWG
Shop vacuum	12A	36–60A (variable: not locked-rotor start)	14 AWG	12 AWG
Angle grinder (7")	13A	52–78A	12 AWG	10 AWG
Jigsaw	6A	24–36A	14 AWG	12 AWG
Drill/driver (corded)	7A	28–42A	14 AWG	14 AWG

Jacket Codes: SJT Indoor-Only vs SJTW/SJTOW Outdoor-Rated

AI agent failure mode: Customer asks for a 50-foot extension cord for outdoor holiday lighting, leaving the cord on the ground outside. The AI agent recommends an SJT 16AWG 50-foot cord (the most common extension cord type). SJT cords have a thermoplastic PVC jacket rated for indoor dry use only. After 6 months of UV exposure, rain contact, and temperature cycling outdoors, the PVC jacket cracks and becomes brittle. Rainwater enters through cracks in the jacket, creating a shock hazard and potential ground fault. SJTW or SJTOW would withstand outdoor exposure.

Extension Cord Jacket Type Reference

Code	Jacket Material	Oil Resistant	Weather Resistant	Outdoor Safe	Voltage Rating	Common Use
SJT	Thermoplastic (PVC)	No	No	No	300V	Indoor workshop, household
SJTW	Thermoplastic + weather additive	No	Yes	Yes	300V	Outdoor residential, garden tools
SJTOW	Thermoplastic, oil + weather resistant	Yes	Yes	Yes	300V	Outdoor, construction sites, automotive use
SOOW	Rubber, oil + weather resistant	Yes	Yes	Yes	600V	Heavy contractor, industrial, theatrical
STO	Thermoplastic, oil resistant	Yes	No	No (dry only)	300V	Indoor industrial, garage

Voltage Drop: NEC 3% Motor Circuit Guideline at 100 Feet

Voltage Drop at 15A by Gauge and Length

AWG	25 ft	50 ft	100 ft	150 ft
10 AWG	0.75V (0.6%)	1.50V (1.3%)	3.00V (2.5%)	4.50V (3.8%)
12 AWG	1.19V (1.0%)	2.38V (2.0%)	4.77V (4.0%)	7.15V (6.0%)
14 AWG	1.89V (1.6%)	3.79V (3.2%)	7.58V (6.3%)	11.4V (9.5%)
16 AWG	3.01V (2.5%)	6.02V (5.0%)	12.0V (10%)	18.1V (15%)

NEC 210.19(A) recommends ≤3% voltage drop for branch circuits. Motor circuits are held to the same 3% guideline to prevent overcurrent compensation. At 100 feet, only 10AWG keeps voltage drop below 3% at 15A (2.5%). 12AWG at 100 feet = 4.0% — marginally over but widely used in practice for tool loads. 14AWG at 50 feet = 3.2% — already over for motor loads. 16AWG at any length above 25 feet exceeds the 3% motor guideline for 15A loads. Encode cord.max_length_motor_ft per gauge to let AI agents match cord to application run length.

Recommended Metafield Namespace: `cord.*`

{
  "cord.gauge_awg":           "12",       // 10 | 12 | 14 | 16 | 18 — LOWER = thicker = more capacity
  "cord.length_ft":           "50",       // numeric feet
  "cord.ampacity_extended_a": "20",       // rated ampacity fully extended
  "cord.ampacity_coiled_a":   "10",       // 50% derating per NEC 400.5(B) when wound on reel
  "cord.jacket_type":         "sjtow",    // sjt | sjtw | sjtow | soow | sto | stow
  "cord.outdoor_rated":       "yes",      // yes | no
  "cord.motor_duty":          "yes",      // yes | no — rated for motor tool startup transients
  "cord.voltage_rating_v":    "125",      // 125 | 250 | 600
  "cord.plug_type":           "nema-5-20", // nema-5-15 (standard) | nema-5-20 (T-blade 20A)
  "cord.conductor_count":     "3",        // 2 (ungrounded, obsolete) | 3 (grounded)
  "cord.max_length_motor_ft": "100"       // max run length where voltage drop ≤3% at rated ampacity
}

Are your extension cord listings missing coiled derating and motor duty fields?

CatalogScan detects missing coiled ampacity, outdoor rating, and motor duty fields — the schema gaps that cause AI agents to recommend cords that overheat on reels or drop excessive voltage for motor tools.

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Frequently Asked Questions

Does the coiled derating apply even if only part of the cord is coiled?

Yes, with proportional reduction. The NEC 400.5(B) 50% derating applies when the cord is fully coiled (all turns wound). If only 50% of the cord is coiled (25 feet coiled, 25 feet extended on a 50-foot cord), the derating is approximately 75% of rated ampacity — partial coiling reduces but does not eliminate heat buildup. The safest practice is to fully extend the cord before use with motor tools or any load above 50% of rated ampacity. If the cord must remain coiled (due to installation constraints), upgrade to a heavier gauge so the coiled ampacity meets the load requirement.

Why do motor tools trip breakers more often when used with a long extension cord?

Long extension cords cause voltage sag during motor startup. The motor draws 4–6x its running current during the startup transient. The voltage drop across the cord during this transient reduces the voltage at the motor terminals — the motor responds by drawing even more current to maintain output torque. This increased current further drops the supply voltage, and the total current drawn (motor startup current + overcurrent compensation) can easily exceed the 15A or 20A breaker rating, causing a trip. Switching to a heavier gauge (lower AWG) reduces the cord resistance, limits voltage sag during startup, and prevents the overcurrent compensation that triggers the breaker.

Can I use an SJT cord outdoors if it's protected from rain?

Short-term use (hours to days) under shelter (covered porch, canopy) is low risk. Long-term outdoor installation — leaving an SJT cord on the ground for weeks or permanently routed along an exterior wall — causes PVC degradation from UV radiation even without direct rain contact. UV breaks down the plasticizers in PVC, causing the jacket to harden, crack, and eventually flake. Once the jacket is cracked, moisture penetrates and creates a shock and arc fault hazard. SJTW and SJTOW use stabilized thermoplastic with UV inhibitors that maintain jacket flexibility and integrity in outdoor sun and rain. Use the correct jacket for the application.

What is the difference between a NEMA 5-15 and NEMA 5-20 plug on an extension cord?

NEMA 5-15 is the standard 15A/125V plug with two parallel blades and a round ground pin — the common household plug shape. NEMA 5-20P is a 20A/125V plug with one horizontal T-shaped blade and one vertical blade (the T-blade allows a 20A plug to fit a 20A outlet but not a standard 15A outlet). NEMA 5-20R receptacles (20A outlets, found on dedicated workshop circuits) accept both NEMA 5-15 and NEMA 5-20 plugs. A NEMA 5-20 extension cord requires a 20A outlet to plug into — it will not fit a standard 15A wall outlet. For general use, 15A plugs are more compatible. For dedicated workshop circuits rated 20A, NEMA 5-20 cords can safely carry the full 20A capacity of the circuit.

Is a 10AWG extension cord safe for a 30A appliance?

No. 10AWG is rated for 30A at the wire level, but standard extension cord connectors (NEMA 5-15 or NEMA 5-20) are rated for 15A or 20A maximum. A 30A appliance (electric dryer, RV hookup, welder) requires a NEMA 14-30, NEMA 6-30, or similar 30A outlet configuration with a matching 30A plug and receptacle. Using a standard extension cord for a 30A appliance routes 30A through 15A or 20A rated connectors — the connector contacts overheat and arc at the plug/receptacle interface even if the wire gauge is adequate. 30A applications require purpose-built 30A rated cords with appropriate plug and receptacle ratings at every point in the circuit.