Electrical Cable Size Calculator

Understand Cable Sizing, Ampacity & Voltage Drop from first principles

This section is intentionally more detailed than a normal calculator page. The aim is to help a learner move from what to enter, to why the formula works, to what must be checked in a real installation or financial decision.

1. Core relationship

A motor cable that carries the running current may still be unacceptable if starting voltage drop is excessive. Long motor feeders must therefore be checked at both running and starting current.

Before calculating, define every symbol, unit and reference point. A technically correct formula can still produce a wrong answer when line/phase quantities, annual/monthly rates, one-way/round-trip length, or input/output power are mixed.

2. Physical or mathematical meaning

Cable size is not selected from amperes alone.

Current-carrying capacity depends on conductor, insulation, ambient temperature, grouping and installation method.

Voltage drop depends on current, route length, conductor resistance, reactance and power factor.

Motor circuits need special attention to starting current and acceleration time.

Short-circuit withstand and protective-device coordination can require a larger conductor than load current alone.

3. Step-by-step reasoning workflow

1. Step 1: Calculate design current. Record the value, its unit and where it came from before moving to the next step.
2. Step 2: Choose Cu/Al conductor. Record the value, its unit and where it came from before moving to the next step.
3. Step 3: Apply installation derating. Record the value, its unit and where it came from before moving to the next step.
4. Step 4: Check current-carrying capacity. Record the value, its unit and where it came from before moving to the next step.
5. Step 5: Calculate voltage drop. Record the value, its unit and where it came from before moving to the next step.
6. Step 6: Verify short-circuit thermal withstand. Record the value, its unit and where it came from before moving to the next step.

4. Worked-example method

Use a worked example in three layers: first write the known values, then substitute them into the equation without rounding, and finally interpret whether the result is plausible. After the calculation, change one input at a time and observe the direction and sensitivity of the result. This develops engineering or financial intuition instead of only producing a number.

5. Sensitivity: what makes the result move?

Do not stop after one answer. Increase and decrease the important inputs and observe what changes. If the result is highly sensitive to an uncertain assumption, that assumption deserves better measurement or a larger design margin.

6. Common mistakes

• Choosing cable from a single generic ampacity table without derating
• Ignoring one-way versus circuit length convention
• Using DC resistance only for a large AC cable without considering reactance
• Not checking starting voltage at the motor terminals
• Assuming a larger breaker can solve nuisance tripping without checking cable protection

7. Field / decision checklist

• Confirm route length and installation method
• Record ambient/soil temperature and grouping
• Use manufacturer resistance/reactance values when available
• Check running and starting voltage drop
• Check protective-device disconnection and short-circuit withstand

8. How to read the diagram

Read the diagram from left to right. Each block is a stage in the reasoning chain. In electrical topics this usually follows energy or signal flow from the source to the load and protection. In finance topics it follows cash flow through time and compounding. If a stage is unknown, the final answer should be treated as provisional.

9. Why measured or real values may differ

Calculators deliberately simplify reality. Electrical equipment has tolerances, temperature effects, waveform distortion, voltage variation and dynamic behaviour. Financial outcomes have timing, fees, taxes, changing rates and market variability. A difference does not automatically mean the formula is wrong; first compare assumptions, units and the exact quantity being measured.

10. Deeper questions to ask