Energy & Workshop Math
Wire Size Calculator – Voltage-Drop Comparison
Compare listed reference gauges against a chosen voltage-drop limit.
Enter your measurements
Results update as you type.
About this calculator
Find the smallest listed Southwire reference gauge meeting a voltage-drop limit for the run. This comparison does not establish a safe conductor size: ampacity, temperature, protection and installation must also be checked.
How to measure
- Measure the one-way run length from supply to load.
- Read the continuous load current and supply voltage.
- For AC, note the power factor and select 3% or 5%.
- Confirm ampacity and termination ratings separately.
How to use this calculator
- Choose DC, single-phase or balanced three-phase.
- Enter current, one-way length and supply voltage.
- Pick the reference conductor material and a comparison drop limit; 3% and 5% are not universal code limits.
- For AC, enter the lagging power factor.
- Read the smallest reference gauge meeting the drop limit. Have a qualified electrician check ampacity, derating, protection, terminations and applicable local requirements separately.
Formula
DC drop = 2 × length × current × resistance
AC drop ≈ circuit factor × length × current × (resistance × PF + reactance × sin(acos(PF)))
Choose the first gauge whose drop ÷ supply ≤ your limit
Receiving voltage = supply − drop
Assumptions
- Reference data is for the named Southwire product series at 75°C. Its 60 Hz reactance assumes the cited non-ferrous conduit and conductor spacing; other cables, frequencies and arrangements require matched data.
- AC reactance is at 60Hz; AC estimates use a balanced, lagging-power-factor approximation.
- The simplified AC projection is supported only when the total series-voltage magnitude is at most 10% of supply. This is a software model limit, not a code or safety limit. Outside it, full circuit/phasor analysis is required.
What the comparison checks
| Item | Included? |
|---|---|
| Chosen voltage-drop limit | Yes |
| Conductor ampacity / derating | No |
| Breaker and termination ratings | No |
| Local installation compliance | No |
The displayed gauge is a reference-product voltage-drop comparison only.
Common mistakes
- Treating the smallest listed voltage-drop gauge as a safe installation size.
- Using a universal 3% or 5% code claim.
- Ignoring conductor temperature, installation conditions and protection.
Worked example
Try it: press “Load example” above the form.
20 A over 30 m at 12 V DC with a 3% comparison limit selects reference 3/0 copper at 75°C. This is a voltage-drop comparison, not ampacity approval.
Limitations
Educational reference-gauge comparison for voltage drop only. The displayed gauge is not a safe or compliant installation size. A qualified electrician must separately assess ampacity, temperature, protection, terminations and applicable requirements. The simplified AC projection is supported only when the total series-voltage magnitude is at most 10% of supply. This is a software model limit, not a code or safety limit. Outside it, full circuit/phasor analysis is required.
Frequently asked questions
What does minimum conductor mean here?
It is the smallest gauge in the listed reference product series meeting the selected voltage-drop limit. It is not the minimum legal or thermally safe conductor size.
Can I treat 3% or 5% as universal code limits?
No. They are comparison limits in this tool. Applicable requirements depend on circuit type, jurisdiction and installation. Have the full design checked rather than choosing cable from a voltage-drop result alone.
What does the worked example show?
20 A over 30 m at 12 V DC with a 3% comparison limit selects reference 3/0 copper at 75°C. This is a voltage-drop comparison, not ampacity approval.
Which assumptions should I check before using the result?
Educational reference-gauge comparison for voltage drop only. The displayed gauge is not a safe or compliant installation size. A qualified electrician must separately assess ampacity, temperature, protection, terminations and applicable requirements.
Sources
Sources explain the methods and any named product presets. Check your product label where coverage, yield or density differs.
- Southwire SPEC10010: copper electrical data, Table 2
Reference copper product series. DC resistance at 25°C, AC resistance at 75°C, inductive reactance at 60 Hz. Checked 8 November 2026.
- Southwire SPEC10015: aluminum electrical data, Table 2
Reference aluminum product series. Listed sizes only. Not a generic NEC ampacity or code approval. Checked 8 November 2026.
- Southwire: Power Cable Manual, fourth edition
Historical manufacturer reference. General resistance-versus-temperature discussion: chapter 2, printed pages 2-16–2-17. The cited product data and operating assumptions do not establish current local-code compliance or complete cable design.
- ABB: electrical handbook
Voltage-drop mathematical context, section 2.2.2. Historical handbook, not current code authority or wire-sizing guidance.
Calculation documentation · Updated 2026-11-12 · Version 1.6.4. Estimates are planning aids. Confirm quantities, coverage and prices with your supplier before purchasing. Found an error? Report it.
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