How to Calculate the Cable Cross-Section for a 100 V PA Line

Cable cross-section for a 100 V public address (PA) line is chosen not for heating, but for permissible voltage drop: on long runs, a thin cable "eats" part of the voltage, and less power reaches the far loudspeakers. Below are the method, a rule-of-thumb formula, and how the VPA online calculator sizes the cable automatically.

Why Voltage Drop Decides Everything on a 100 V Line

A 100 V PA line is a high-voltage scheme with matching transformers: the amplifier steps the voltage up to 70–100 V, the line current stays low, and every loudspeaker carries a transformer that steps the voltage back down to the required level. This scheme was designed precisely to carry a signal tens and hundreds of meters over thin cable without major losses.

Losses still occur, though. Voltage drops across the conductor resistance, so less than 100 V reaches the far points — and loudspeaker power falls with the square of the voltage. A 10% voltage drop already means roughly a 19% power loss and noticeably quieter sound at the end of the line. That is why the cross-section is chosen to stay within a permissible drop, usually 10–20% depending on the speech-intelligibility requirements.

For more on the scheme itself and its limitations, see 70/100 V PA lines.

What Determines the Cross-Section: Length, Load, Voltage

The required cable cross-section grows with line length and total load power, and shrinks as the permissible drop increases. The key inputs for the calculation:

• Line length — measured along the actual cable route to the farthest loudspeaker, not as the crow flies;
• Total load power — the sum of the power of all loudspeakers on the line (per their transformer taps);
• Line voltage — 70 or 100 V;
• Permissible drop — set as a percentage (10–20%) based on the loudness and intelligibility requirements at the far point;
• Conductor material — copper (resistivity ≈ 0.0175 Ω·mm²/m).

Load distribution is checked separately: if the loudspeakers sit closer to the amplifier, the drop is smaller; if most of the load is at the end of a long run, you will have to upsize the cable or split the line into several branches.

A Rule-of-Thumb Formula for a Quick Estimate

For a back-of-the-envelope estimate, the voltage drop in a two-wire line is calculated as:

ΔU = (2 × ρ × L × P) / (S × U), where ρ is the resistivity of copper (≈ 0.0175 Ω·mm²/m), L is the line length (m), P is the total load power (W), S is the conductor cross-section (mm²), and U is the line voltage (V). The factor of 2 accounts for both conductors (out and back).

If you set a permissible drop ΔU_max (in volts — for example, 10% of 100 V is 10 V), the minimum cross-section follows from S ≥ (2 × ρ × L × P) / (ΔU_max × U). Round the result up to the nearest standard cross-section.

This is strictly a guide: it ignores reactive components, load distributed along the route, and design margin. A full electroacoustic calculation with compliance checks is easier to run in the VPA online calculator, while the formula helps you understand what drives the result.

Which Cable Is Used: Fire-Resistant КПСнг(А)-FRLS

Cable for a voice evacuation system must remain operational during a fire for the code-mandated time — the line has to "survive" until the evacuation is complete. That is why fire-resistant cables with the FRLS index (fire resistant, low smoke) are used — the typical solution is КПСнг(А)-FRLS (KPSng(A)-FRLS): a fire-safe, low-smoke design that meets the fire-safety requirements for cable products under GOST 31565 (Russian cable fire-safety standard).

Selection factors include: the number and cross-section of conductors for the design load, the fire-resistance class for the required line operating time, and routing separate from power circuits. The specific cable type, fire-resistance time, and installation method are defined in the project design: VPA supplies the equipment; design and installation are performed by a licensed contractor.

A Worked Example

Take a 100 V line, a cable route of 500 m to the farthest point, total loudspeaker power of 200 W, and a permissible drop of 10% (that is, 10 V). Plugging into the formula:

S ≥ (2 × 0.0175 × 500 × 200) / (10 × 100) = 3500 / 1000 = 3.5 mm².

The nearest larger standard cross-section is 4 mm². If that cable is overkill for the route, there are alternatives: split the line into two branches (reducing each one's length and load), move some loudspeakers closer to the amplifier, or raise the permissible drop to 15–20% if intelligibility allows. Each option changes both the equipment count and its power — so the cable is sized not in isolation, but together with the loudspeaker layout and the amplifier.

How the VPA Calculator Sizes the Cable Automatically

The online tool at voice-pa.com performs the complete electroacoustic calculation: sound pressure level point by point and speech intelligibility (STI per IEC 60268-16), automatic selection of loudspeaker count and layout, amplifier power with a 25% margin, and then the cable cross-section per GOST 31565 for the specified voltage drop, plus UPS and battery capacity. It offers floor-plan upload with a heat map and export to Word, Excel, PDF, and DXF (AutoCAD).

Compliance is checked against SP 3.13130.2026 (Russian fire-safety code, for indoor spaces) and GOST R 42.3.01-2021 (Russian standard for outdoor civil alert zones). The tool is free and requires no sign-up — you can calculate the cable cross-section online and instantly see how it changes with a different line length or load. The resulting calculation is a basis for engineering design: VPA supplies the equipment; design and installation are performed by a licensed contractor, and compliance with the codes is documented in the project design.

VPA Equipment and Supply

For the calculated line, VPA supplies 70/100 V amplifiers, mixers, and loudspeakers — the mixer amplifiers section helps you select an amplifier of the right power. VPA brand equipment (VOICEPA LLC) is certified to EAEU (Eurasian Economic Union) requirements, with a 24-month warranty and a service life of at least 10 years.

Warehouse in Moscow with delivery across all of Russia; corporate (B2B) orders are placed via "Request a Quote".

Frequently Asked Questions

Build your line in a couple of minutes: enter the route length, load, and permissible drop — and the VPA calculator will size the cable along with the loudspeaker count, amplifier power, UPS, and export to Word, Excel, PDF, and DXF. Once the bill of materials is ready, place your order via "Request a Quote": warehouse in Moscow with delivery across all of Russia, and VPA equipment certified to EAEU requirements with a 24-month warranty.