DC Voltage Drop Wire Gauge (AWG) Calculator: 12V, 24V & 48V
Undersized DC wiring is one of the most common causes of dim lights, weak inverters and overheating connectors in solar, RV and marine systems. This calculator applies the standard circular-mil formula to find the minimum wire gauge for your amperage, distance and allowable voltage drop.
Use at least 4 AWG for this run — it keeps voltage drop to 2.5%, within your 3% target.
| Measure | Value |
|---|---|
| System voltage | 12 V |
| Load current | 30.0 A |
| One-way distance | 20.0 ft |
| Max allowed drop | 3% (0.36 V) |
| Min circular mils | 35,833 |
| Recommended gauge | 4 AWG |
| Actual drop | 0.30 V (2.5%) |
| Power lost as heat | 8.9 W |
- 1.
Min_Circular_Mils = (2 × Distance_Ft × Amps × 10.75) / Max_Drop_Volts - 2.
Voltage_Drop = (2 × Distance_Ft × Amps × Resistance_per_1000ft) / 1000 - 3.
Power_Loss_W = Voltage_Drop × Amps
How to Use This Calculator
1. Enter your details
Use the labelled fields and units that match your situation.
2. Calculate instantly
The result, visual comparison, and working update as values change.
3. Review your result
Check assumptions, then copy, print, or share the calculation.
Formula & step-by-step maths
- Distance_Ft
- One-way wire run length
- Amps
- Total circuit load current
- Max_Drop_Volts
- System voltage × allowed drop %
Why voltage drop matters more at low voltage
A 3% drop on a 120V AC circuit is trivial, but the same percentage drop on a 12V DC circuit represents a much larger share of your usable voltage. That's why DC systems typically need much heavier wire gauge for the same amperage and distance than AC household wiring.
The circular mil formula explained
Circular mils measure cross-sectional wire area. The formula multiplies distance (round-trip, hence the factor of 2), current and a constant (10.75, the resistivity of copper) then divides by your allowed voltage drop — larger mil values mean thicker, lower-resistance wire is required.
Critical vs non-critical circuits
ABYC and NEC guidance treats circuits like bilge pumps, navigation lights and engine controls as critical (3% max drop), while interior lighting and accessories tolerate up to 10%. Choosing the wrong tolerance can leave safety-critical equipment underpowered.
Heat and power loss consequences
Every volt of drop across a wire converts directly to wasted heat (Watts = Volts × Amps). Beyond wasted energy, that heat can degrade insulation over time, so oversizing wire slightly beyond the bare minimum is cheap insurance.
Common AWG gauge amp capacity (10ft one-way, 3% drop, 12V)
| AWG | Max amps (10ft) | Max amps (25ft) |
|---|---|---|
| 18 AWG | 6A | 2.5A |
| 14 AWG | 15A | 6A |
| 10 AWG | 30A | 12A |
| 8 AWG | 45A | 18A |
| 6 AWG | 75A | 30A |
| 4 AWG | 110A | 45A |
| 2 AWG | 150A | 60A |
Frequently Asked Questions
Three worked examples
Same engine, three different starting points — useful if you want to see how sensitive the answer is before you type your own numbers in.
Example 1: total amperage load 23.4 A, system voltage "12V"
On the lower / more conservative end. Use at least 4 AWG for this run — it keeps voltage drop to 1.9%, within your 3% target.
Example 2: total amperage load 30 A, system voltage "12V"
A typical middle-of-the-road setup. Use at least 4 AWG for this run — it keeps voltage drop to 2.5%, within your 3% target.
Example 3: total amperage load 39 A, system voltage "24V"
On the higher / more demanding end. Use at least 6 AWG for this run — it keeps voltage drop to 2.6%, within your 3% target.
Quick answers about the DC Wire Voltage Drop
What exactly does the DC Wire Voltage Drop work out?
Undersized DC wiring is one of the most common causes of dim lights, weak inverters and overheating connectors in solar, RV and marine systems. You enter system voltage, total amperage load, one-way wire distance and max allowable voltage drop and the result panel updates straight away, so you can compare two or three versions of the same question in a few seconds.
What do I need before I start?
Only 4 fields: system voltage, total amperage load, one-way wire distance and max allowable voltage drop. Nothing else is needed and nothing is stored.
How is it calculated — why voltage drop matters more at low voltage?
A 3% drop on a 120V AC circuit is trivial, but the same percentage drop on a 12V DC circuit represents a much larger share of your usable voltage. The same maths runs inside this page, so hand-checking the result on paper gives you the identical figure.
Why do two calculators give me different answers for dC Wire Voltage Drop?
Circular mils measure cross-sectional wire area. Different sites pick different assumptions, so always check which method a calculator states before you trust the gap between two numbers.
What does the "Common AWG gauge amp capacity (10ft one-way, 3% drop, 12V)" table on this page tell me?
It is the reference range this tool works against — 7 rows from "18 AWG" (6A) up to "2 AWG" (150A). Use it to sanity-check whether the number you just calculated sits where you expected it to.
Which system voltage should I pick?
The dropdown offers 4 choices — 12V, 24V, 36V and 48V. Pick the one that matches your real situation rather than the one you would like to be true; system voltage usually moves the final figure more than any other single input, so it is worth running it twice with the option above and below your guess.
Does it work in both metric and imperial (or another currency)?
Yes. The "System voltage" control converts every field and every result, so you never have to convert anything by hand before typing it in. Switch it after entering your numbers and the output re-renders instantly in the new system.
Do I have to press a button or reload the page to see the result?
No. DC Wire Voltage Drop runs completely inside your browser, so the moment you change a value the cards recalculate — there is no submit step, no page reload and no waiting for a server round trip. That also means it keeps working on a weak or intermittent mobile connection.
Is it free, and do you keep what I type?
It is free with no sign-up, no app install and no usage limit. Nothing you enter into DC Wire Voltage Drop leaves your device — the calculation is JavaScript running locally, so there is no upload of your figures to DrHint or anyone else.
Can I use it on a phone?
Yes — the layout stacks to a single column on small screens and the number fields open the numeric keypad on both Android and iOS. Many people bookmark this page or add it to their home screen and re-open it whenever the question comes up.
Anything to be careful about with the result?
Because current must travel out to the load and back through the negative/ground conductor, so the total resistive path is twice the one-way distance you measure. Treat the output as a well-grounded estimate for planning, not as a professional, legal or medical decision on its own.
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