Transformer Secondary Tool

Transformer SecondaryCalculator

Calculate transformer secondary full-load current, conductor voltage drop, and a practical conductor recommendation for panelboard and MCC secondary runs.

Estimate secondary full-load current, conductor voltage drop, and the first conductor that meets a target drop. This is an engineering check only; transformer secondary conductor protection still requires NEC 240.21(C), 450.3, 310.16, and local amendments.

Secondary Inputs

kVA, secondary voltage, route length, and conductor choice

Transformer phase

Transformer size (kVA)Secondary voltage (V)Load on transformer (%)One-way secondary length (ft)Target voltage drop (%)Parallel setsConductor materialSelected conductor

Calculated Results

Secondary current and selected conductor drop

Full-load current

124.9 A

Design current

124.9 A

Selected drop

2.90 V

Drop percent

1.40%

Selected conductor status

The selected 1/0 copper conductor leaves about 205.1 V at the secondary load terminals. First conductor meeting 3.0% drop is 1.

Conductor Comparison

Nearby sizes using NEC Chapter 9 Table 8 resistance

Conductor	Resistance	Drop	Drop %	Load voltage	Status
1 Cu	0.1540 ohm/kft	3.66 V	1.76%	204.3 V	Meets target
2 Cu	0.1940 ohm/kft	4.62 V	2.22%	203.4 V	Meets target
3 Cu	0.2450 ohm/kft	5.83 V	2.80%	202.2 V	Meets target