Feeder sizing with the math shown.
The voltage drop and conduit fill engines are live and free to use. Automated feeder extraction from drawings is in development.
The two constraints
Ampacity
The conductor must carry the load current. VoltDrop compares load against the 75°C column of NEC 310.16, which is the terminal rating most equipment is listed for.
Voltage drop
NEC 215.2(A)(1)(b) Informational Note recommends feeders be sized so voltage drop does not exceed 3%, with combined feeder and branch drop within 5% per 210.19(A) Informational Note No. 4.
Raceway
Once conductors are selected, the raceway has to accept them. Conduit fill is checked against NEC Chapter 9 Tables 1, 4, and 5.
Termination temperature
Conductor ampacity may be limited by the temperature rating of the terminations per NEC 110.14(C), independent of the conductor insulation rating.
How the calculation works
Voltage drop uses the simplified formula Vd = factor × K × I × L × PF ÷ CM, where the factor is 2 for single phase and √3 for three phase, K is the specific resistance of the conductor material corrected for operating temperature, L is one-way length in feet, and CM is circular mils from NEC Chapter 9 Table 8.
The recommended conductor is the smallest size that satisfies both ampacity and the 3% voltage drop target. Where those two constraints disagree, the calculator says which one is driving the recommendation.
- Copper and aluminum, single and three phase
- Temperature correction from the 75°C reference
- Conduit type adjustment factor
- Ampacity check against NEC 310.16
- Voltage drop versus distance curve
Important limits
This is a planning tool. It does not perform a short-circuit or coordination study, does not apply ampacity adjustment factors for more than three current-carrying conductors in a raceway per NEC 310.15(C)(1), and does not account for continuous-load factors, derating for ambient conditions beyond the stated correction, or termination limitations specific to the equipment being served.
Results must be reviewed by a qualified professional before use on a real installation.
RELATED
- Voltage Drop CalculatorFree voltage drop calculator for copper and aluminum conductors. Single and three phase, NEC Chapter 9 circular mils, temperature correction, ampacity check, and conductor recommendation.
- Conduit Fill CalculatorFree conduit fill calculator using NEC Chapter 9 Table 4 raceway areas and Table 5 conductor dimensions. Checks the 40% fill limit for three or more conductors.
- Electrical Homerun SoftwareOrganize homeruns and branch circuiting as structured runs. Connect devices to circuits, circuits to panels, and panels to the distribution system.
- Electrical Estimating SoftwareElectrical estimating software built around the electrical system, not just object counts. Structured scope, feeders, runs, and reviewable calculations that carry into your estimate.
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