100 Volt Cable Loss Calculator and Voltage Drop

This calculator estimates voltage loss in a 100-volt constant-voltage speaker cable from the cable length, conductor size, copper resistivity, and amplifier load. Enter one-way distance and total connected speaker power to see the cable’s voltage drop, the voltage at the far end, and the percentage lost.

Formula

The calculator treats the amplifier as a 100 V RMS source feeding a resistive cable loop and a resistive load. It estimates the load resistance from power at 100 V, then uses the voltage divider relationship:

R_loop = 2ρL/A
R_load = V²/P
V_drop = V × (1 − R_load/(R_load + R_loop))

The calculator’s equivalent drop expression is V × (1 − 1/√(1 + R_loopP/V²)). Here V is nominal line voltage in volts, ρ is conductor resistivity in ohm·mm²/m, L is one-way distance in metres, A is one conductor’s cross-sectional area in mm², P is total speaker power in watts, and R is resistance in ohms. The factor of two in loop resistance accounts for the outgoing and return conductors. It uses V = 100 V.

Worked Example

Using the defaults: L = 100 m, A = 1.5 mm², P = 100 W, and ρ = 0.0172 ohm·mm²/m.

First, loop resistance is 2 × 0.0172 × 100 / 1.5 = 2.293 ohms. The load resistance at 100 V is 100² / 100 = 100 ohms. The estimated drop is 100 × (1 − 1/√(1 + 2.293/100)) = 1.13 V. Rounded to the calculator’s two decimals, the far-end voltage is 98.87 V and voltage loss is 1.13%. The loop-resistance output is 2.293 ohms, shown to three decimals.

When to Use This Calculator

Use it for an initial estimate when planning a nominal 100 V distributed-audio run, comparing cable sizes, or checking how a longer route or larger speaker load could affect voltage at the end. The distance input is one-way route length, not the combined length of both conductors. The formula doubles that distance internally to account for the full electrical loop.

Speaker taps determine the power drawn from the line. Add the selected tap wattages for all speakers connected to the run and enter that total, rather than the amplifier’s nameplate rating. If taps or speaker counts change, recalculate the total load. The estimate assumes the full load is at the far end; speakers distributed along the cable draw current at different points, so an actual distributed system may have a different drop.

How to Read the Results

“Cable voltage drop” is the estimated difference between the nominal 100 V source and the voltage at the modeled load. “Estimated far-end voltage” subtracts that drop from 100 V. “Voltage loss” expresses the drop as a percentage of the nominal source voltage. “Loop cable resistance” is the calculated resistance of both conductors together.

For comparison, a 1% voltage loss on a 100 V line equals 1 V. This calculator reports electrical voltage loss, not a guaranteed change in sound level or a pass/fail judgment. Check the installation’s design criteria and equipment documentation when deciding whether a result is acceptable.

Common Mistakes and Limits

Keep units consistent: enter cable area in mm², distance in metres, power in watts, and resistivity in ohm·mm²/m. For US customary measurements, convert feet to metres (1 ft = 0.3048 m) and AWG wire size to its conductor area in mm² before entering it. Do not enter AWG numbers directly as area.

The default resistivity, 0.0172 ohm·mm²/m, approximates copper near room temperature. Copper resistance rises as it warms, and cable specifications can differ; use the conductor manufacturer’s resistance data when available. This model also assumes a resistive load, a stable 100 V RMS source, and no transformer, connector, splice, or other wiring losses. Real loudspeaker transformers and audio signals vary, so treat the result as an estimate, not a field measurement.

The calculation is general arithmetic, not cable sizing or safety advice. It does not check amplifier capacity, installation conditions, applicable codes, or manufacturer requirements. Confirm cable selection and installation details with a qualified audio-system professional, especially on long runs or high loads.

How to Use This List

Enter the actual one-way cable route, the area of one conductor, and the sum of speaker tap powers on that run. If the cable specification gives resistance rather than area, this calculator cannot use that figure directly; obtain the conductor area and suitable resistivity, or use a method based on the specified resistance. Compare results across plausible cable sizes, then verify the design against the equipment documentation and a qualified professional’s review.

Frequently Asked Questions

Is a 100 V speaker line actually 100 volts?
It is a nominal constant-voltage audio distribution system; the exact signal voltage varies with the audio signal and load.

Should distance mean one-way or round-trip?
Enter one-way distance. The formula doubles it to include both conductors.

What power should I enter?
Add the wattage selected at each speaker transformer tap connected to that cable run.

Can I enter AWG wire size?
No. Convert the wire’s conductor area to mm², or use a calculator designed for AWG inputs.

Does this predict sound-level loss exactly?
No. It estimates voltage loss under simplified resistive assumptions; transformers, speaker distribution, and signal conditions affect real performance.

Final Thoughts

This estimate is useful for comparing cable length, conductor area, and connected load on a 100 V audio run. Treat it as a planning aid, check the assumptions, and confirm the final design with qualified guidance.

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in d | One-way cable distance | m | 100 | 0 | 100000
in a | Conductor area | mm² | 1.5 | 0.01 | 1000
in p | Connected speaker power | W | 100 | 0 | 10000000
in r | Copper resistivity | ohm·mm²/m | 0.0172 | 0.01 | 0.02
out drop | Cable voltage drop | V | 100 * (1 - 1 / sqrt(1 + 2 * r * d * p / (a * 10000))) | 2
out end | Estimated far-end voltage | V | 100 - drop | 2
out loss | Voltage loss | % | 100 * drop / 100 | 2
out resistance | Loop cable resistance | ohm | 2 * r * d / a | 3