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Cable Calculator

Last reviewed: June 24, 2026 · Formula version: CABLE-1.0 · Safety-sensitive estimate — check product labels and professional guidance before acting

Estimate voltage drop and wire loss

Estimate cable voltage drop from voltage, current, one-way length, wire gauge, material, and phase. This is a safety-sensitive planning aid, not electrical approval.

Server-rendered example result

Example: Input: 120 V, 15 A, 50 ft, 12 AWG copper, single-phase. Output: about 2.38 V drop, 1.99% drop, and 35.7 W power loss.

The interactive result cards update this example when JavaScript runs, but this default snapshot is crawlable and printable.

Quick answer

For 120 V, 15 A, 50 ft one-way, 12 AWG copper, the estimate is about 2.38 V drop or 1.99%, usually within a common 3% planning target.

Best for: comparing realistic scenarios before acting. Not for: final professional approval, emergency decisions, or replacing product labels and local requirements.

How to use the calculator

  1. Enter the measurements, quantities, costs, or target values requested above.
  2. Adjust optional assumptions such as waste, overhead, product strength, retention days, or multipliers.
  3. Read both the main result cards and any warning notes. The warnings are part of the answer, not fine print.
  4. Use the share, copy, or print/PDF controls when you want to save the scenario.

What the results mean

The first result card is the primary decision number. Supporting cards explain capacity, cost, efficiency, safety margin, input assumptions, or the next value to check. When the page returns a range, treat it as a planning envelope rather than a guaranteed price.

Formula / methodology

Voltage drop = current × conductor resistance × length × phase factor. Single-phase/DC pair uses a round-trip factor; three-phase uses 1.732.

Assumptions and limitations

  • Inputs are assumed to be measured accurately and entered in the units shown.
  • Rounding is intentional so the result is easier to use in real decisions.
  • Vendor-specific behavior, local code, product labels, and regional pricing can override a generic calculator.
  • High-risk medical, legal, tax, and emergency calculators are intentionally not published without expert review.

Example calculation

Input: 120 V, 15 A, 50 ft, 12 AWG copper, single-phase. Output: about 2.38 V drop, 1.99% drop, and 35.7 W power loss.

Server-rendered example result

Example: Input: 120 V, 15 A, 50 ft, 12 AWG copper, single-phase. Output: about 2.38 V drop, 1.99% drop, and 35.7 W power loss.

The interactive result cards update this example when JavaScript runs, but this default snapshot is crawlable and printable.

Common mistakes

  • Using one-way vs round-trip length incorrectly.
  • Ignoring aluminum vs copper resistance.
  • Treating a voltage-drop estimate as code approval.
  • Forgetting temperature, conduit fill, breaker size, and local code.

FAQ

What formula does this page use?

Voltage drop = current × conductor resistance × length × phase factor. Single-phase/DC pair uses a round-trip factor; three-phase uses 1.732.

What changes the result the most?

The most important inputs are the size, count, rate, target, or unit assumptions shown in the calculator.

Is this a final professional answer?

No. Treat it as a planning result and verify important decisions against product documentation, labels, quotes, local code, or professional guidance.

Why does this page show warnings?

Warnings call out assumptions that can materially change the result or create safety, cost, or reliability problems.

Can I share this scenario?

Yes. The share button copies a URL with the current inputs, while the canonical page remains the base calculator URL.

Last reviewed

Last reviewed: June 24, 2026. Formula version: CABLE-1.0. Index status: indexable.