Cable Sizing Calculator per NEC/NFPA 70:2023 for United States
Conductor sizing in the United States follows NEC/NFPA 70:2023 (National Electrical Code). Electricians select wire gauges using Table 310.16 ampacity ratings for 60 °C, 75 °C, or 90 °C insulation, apply adjustment factors from 310.15(C)(1) for ambient temperature and 310.15(B)(3)(a) for conduit fill, and follow NEC 210.19(A) informational note voltage drop guidance of 3% branch / 5% total.
Quick Reference Table
| US Cable Sizing Key References — NEC/NFPA 70:2023 — NEC/NFPA 70:2023 (2023 Edition) | ||
|---|---|---|
| Parameter | Value / Requirement | Clause Reference |
| Supply voltage | 120/240 V split-phase, 120/208 V or 277/480 V three-phase | NEC 220.5(A) |
| Frequency | 60 Hz | NEC general provisions |
| Ampacity table | Allowable ampacities for insulated conductors (copper & aluminum) | NEC Table 310.16 |
| Voltage drop guidance | 3% for branch circuits, 5% total (feeder + branch) — informational | NEC 210.19(A) Informational Note No. 4 |
| Ambient temperature adjustment | Correction factors for temperatures above 30 °C | NEC 310.15(C)(1), Table 310.15(C)(1)(1) |
| Conduit fill adjustment | Derating for more than 3 current-carrying conductors | NEC 310.15(B)(3)(a), Table 310.15(C)(1) |
| Wire gauges (AWG/kcmil) | 14, 12, 10, 8, 6, 4, 3, 2, 1, 1/0, 2/0, 3/0, 4/0, 250 kcmil+ | NEC Table 310.16 |
| Minimum branch circuit sizes | 14 AWG (15 A), 12 AWG (20 A), 10 AWG (30 A) | NEC 210.3, Table 310.16 |
How to Calculate Cable Sizing for United States
- 1
Calculate connected load and demand
Determine the load per NEC Article 220. For dwellings, apply demand factors from Table 220.42 (lighting), Table 220.55 (cooking), and 220.54 (dryers). For commercial, use NEC Article 220 Part IV.
- 2
Select overcurrent protection device
Choose a circuit breaker or fuse per NEC 240.6(A) standard sizes: 15, 20, 25, 30, 35, 40, 50, 60, 70, 80, 100 A. The OCPD must not exceed the conductor ampacity per 240.4.
- 3
Determine raceway type and installation
Identify the wiring method — NM cable (Romex), MC cable, EMT/RMC conduit, cable tray. The ampacity column in Table 310.16 depends on insulation type (THWN-2 at 90 °C, THHN, etc.).
- 4
Apply NEC adjustment and correction factors
Adjust for ambient temperature per 310.15(C)(1) and for conduit fill per 310.15(B)(3)(a). More than 3 current-carrying conductors in a raceway requires derating (e.g., 4-6 conductors = 80%).
- 5
Check voltage drop per NEC recommendations
Though not mandatory, NEC 210.19(A) Informational Note recommends keeping branch circuit voltage drop under 3% and total drop (feeder + branch) under 5%. At 120 V, that is 3.6 V branch / 6 V total.
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Calculate Cable Sizing NowNEC vs IEC 60364 Cable Sizing Comparison
| Parameter | NEC | IEC 60364 |
|---|---|---|
| Conductor sizing unit | AWG/kcmil | mm² |
| Voltage drop recommendation | 3% branch / 5% total | 4% lighting / 5% other |
| Reference ambient temp | 30°C | 30°C (air), 20°C (ground) |
| Continuous load multiplier | 1.25x required | Not explicitly required |
| Ampacity table | Table 310.16 (60/75/90°C) | Tables B.52.2–B.52.13 |
| Conduit fill limit | 40% for 3+ conductors | Not specified (derating instead) |