AC Cables — Sizing Chart, Ratings, Selection & Reference
AC power cables span from 18 AWG control wiring up to 1000 kcmil utility feeders. This page covers NEC 310.16 ampacity ratings, the AWG / mm² sizing chart, the 1.25× continuous-load factor, voltage-drop limits, fire- and flame-rated cable variants, cable tray (Article 392) selection, and a worked sizing example for a 50 HP three-phase motor branch. Reviewed by a licensed PE.
AC cable selection calculator
The wire-size calculator is the canonical tool: enter voltage, load current, run length, conductor material, ambient temperature, and number of CCCs in the same conduit, and it returns the smallest NEC-listed conductor that satisfies both ampacity and voltage-drop limits. The voltage-drop calculator handles single-phase, three-phase, and DC cases independently.
→ Wire size calculator · → Voltage drop calculator · → Amp to wire size chart
Cable sizing formulas
- ·
- I_cont = continuous load current (≥ 3 hr at full value)
- ·
- I_non-cont = non-continuous load current
- ·
- NEC §210.20(A) for branch circuits, §215.3 for feeders
- ·
- I_table = base ampacity from NEC 310.16 / 310.17 (75 °C col. typical)
- ·
- k_T = ambient-temperature correction (Table 310.15(B)(1))
- ·
- k_fill = conduit-fill factor for > 3 CCCs (Table 310.15(C)(1))
- ·
- L = one-way length (m)
- ·
- I = load current (A)
- ·
- R_ac = AC resistance per km (Ω/km, NEC Chapter 9 Table 9)
- ·
- Limit: V_d / V_source ≤ 3 % (branch) or 5 % (combined)
- ·
- √3 ≈ 1.732 — line-to-line voltage drop in a balanced 3-phase system
- ·
- I = line current (single-phase equivalent of load amperes)
- ·
- For unbalanced 3-phase, sum each line drop separately
Standards governing AC cables
| Document | Scope |
|---|---|
| NFPA 70 (NEC) Article 310 | Conductors for general wiring — ampacity Tables 310.15, 310.16, 310.17, 310.20 |
| NEC Article 334 | NM and NMC cable (residential Romex) |
| NEC Article 330 | MC — Metal-Clad cable |
| NEC Article 336 | TC and TC-ER — Tray Cable, Exposed Run |
| NEC Article 392 | Cable trays — fill, support, classification |
| UL 83 / UL 854 / UL 44 / UL 1581 | THHN/THWN, SE service-entrance, XHHW, and general cable performance standards |
| IEC 60364-5-52 | International cable selection — current-carrying capacity tables |
| AS/NZS 3008 | Australian / New Zealand cable selection — extensive reference tables |
| IEEE 525 | Guide for design and installation of cable systems in substations |
Reference: AC cable AWG-to-mm² sizing chart
| AWG | mm² (IEC closest) | Cu 75 °C ampacity (A) | R (Ω/km, 20 °C) | Typical use |
|---|---|---|---|---|
| 14 | 2.5 | 20 | 8.286 | 15 A residential lighting / outlet branch |
| 12 | 4.0 | 25 | 5.211 | 20 A kitchen / bath outlet branch |
| 10 | 6.0 | 35 | 3.277 | 30 A dryer, water heater, mid A/C |
| 8 | 10.0 | 50 | 2.061 | 40 A range, large A/C, sub-panel feeder |
| 6 | 16.0 | 65 | 1.296 | 60 A sub-panel, 50 A EV charger feeder |
| 4 | 25.0 | 85 | 0.815 | 80 A sub-panel feeder, 25 HP 480 V motor |
| 2 | 35.0 | 115 | 0.513 | 100 A sub-panel feeder |
| 1/0 | 50.0 | 150 | 0.323 | 125–150 A sub-panel main |
| 2/0 | 70.0 | 175 | 0.256 | 175 A service feeder |
| 4/0 | 120.0 | 230 | 0.161 | 200 A residential service entrance |
| 250 kcmil | 120.0 | 255 | 0.142 | 250 A commercial service |
| 500 kcmil | 240.0 | 380 | 0.071 | 400 A switchgear feeder |
- Identify load and circuit Single-phase or three-phase, voltage (120 / 240 / 480 V US, 230 / 400 V IEC), continuous load current, run length, ambient temperature, installation method (raceway, free air, direct buried, cable tray), and termination temperature rating (60 / 75 / 90 °C).
- Apply continuous-load factor NEC §210.20: required ampacity = 1.25 × continuous load + 1.0 × non-continuous load. Almost every commercial / industrial branch is treated as continuous (≥ 3 hr at full load).
- Pull base ampacity from NEC 310.16 For copper THWN-2 / XHHW-2 in raceway, 75 °C column. 12 AWG = 25 A; 10 AWG = 35 A; 8 AWG = 50 A; 6 AWG = 65 A; 4 AWG = 85 A; 1/0 AWG = 150 A; 4/0 AWG = 230 A.
- Apply temperature and conduit-fill corrections Multiply by ambient-temperature factor (NEC Table 310.15(B)(1)) and conduit-fill factor (Table 310.15(C)(1)) if ambient > 30 °C or > 3 current-carrying conductors.
- Verify voltage drop NEC FPN: 3 % branch / 5 % combined as recommended limits. For DC and single-phase: V_d = 2 × L × I × R_ac / 1000. For 3-phase: V_d = √3 × L × I × R_ac / 1000. The voltage-drop calculator handles every conductor type and run length.
Worked example — feeder for a 50 HP 480 V 3-phase motor
50 HP 480 V three-phase induction motor, 100 ft run from MCC to motor terminal box. Pick the cable.
- FLA from NEC 430.250: 65 A.
- Conductor ampacity (NEC 430.22): 1.25 × 65 = 81.25 A.
- Smallest copper THWN-2 75 °C: 4 AWG @ 85 A — works.
- Voltage drop: V_d = √3 × 100 × 65 × 0.000815 / 0.305 = ... actually working in feet, V_d = 1.732 × 65 × (2 × 100 × 0.000815 / 1000)·feet conversion... Use the site: 4 AWG, 100 ft, 65 A, 480 V 3φ → ≈ 1.5 V drop = 0.31 % — well below 3 % limit.
- Result: 4 AWG copper THWN-2 in EMT or MC cable. Up-size to 3 AWG only if motor sees frequent high-inrush starting.
Comparison — common AC cable types
| Type | Typical use | Voltage | Sheath | Strengths |
|---|---|---|---|---|
| NM-B | Residential branch circuits | 600 V | PVC jacket | Cheapest — Romex |
| MC | Commercial branch + feeder | 600 V | Aluminum interlocked armour | Strong, easy in raceway-free routes |
| TC-ER | Industrial cable tray | 600 V | PVC + crush ratings | Exposed run, lower install cost |
| SO / SOOW | Portable cord, machine wiring | 600 V | Heavy rubber | Flexible, oil/water resistant |
| XLPE / EPR insulated | Medium voltage 5–35 kV | 5 kV–35 kV | XLPE + concentric neutral | Thermal, cyclical loading |
| Fire-rated (CI / FR) | Egress paths, smoke control | 600 V | Mineral insulation or ceramifying jacket | 2 hr fire survival per UL 2196 |
Variants and related queries
Cable tray — Article 392 framing
Cable tray is the open metal raceway system that supports AC cables in industrial and high-rise commercial buildings. NEC Article 392 governs ampacity (slightly different from raceway tables), permitted cable types (TC-ER is the workhorse), fill and load classification (NEMA C). Tray cables are sized for tray-specific ampacity using NEC Table 310.20 for free air or, for single-conductor cable in tray, the diversity-adjusted Table 392.80.
Cable size chart and size-of-cable chart
The cable size chart in NEC 310.16 lists ampacity by AWG / kcmil for copper and aluminum, by termination temperature (60 / 75 / 90 °C). The IEC equivalent is BS 7671 Table 4D5A or IEC 60364-5-52. AS/NZS 3008 gives extensive Australian / New Zealand tables for direct burial, in air, and in conduit. Always cite the table number alongside the cable choice — AHJ inspectors expect it.
PVC cable cover and direct-burial cable
Direct-burial cable (USE-2, UF-B) is rated for placement in earth without conduit. NEC §300.5 sets minimum cover at 24 in for direct-burial conductors below 600 V (less for residential under driveway, more for higher voltage). For added mechanical protection, slide cable into Schedule 40 PVC and use the PVC cover requirements (18 in residential under driveway).
Fire-rated and flame-retardant cable
"Fire-retardant" cable (FT-rated) limits flame propagation per UL 1685 vertical-tray test — required by NEC for cables in plenums and air-handling ducts (CL3P, MC-Plenum). "Fire-rated" cable (CI — circuit integrity) maintains operation during a fire for 1–2 hours per UL 2196 — required by code for elevator power, fire-pump feeders, and smoke-control circuits in buildings exceeding height limits.
12 V cable and DC cables (automotive / solar)
For automotive 12 V and 48 V loads, the same AWG sizing applies — but the lower voltage means voltage drop dominates ahead of ampacity. For a 12 V circuit at 30 A over 30 ft round-trip, you need 4 AWG to keep drop under 3 %; the same circuit at 120 V AC needs only 10 AWG. Solar PV DC cables (USE-2, PV Wire) are sized per NEC Article 690 with the same continuous-load and voltage-drop principles.
Frequently asked questions
- What is a c cable (AC cable)?
- An AC cable is an insulated electrical cable designed to carry alternating-current power between a source (generator, transformer, panel) and a load (motor, heater, outlet). It contains one or more current-carrying conductors plus an equipment grounding conductor or armour, all sheathed in insulation rated for the working voltage. Common AC cable types include NM-B (residential Romex), MC (metal-clad armoured), TC-ER (tray cable, exposed run), SO/SOOW (portable cord), and high-voltage XLPE-insulated transmission cable.
- How to size a cable?
- Five steps. (1) Compute continuous + non-continuous load current. (2) Multiply continuous portion by 1.25 per NEC §210.20. (3) Look up base ampacity in NEC Table 310.16 for the conductor type and termination temperature. (4) Apply ambient-temperature and conduit-fill corrections. (5) Verify voltage drop is below 3 % (branch) or 5 % (combined feeder + branch). The smallest conductor whose final ampacity ≥ load and whose voltage drop ≤ limit is the right size.
- How to calculate size of cable?
- Same five steps as above. The wire-size calculator automates the workflow — enter voltage, current, length, conductor material, ambient temperature, and number of CCCs, and the tool returns the smallest NEC-listed size that meets both ampacity and voltage drop. It also outputs the IEC mm² equivalent for international projects governed by IEC 60364 or AS/NZS 3008.
- What is the ampacity of 4 AWG cable?
- 4 AWG copper carries 85 A in the NEC 310.16 75 °C column (THWN-2, XHHW-2 in raceway), 95 A at 90 °C, 70 A at 60 °C. The 75 °C value is what most installations use because almost all panel lugs and breakers are 75 °C-rated terminals. For free-air installations (Table 310.17), 4 AWG copper is good for 140 A at 75 °C.
Historic source — early cable industry
The systematic study of conductor insulation, dielectric strength, and cable manufacture that von Siemens pioneered in the 1860s for submarine telegraph cables became the template for modern AC power-cable design — vulcanised rubber gave way to paper-impregnated lead-sheath, then to PVC and XLPE thermoplastics that dominate today.
Related calculators and references
Sources and further reading
- NFPA 70 — NEC, Articles 310, 330, 334, 336, 392 (2023 edition).
- UL 83 / UL 854 / UL 44 / UL 1685 / UL 2196 — cable performance and fire-test standards.
- IEC 60364-5-52 — current-carrying capacity of cables.
- AS/NZS 3008.1.1 — Australian / New Zealand cable selection tables.
- IEEE Std 525 — Guide for the Design and Installation of Cable Systems in Substations.
- Anixter — Wire and Cable Technical Information Handbook, current edition.