NEC 400.5 · Flexible cord ampacity · 2023 NEC
How to read NEC Table 400.5(A)(1) flexible cord ampacity
What Column A and Column B mean, how to count current-carrying conductors, the adjustment for four or more, and what SO, SOOW, SJOOW, and SJTW actually stand for.
Need the actual table? Look up Table 400.5(A)(1) in the free NFPA viewer →
Decoding cord type letters (our own shorthand)
| Letter(s) | Meaning | Field takeaway |
|---|---|---|
| S | Hard service (extra-hard usage family), 600 V | Heavier jacket — temporary power, equipment leads |
| SJ | Junior hard service, 300 V | Lighter jacket — extension cords, tool cords |
| E | Thermoplastic elastomer (TPE) | Flexible, resists abrasion |
| T | Thermoplastic | Common on consumer and light-duty cords |
| O | Oil-resistant outer jacket | One O = jacket only |
| OO | Oil-resistant jacket and insulation | Both layers resist oil |
| W | Weather and water resistant | Suitable outdoors / wet |
Put together: SOOW = hard service, oil-resistant jacket and insulation, weather-rated, 600 V. SJOOW = the same, junior (300 V) jacket. SJTW = junior, thermoplastic, weather-rated.
How to read Table 400.5(A)(1), step by step
| Step | What you do | Watch out for |
|---|---|---|
| 1 | Find the cord type and conductor size (AWG) printed on the jacket | Size is per conductor — 12/3 means three 12 AWG conductors, including the ground |
| 2 | Pick the column for the cord's insulation family | The table groups cord types; use the group yours is in |
| 3 | Count current-carrying conductors: 3 → Column A, 2 → Column B | The equipment grounding conductor never counts |
| 4 | More than three current-carrying? Start from the three-conductor value and apply the 400.5(A) adjustment percentage | A neutral carrying only unbalanced current usually doesn't count |
| 5 | Correct for ambient temperature if it isn't near 30 °C | Hot rooftops and generator enclosures |
| 6 | Protect the cord per 240.5 and use it only as Article 400 permits | Cord is not a substitute for fixed wiring |
Our own summary of the 2023 NEC, not the code text. Look up 400.5 and Table 400.5(A)(1) in the free NFPA viewer (nfpa.org/freeaccess) and verify against the NEC edition adopted by your AHJ.
Column A or Column B? Common cords
| Circuit | Current-carrying | Column |
|---|---|---|
| 120 V tool cord: hot, neutral, ground | 2 | B |
| 240 V single-phase, 2 hots + ground | 2 | B |
| 208/480 V three-phase, 3 hots + ground | 3 | A |
| 120/240 V: 2 hots, neutral, ground (neutral carries only imbalance) | 2 | B |
| Three-phase wye with a neutral serving mostly nonlinear loads | 4 | A, then adjust |
Our own summary of the 2023 NEC, not the code text. Look up 400.5 and Table 400.5(A)(1) in the free NFPA viewer (nfpa.org/freeaccess) and verify against the NEC edition adopted by your AHJ.
Worked example — 12 AWG portable cord (SJOOW), two loads
| Input / step | Value | Source |
|---|---|---|
| Cord A: 12/3 on a 120 V load (hot, neutral, ground) | 2 current-carrying | Count |
| Column B ampacity, 12 AWG | 25 A | Table 400.5(A)(1) |
| Cord B: 12/5 on a 208Y/120 V load, neutral carrying harmonics | 4 current-carrying | Count |
| Start from Column A (three-conductor) value, 12 AWG | 20 A | Table 400.5(A)(1) |
| Adjustment for 4–6 current-carrying | 80% | 400.5(A) |
| Cord B allowable ampacity | 20 × 0.80 = 16 A | Calculation |
Worked example values only. Look up the full table in the free NFPA viewer.
Common field mistakes
| Mistake | What to do instead |
|---|---|
| Counting the green ground as current-carrying | The EGC never counts toward Column A/B |
| Using the building-wire ampacity (310.16) for cord | Flexible cord uses Table 400.5(A)(1) — values differ |
| Using SJ cord where extra-hard usage is required | Temporary power and heavy-duty sites often need S-type (extra-hard) cord |
| Adding more conductors without derating | Four or more current-carrying conductors take the 400.5(A) adjustment |
| Assuming "W" means sunlight/UV resistant for every product | Check the jacket print and listing |
| Running cord through walls, ceilings, or doorways as permanent wiring | Article 400 prohibits cord as a substitute for fixed wiring |
Our own summary of the 2023 NEC, not the code text. Look up 400.5 and Table 400.5(A)(1) in the free NFPA viewer (nfpa.org/freeaccess) and verify against the NEC edition adopted by your AHJ.
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Cord ampacity, in plain language
Look up Table 400.5(A)(1) in the free NFPA viewer. NFPA offers read-only access to current NEC editions at no cost. This page explains how to use the table; it does not reproduce it.
Flexible cord has its own ampacity table because it’s built differently from building wire — finely stranded conductors, a jacket that holds heat, and conductors bundled tight in one sheath. Table 400.5(A)(1) gives ampacities at a 30 °C ambient, split into Column A (three current-carrying conductors) and Column B (two current-carrying conductors). Fewer conductors sharing the jacket means less heat, so Column B is higher.
Counting. The green equipment ground never counts. On a 120 V cord, hot and neutral both carry current — that’s two, Column B. A three-phase motor lead with three hots is Column A. Past three current-carrying conductors, 400.5(A) has an adjustment table: start from the three-conductor value and multiply by the percentage.
Worked example. A 12/3 SJOOW on a 120 V heater has two current-carrying conductors, so it reads Column B: 25 A. A 12/5 cord feeding a 208Y/120 V load whose neutral carries harmonic current has four current-carrying conductors: start at the 12 AWG Column A value of 20 A and take 80% — 16 A. If the cord runs somewhere hot, apply the temperature correction factors too.
Cord is not a substitute for fixed wiring — compare the NEC 310.16 building-wire ampacities when you’re deciding between a cord drop and a hard-wired circuit, and match plugs and receptacles with the NEMA plug & receptacle chart. Protect cords per 240.5, and on temporary power follow Article 590 and your GFCI or assured-grounding program. Not engineering advice.
FAQ
What do Column A and Column B mean in Table 400.5(A)(1)?+
Column A is for cords with three current-carrying conductors, such as a three-phase load. Column B is for cords with only two current-carrying conductors, such as a 120 V tool cord (hot and neutral). The equipment grounding conductor never counts. Look up the table in the free NFPA viewer at nfpa.org/freeaccess.
How many amps is 12/3 SJOOW cord rated for?+
On a typical 120 V load with hot, neutral, and ground, only two conductors carry current, so you read Column B — 25 A for 12 AWG in the 2023 table. The cord still has to be protected per 240.5, and the plug, receptacle, and branch circuit set the practical limit.
What does SOOW stand for?+
S = hard (extra-hard usage) service cord rated 600 V, OO = oil-resistant outer jacket and insulation, W = weather and water resistant for outdoor use. SJOOW is the same construction with a junior (300 V) jacket.
Do I derate cord with more than three current-carrying conductors?+
Yes. Section 400.5(A) reduces the three-conductor ampacity by a percentage when a cord has four or more current-carrying conductors. For example, 12 AWG with four current-carrying conductors takes 80% of its three-conductor value: 20 × 0.80 = 16 A.
What is the difference between SO and SJ cord?+
The S family is extra-hard usage, rated 600 V, with a thicker jacket. SJ (junior) is hard usage, rated 300 V, lighter and more flexible. Jobsite temporary power and heavy equipment typically call for the S family; check Article 590 and the equipment instructions.
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