Cable losses and cable length in the secondary circuit

How many VA does the measuring cable between current transformer and instrument consume? Calculates cable burden, total burden against the rated output and the maximum cable length – with a recommendation on 1 A or a larger cross-section.

Input values

Secondary current

One-way cable length (m)

Distance from transformer to instrument – the calculator adds the return conductor itself.

Cross-section (copper)

Instrument/meter burden (VA, optional)

Current circuit from the datasheet; leave empty = cable only.

Rated output of the transformer

Result

Cable resistance (out and back): 0.142 Ω
Cable burden: 3.56 VA
Total burden: 3.56 VA
Share of rated output: 71 %
Maximum cable length with this cross-section: 14 m
Same cable with 1 A secondary: 0.14 VA

Fits: the total burden is between 25 % and 100 % of the rated output – the transformer keeps its accuracy class.

Recommendation

With a transformer for 1 A secondary the total burden drops to 0.14 VA – the cable losses are one twenty-fifth.

How it is calculated

P = 2 · L · I² · ρ / A

Example: 5 A, 10 m, 2.5 mm² → 2 · 10 · 25 · 0.0178 / 2.5 = 3.56 VA

ρ = 0.0178 Ω·mm²/m (copper at 20 °C). Terminals and contacts are not included; the VA calculator adds a flat 0.1 VA for them.

Chart of cable burden in VA versus one-way cable length for 2.5 mm² copper: with 5 A secondary 3.56 VA at 10 m, with 1 A only 0.14 VA; a 5 VA transformer is sufficient at 5 A up to 14 m — The cable burden grows linearly with length and with the square of the secondary current: P = 2 · L · I² · ρ / A. With 1 A secondary it is one twenty-fifth of the value at 5 A.

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