Voltage Drop Calculator
Calculate the voltage drop, voltage at the load, and heat loss across a cable run from its load current, wire gauge, and one-way length — and check it against the NEC's recommended voltage drop limits.
Input
Load
Cable
NEC recommends ≤3% for branch circuits, ≤5% combined feeder + branch.
Output
| Property | Value |
|---|---|
| No data yet | |
More ways to use this tool
REST API
curl -X POST https://api.iotools.cloud/v1/tool/voltage-drop-calculator \
-H "Authorization: Bearer YOUR_API_KEY" \
-H "Content-Type: application/json" \
-d '{
"loadCurrent": "15",
"supplyVoltage": "120",
"cableLength": "50",
"lengthUnit": "ft",
"wireGauge": "12",
"conductor": "copper",
"voltageSystem": "ac1",
"maxVdPercent": "3"
}'Swap in your own key from your account. The tool's fields are the body — no wrapper.
Ask an AI agent
Use the IOTools `voltage-drop-calculator` tool (Voltage Drop Calculator) on this input:
YOUR_INPUT_HEREPaste this at any agent connected to the IOTools MCP server, then add your input.
Embed widget
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src="https://iotools.cloud/embed/voltage-drop-calculator/"
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sandbox="allow-scripts allow-forms allow-same-origin allow-downloads allow-popups allow-popups-to-escape-sandbox"
allow="clipboard-write"
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<script src="https://iotools.cloud/embed.js" async></script>Drop this into your own page — free, no key required, just a link back.
| Cost per API/MCP call | From 5 credits |
|---|---|
| Need more credits? | View pricing |
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Guides
The Voltage Drop Calculator tells you how much voltage a circuit loses over a cable run — and whether that loss stays within the NEC's recommended limits — from a known load current, wire gauge, and length. It's the complement to the Wire Size Calculator: use that tool first if you need a gauge recommendation, then use this one to double-check a specific wire size you already have (or already installed).
How to use it
- Enter the load current in amps and the supply voltage.
- Enter the one-way cable length and pick feet or meters — this is the distance to the load, not the round-trip length.
- Choose the wire gauge (AWG) and conductor material (copper or aluminum).
- Choose the voltage system — AC single-phase, AC three-phase, or DC.
- Set your maximum acceptable voltage drop as a percentage (the NEC recommends ≤3% for a branch circuit, ≤5% combined feeder + branch).
The voltage drop in volts and percent, the voltage actually reaching the load, the power lost as heat, and a pass/fail status against your limit appear instantly.
The voltage-drop formula
Voltage drop is the voltage lost to the resistance of the conductors. Because current flows out and back, both conductors count:
- DC and AC single-phase:
Vdrop = 2 × L × I × R - AC three-phase:
Vdrop = √3 × L × I × R
where L is the one-way length in thousands of feet, I is the current in amps, and R is the conductor's resistance in ohms per 1000 ft (from the NEC Chapter 9, Table 8 resistance tables). Voltage-drop percentage is Vdrop ÷ supply voltage × 100, and power lost as heat is Vdrop × I.
For example, a 15 A load on 12 AWG copper over a 50 ft run at 120 V single-phase: 2 × 0.05 × 15 × 1.93 = 2.9 V, or 2.41% — within a 3% limit.
What if my exact wire size isn't listed?
The calculator covers the standard sizes from 14 AWG to 4/0. For anything outside that range, use the Wire Size Calculator, which covers the same table and additionally recommends the smallest size that satisfies both ampacity and voltage-drop limits for your circuit.
Is this a substitute for an electrician?
No. This tool is a planning aid using standard 75°C table values. Real installations must account for ambient temperature, conductor bundling, termination ratings, and local code — always confirm with a licensed electrician and the current edition of the NEC or your local electrical code.
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