Maximum Wire Length Calculator
Calculate voltage-drop-limited maximum one-way wire length from conductor size, material, current, supply voltage, and allowed voltage drop. Results are voltage-drop limits only and do not verify ampacity, safe current, NEC compliance, or IEC compliance.
Engineering tool
Maximum Wire Length Calculator
Calculate the voltage-drop-limited maximum one-way cable length for a known conductor size, material, current, and allowed drop. This is not an ampacity or code-compliance calculator.
Used as the voltage-drop percentage denominator and load-voltage reference.
Current must be greater than zero; zero current has no finite voltage-drop-limited length.
Percentage is based on supply voltage: Vdrop / Vsupply × 100%.
Material presets reuse the shared Wires AWG reference data.
Conductor metal area, not cable outer diameter.
Result console
- Maximum One-Way Cable Length
- 4.35035m
- Maximum Total Conductor Length
- 8.7007m
- Wire Size
- 2.5mm²
- Selected Wire Area
- 2.5mm²
- Loop Resistance at Maximum Length
- 0.06Ω
- Voltage Drop at Maximum Length
- 0.6V
- Voltage Drop Percentage
- 5%
- Minimum Load Voltage
- 11.4V
- Wire Power Loss
- 6W
- Resistance per Meter
- 0.0069Ω
- Resistance per Kilometer
- 6.896Ω
- Load Current
- 10A
- Supply Voltage
- 12V
- Maximum Allowed Drop
- 0.6V
- Material
- Copper
- Resistivity
- 1.724e-8Ω·m
Maximum length is based on the specified voltage-drop limit only. Ampacity, code compliance, insulation rating, installation method, bundling, termination rating, and fault protection are not verified.
Formula reference
Voltage-drop maximum length formulas
WIR-004 solves the inverse of the WIR-002 voltage-drop model for a two-conductor DC circuit. The displayed maximum length is one-way cable length.
Rloop = 2ρL/AVdrop = I × RloopLmax = Vdrop,max A/(2ρI)Vdrop,max = Vsupply × p/100Rloop,max = Vdrop,max/IVload,min = Vsupply - Vdrop,maxPloss = I²Rloop = I × Vdrop,maxVariable definitions
- ρ
- conductor resistivity
- Lmax
- maximum one-way source-to-load cable length
- A
- conductor cross-sectional area
- I
- load current
- p
- maximum voltage-drop percentage
WIR-004 Formula Audit
| Core Maximum Length Formula | Lmax = Vdrop,max A / (2ρI). |
|---|---|
| One-Way Length Definition | Lmax is the source-to-load physical cable length. |
| Total Conductor Length Formula | Ltotal,max = 2 × Lmax for two equal DC conductors. |
| Loop Resistance Formula | Rloop = 2ρL/A. |
| Voltage Drop Formula | Vdrop = I × Rloop. |
| Percentage Formula | %drop = Vdrop / Vsupply × 100%. |
| Allowed Drop Conversion | Vdrop,max = Vsupply × p / 100. |
| Load Voltage Formula | Vload,min = Vsupply - Vdrop,max. |
| Power Loss Formula | Ploss = I²Rloop = I × Vdrop,max. |
| Resistance Identity | Rloop,max = Vdrop,max/I = 2ρLmax/A. |
| Power Identity | I²Rloop equals I × Vdrop,max. |
| AWG Dataset Reuse | AWG input uses data/reference/awg.ts. |
| AWG Precision Policy | Raw AWG area is used internally; rounded display values are not fed back into formulas. |
| Material Data Reuse | Copper/aluminum presets reuse WIR material data. |
| Reference Temperature Policy | Preset resistivity is treated as a reference value near 20°C. |
| Zero Current Policy | I = 0 is rejected because no finite voltage-drop-limited length exists. |
| Zero Drop Policy | Vdrop,max must be greater than zero, matching WIR-003. |
| DC / AC Boundary | DC and purely resistive conductor model only. |
| Three-Phase Boundary | No incomplete √3 shortcut is used. |
| Ampacity Boundary | No current capacity or thermal approval. |
| Code Compliance Boundary | No NEC, IEC, fuse, breaker, or safety compliance result. |
| WIR-002 Boundary | WIR-002 calculates actual drop for known length. |
| WIR-003 Boundary | WIR-003 solves required area for known length. |
| WIR-005 Boundary | WIR-004 reports power loss but does not optimize energy/loss design. |
Worked Examples
| Case | Calculation | Result |
|---|---|---|
| Critical spot 1 | 0.5 × 2.5e-6 / (2 × 1.724e-8 × 10) | 3.62529m one-way |
| Total length | 2 × 3.62529m | 7.25058m total conductor |
| 12V at 5% | Vdrop,max = 12 × 5 / 100 | 0.6V |
| 12V 5% length | 0.6 × 2.5e-6 / (2 × 1.724e-8 × 10) | 4.35035m |
| 24V at 5% | Vdrop,max = 24 × 5 / 100 | 1.2V |
| 24V 5% length | same wire/current | 8.70070m |
| Half current | I = 5A | length doubles to 7.25058m |
| Double current | I × 2 | length halves |
| Double area | A × 2 | length doubles |
| Double resistivity | ρ × 2 | length halves |
| Double drop | Vdrop,max × 2 | length doubles |
| Forward resistance | 2ρL/A | 0.05Ω |
| Forward voltage drop | 10A × 0.05Ω | 0.5V |
| Resistance identity | Vdrop/I | 0.05Ω |
| Power identity | I²R = I × Vdrop | 5W |
| WIR-003 round trip | Lmax into area solver | recovers 2.5mm² |
| AWG input | AWG 13 | uses raw area from shared table |
| Copper vs aluminum | same area/drop/current | copper allows longer run |
| mV conversion | 1mV | 0.001V |
| kV conversion | 1kV | 1000V |
| mA conversion | 1mA | 0.001A |
| ft conversion | 1ft | 0.3048m |
| in² conversion | 1in² | 0.00064516m² |
| 100% drop | p ≥ 100 | warning shown |
| Zero current | I = 0 | rejected, no Infinity |
Engineering Notes
- WIR-004 returns maximum one-way length, not total conductor length.
- The total conductor length is shown separately to make the return path explicit.
- The formula is the inverse of WIR-002's voltage-drop model.
- WIR-003 and WIR-004 form a round-trip pair: known length solves area; known area solves length.
- AWG input reuses the shared raw AWG table.
- Conductor area input is metal cross-sectional area, not cable outside diameter.
- Copper and aluminum presets use shared resistivity data.
- Reference resistivity is not hot operating resistance.
- Higher current shortens the allowed run.
- Higher conductor area lengthens the allowed run.
- Higher supply voltage only helps in percentage mode because it increases allowed absolute drop.
- The calculator does not check ampacity, breaker size, fuse size, insulation rating, or electrical code.
- AC reactance, power factor, skin effect, and proximity effect are outside this V1 model.
- Critical installations should be verified against standards and measured under real conditions.
Common Mistakes
- Confusing one-way length with total conductor length.
- Forgetting the return conductor.
- Entering loop length where one-way length is expected.
- Doubling length twice.
- Using voltage-drop percentage relative to load voltage.
- Entering 5% as 0.05.
- Using rounded AWG display area as a design basis.
- Assuming a larger AWG number means larger wire.
- Using cable outside diameter as conductor area.
- Calling voltage-drop-limited length a safe maximum length.
- Ignoring ampacity.
- Ignoring conductor temperature.
- Using this DC model as a complete AC cable design.
- Expecting a finite maximum length at zero current.
- Mixing mA and A.
- Mixing mV and V.
Support reference
FAQ
How do I calculate maximum wire length?
For a two-conductor DC cable, use Lmax = Vdrop,max A / (2ρI), where Lmax is one-way length, A is conductor area, ρ is resistivity, and I is load current.
What is the maximum cable length formula?
The voltage-drop-limited formula is Lmax = Vdrop,max A / (2ρI) for one-way source-to-load length in a two-wire DC loop.
How do I calculate cable length from voltage drop?
Convert conductor area, current, resistivity, and allowed voltage drop to SI units, then divide Vdrop,max A by 2ρI.
Why is the return conductor included?
Current travels out through one conductor and returns through the other, so loop resistance uses twice the one-way cable distance.
What is one-way cable length?
One-way cable length is the physical distance from source to load. This is the primary output of the calculator.
What is total conductor length?
Total conductor length is the outgoing conductor plus the return conductor, so it is twice the one-way length for two equal conductors.
How does AWG affect maximum cable length?
Larger conductor area gives lower resistance per meter and allows a longer run for the same current and voltage-drop limit.
How does conductor area affect maximum length?
Maximum length is directly proportional to conductor area. Doubling area doubles the voltage-drop-limited length.
How does current affect maximum wire length?
Maximum length is inversely proportional to current. Doubling current halves the maximum length for the same conductor and drop limit.
How does supply voltage affect maximum length?
In percentage mode, higher supply voltage increases the allowed absolute voltage drop and therefore increases maximum length.
How does allowed voltage drop affect maximum length?
Maximum length is directly proportional to the allowed voltage drop. A stricter drop limit gives a shorter maximum run.
How do I calculate maximum length using a percentage drop?
First convert the percentage using Vdrop,max = Vsupply × percentage / 100, then apply the maximum length formula.
Is voltage-drop percentage based on supply voltage?
Yes. The project convention is %drop = Vdrop / Vsupply × 100%, matching WIR-002 and WIR-003.
Can I calculate copper wire length?
Yes. Copper is supported through the shared AWG material dataset.
Can I calculate aluminum wire length?
Yes. Aluminum is supported and usually produces a shorter maximum length than copper for the same area.
Does wire temperature affect maximum length?
Yes. Resistance changes with conductor temperature, but this calculator uses reference resistivity only.
Does this calculator correct for conductor temperature?
No. Full temperature correction is reserved for WIR-011 and should be handled separately for critical designs.
Does this calculator check ampacity?
No. It only checks voltage drop and does not verify thermal current capacity.
Is this the maximum safe wire length?
No. It is a voltage-drop-limited length, not a safety or code-compliance approval.
Is the result NEC compliant?
No NEC compliance is implied. Verify ampacity, installation method, protection, and all applicable code requirements separately.
Is the result IEC compliant?
No IEC compliance is implied. The result is only a DC resistive voltage-drop estimate.
Can I use this calculator for AC?
Only as a simple low-frequency resistive estimate. Complete AC cable design may require reactance, skin effect, proximity effect, and power factor.
Can I use this calculator for three-phase systems?
No. This V1 tool intentionally avoids partial three-phase shortcuts such as adding √3 without a complete model.
Why is maximum length undefined at zero current?
At zero current, ideal resistive voltage drop is zero, so voltage drop alone does not impose a finite maximum length.
What happens if allowed drop is greater than the supply voltage?
The math can still be evaluated, but the page warns that the limit may imply zero or negative load voltage.
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