Wire Size Calculator
Pick AWG for ampacity and max voltage drop.
Wire Size Calculator
Wire sizing for DC solar and battery circuits must satisfy two limits at once: the cable must carry the current safely (ampacity), and voltage drop along the run must stay low enough that loads and chargers see adequate voltage.
This calculator steps through common copper AWG sizes from 14 to 4, checks each against your amp draw and one-way cable length, and recommends the smallest gauge that meets both ampacity and your maximum voltage drop percent.
Use it when you know amps and run length but not which AWG to buy. Pair with the voltage drop calculator to verify a specific wire size you already have on hand, or to see drop volts at the recommended gauge.
How this calculator works
- Enter load current in amps—the maximum steady DC current on the circuit.
- Enter system voltage and one-way cable length in feet (source to load; round trip is calculated internally).
- Set max voltage drop percent—3% is a common target for critical DC runs; use 2% on long 12 V circuits if you want extra headroom.
- The tool tests AWG 14, 12, 10, 8, 6, and 4 in order. Each size must meet ampacity ≥ your amps.
- Among sizes that pass ampacity, the first that also meets your drop limit is recommended. Results show AWG, ampacity, and achieved drop %.
Core wire sizing relationships
Ampacity ≥ load amps AND Drop % ≤ max % | Drop % = (2 × I × R/ft × length) ÷ system V × 100
Ampacity is a safety ceiling—wire must not overheat under sustained current. Voltage drop is a performance limit—excessive loss wastes power and depresses voltage at the load. The calculator picks the smallest standard AWG that satisfies both. If even 4 AWG exceeds your drop limit, the result still shows 4 AWG with the actual higher drop—shorten the run, raise voltage, or accept the loss.
Formulas used by this tool
- Checks AWG ampacity first, then voltage drop at your length.
- Recommends the smallest wire that meets both limits.
- Drop limit is compared against your max % voltage drop input.
Worked example: 15 A on 12 V, 30 ft
A 12 V circuit draws 15 A maximum with a 30 ft one-way run. You set a 3% max voltage drop.
- 14 AWG (15 A ampacity) passes current but drop is too high—skipped for drop.
- 12 AWG (20 A) still exceeds 3% drop at 30 ft for 15 A.
- 10 AWG (30 A) typically meets both 15 A ampacity and ≤3% drop—recommended size.
- Result shows recommended AWG, ampacity rating, and actual drop % at that gauge.
Practical tips
- Size for maximum current, not average—solar strings and inverter input use peak or continuous ratings.
- Higher system voltage (24 V or 48 V) lowers amps for the same watts and often allows smaller AWG at the same drop target.
- Use copper UL-listed DC cable; undersized terminals and long fuse leads add resistance beyond this model.
- If the tool returns 4 AWG with drop above your limit, no size in the table fixes the run—shorten cable or split circuits.
- Marine and welding cable have flexible stranding but follow the same ampacity and drop math.
Frequently asked questions
What is ampacity?
Ampacity is the maximum current a wire can carry continuously without exceeding safe temperature limits. This calculator uses conservative planning values for common copper AWG in DC applications. Local electrical code, bundling, and ambient temperature can require larger wire than these tables alone.
What max voltage drop should I use?
3% is widely used for solar DC homeruns (array to controller, battery to inverter). Some designers use 2% on 12 V systems because absolute voltage loss is small. Enter the stricter percent if you want the tool to upsize wire accordingly.
Why smallest wire that meets both limits?
Heavier wire costs more and is harder to route. Once ampacity and drop are satisfied, the next larger AWG adds little benefit for steady DC loads. The tool stops at the first passing size in the table (14 → 4).
Does this replace NEC or local code?
No. It is a planning aid for off-grid, RV, and solar DC wiring using simplified ampacity and resistance data. Permitted installations must follow applicable codes, conduit fill rules, and manufacturer instructions.
How is this different from the voltage drop calculator?
Voltage drop calculator: you pick AWG and see drop. Wire size calculator: you set a max drop % and the tool picks AWG. Use wire size when shopping; use voltage drop when verifying existing cable.
What if my amps exceed 4 AWG ampacity?
The built-in table tops out at 4 AWG (70 A planning ampacity). For higher current, consult code tables or manufacturer data for larger gauges, parallel conductors, or busbar—this tool will recommend 4 AWG but drop may still fail your percent limit.
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