Cable Sizing Calculator per NEC (NFPA 70) for Commercial Buildings
Conductor sizing for NEC 2026 commercial buildings applies Table 310.16 ampacities at 75 °C termination rating per Section 110.14(C)(2), uses Article 220 Part V for feeder and service load calculations, applies Article 310.15(C) adjustment factors for conduit runs with more than three current-carrying conductors, and requires coordination with overcurrent protection per Article 240.
Quick Reference Table
| NEC 2026 Commercial Conductor Sizing References — NEC (NFPA 70) (2026 Edition) | ||
|---|---|---|
| Parameter | Value / Requirement | Clause Reference |
| Conductor ampacities | 75 °C column for commercial terminations | Table 310.16 |
| Conduit fill adjustment | Factors for 4-30+ current-carrying conductors | Article 310.15(C)(1), Table 310.15(C)(1) |
| Commercial load calculation | Feeder and service loads for commercial occupancies | Article 220, Part V |
| Overcurrent protection | Standard device sizes and next-size-up rule | Article 240.4(B), Article 240.6(A) |
| Conductor termination temp | 75 °C for circuits over 100 A | Section 110.14(C)(2) |
| Parallel conductors | Requirements for paralleled conductors 1/0 AWG+ | Section 310.10(G) |
How to Calculate Cable Sizing for Commercial Buildings
- 1
Calculate the commercial building load
Use Article 220 Part V to calculate feeder and service loads. Apply lighting load densities from Table 220.12 (e.g., 3.5 VA/ft² for office buildings, 1.5 VA/ft² for warehouses) with demand factors from Table 220.44.
- 2
Design the electrical distribution
Plan the feeder hierarchy from service entrance to panelboards. Determine the load at each distribution level and size feeders individually from Article 215.
- 3
Select conductors from Table 310.16
Use the 75 °C column for conductors with appropriately rated terminations per Section 110.14(C)(2). For large feeders, consider paralleled conductors per Section 310.10(G) — minimum 1/0 AWG per phase.
- 4
Apply adjustment factors for conduit runs
Where more than three current-carrying conductors share a conduit, apply Table 310.15(C)(1) adjustment. For 4-6 conductors the factor is 0.80; for 7-9 it drops to 0.70.
- 5
Select overcurrent protection per Article 240
Choose standard overcurrent device sizes from Article 240.6(A). Apply the next-size-up rule of Article 240.4(B) where the conductor ampacity does not match a standard device rating, up to 800 A.
- 6
Verify voltage drop for longest feeders
Calculate voltage drop on the longest feeder run and combine with branch circuit drop. Target 5% total per Section 215.2(A) Informational Note for efficient operation of lighting and equipment.
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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) |