SpecCalc Hub Guide
Why wire size estimate is not final cable selection
Voltage-drop-based sizing is only one part of cable selection.
Only voltage drop
A conductor can pass a voltage-drop estimate and still fail ampacity or installation rules.
Professional verification
Final cable selection requires qualified review and local code compliance.
Why it matters
Cable selection has multiple constraints. A conductor may look acceptable for voltage drop while still being unsuitable for current capacity, temperature, grouping, mechanical protection or the specific wiring method.
Formula or method
The calculator compares standard conductor sizes against a simplified voltage-drop target. It uses material, length, current, system type and supply voltage assumptions, then reports a preliminary matching standard size where possible.
Worked example
If a small conductor causes a voltage drop above the selected target, the estimate moves to larger standard sizes until the simplified drop is within the target. If no standard size in the model fits, the result is a planning warning rather than a final answer.
Comparison table
| Topic | Value | Note |
|---|---|---|
| Voltage-drop check | Electrical loss over length | Useful, but not complete. |
| Ampacity check | Current and heating | Requires installation and code context. |
| Protection check | Breaker/fuse coordination | Must be verified separately. |
Common mistakes
- Calling the estimate a final cable size.
- Ignoring local cable tables and installation derating.
- Using the result for protection-device selection.
Limitations
- The estimate is simplified and informational.
- AC reactance and low power factor are not modeled.
- Qualified professional review is required before real installation decisions.
When to use the calculator
Use the Wire Size Estimate Calculator for early comparison and documentation of assumptions, then verify with proper cable tables and a qualified professional.
FAQ
Can this page select a cable for installation?
No. It is a preliminary estimate only and not a final engineering selection.
Why can the result say no standard size was found?
The entered load, length or target may exceed the simplified standard-size range included in the MVP model.
Related guides
Last reviewed: 2026-05-29