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Voltage Drop Calculator (12/24 V DC & 230 V AC)

Cable voltage drop, end-of-line voltage and minimum conductor size for fire alarm sounder circuits, CCTV and access control power, and 230 V single-phase feeds.

FreeNo sign-upIEC 60228 conductor dataDaisy-chain mode

⚡ Supply

V
Treated as DC / resistive load (2-wire loop).
Fire alarm panels must work on standby batteries; test the worst case at end of discharge (typical 1.75 V/cell lead-acid).

🧵 Cable

mm²
Ω/km
m
Loop resistance uses 2 × this length.
Length = distance of the last device in the daisy-chain list.
°C
Rt = R20 × (1 + α(t − 20)); α = 0.00393 Cu, 0.00403 Al. Hot ceiling voids and fire-resistant cable runs are often 30–50 °C.

💡 Load

Load distribution
A
W
I = P / V (× PF for AC), at the supply voltage. Constant-power loads (PoE switches, DC-DC converters) draw more at low voltage, so keep a margin.
1.0 for resistive loads; LED drivers are typically 0.9–0.95.
mA
Use the alarm (worst-case) current from the datasheet at the lowest volume/flash setting you will not exceed.
Unticked: the ½ rule for a load spread evenly along the cable. Ticked is slightly more conservative.
#Distance from PSU (m)Current (mA)Remove
Distances are cumulative cable distance from the supply, not spacing. Max 200 devices.

🎯 Limits

%
V
0 = don't check. Many 24 V sounders/beacons are rated down to about 16–17 V (typical; check the datasheet).

📈 Circuit & voltage profile

🔔 Voltage at each device

Inputs

🧮 Calculation steps for your inputs
📘 Worked example
🎓 How to use this tool
  1. Pick the system voltage. For a fire alarm circuit on batteries, tick end-of-discharge battery voltage to test the worst case (21.0 V on a 24 V system).
  2. Choose the conductor size (mm² or AWG) and material. Use Custom with the datasheet Ω/km for unusual cables.
  3. Enter the one-way cable length: the route distance from the supply to the far end. Do not double it; the tool does.
  4. Choose how the load sits on the cable: at the end (one camera, one lock), evenly spread (sounders on a loop-less radial circuit), or daisy chain to list each device's distance and current.
  5. Enter the load as total amps, watts, or mA per device × number of devices. Use alarm current for sounders and inrush/heater current for cameras with heaters.
  6. Set the allowed drop % and the device minimum voltage. The tighter of the two governs.
  7. Read the verdict, then use Max length and Min conductor size to fix a failing circuit. Save or print the result for your submittal.

Tips & common mistakes

  • Entering the loop length as the one-way length doubles the result. Enter the route distance only.
  • Checking at 24 V nominal hides failures: fire alarm devices must still work at the end of the battery standby period.
  • For sounders, use the alarm current at the tone and volume you will commission, not the standby current.
  • Terminal, fuse and connector resistance add to the drop. Keep a margin rather than designing to exactly 100% of the limit.
  • 12 V CCTV over long runs rarely works: the same cable carries 4× the power at 24 V or 48 V for the same drop.

Standards & assumptions

Conductor resistance at 20 °C from IEC 60228 (class 1/2 plain copper; aluminium from 10 mm²). AWG values are typical solid annealed copper (ASTM B258). Aluminium below 10 mm² and custom areas are typical estimates. Temperature correction Rt = R20(1 + α(t − 20)). The 230 V calculation is resistive only (reactance ignored, which is small for ≤ 16 mm²; ΔV = I × Rloop is conservative versus R·cosφ). Typical 230 V limits: 3% lighting, 5% other uses (e.g. IEC 60364-5-52 Annex G / BS 7671 Appendix 4, from the origin of the installation). For fire alarm circuits, check the end-of-line voltage against the device's minimum operating voltage (EN 54-3 / BS 5839-1 practice, NFPA 72 in the US). This result is for guidance only and must be verified against the manufacturer's data and local codes.

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