EngiMetric Arena / Engineering Trade-off
Choose a cable size that satisfies drop and ampacity
Competency Constraints and trade-offs primary Quantitative reasoning supporting
What you should take away: Evaluate a cable choice against two independent electrical criteria and a physical termination limit, accepting that more than one standard size is defensible.
The situation
What is happening
A 400 V, 100 A balanced three-phase feeder runs 180 m. The project allows 4 % voltage drop, and the switchgear terminals accept conductors up to 95 mm2. Specify a standard conductor size that satisfies voltage drop, ampacity and the terminal limit.
Given
| Quantity | Value | Unit | Note |
|---|---|---|---|
| System voltage | 400 | V | Line-to-line, balanced three-phase. |
| Design current | 100 | A | Full-load current. |
| One-way run | 180 | m | Cable route length. |
| Allowable drop | 4 | % | Project limit. |
| Terminal limit | 95 | mm2 | Largest conductor the switchgear accepts. |
Assumptions this challenge makes
- The load is balanced and the conductor resistance is taken from the calculator table.
- Reactance is neglected, as in the voltage-drop engine.
- Ampacity is the tabulated free-air value; installation method and grouping are not applied.
Limits this decision must satisfy
| Limit | Requirement | If it is breached |
|---|---|---|
| Voltage drop | not more than 4 % | Above 4 % the load voltage is outside the project limit and the feeder must be enlarged or shortened. |
| Ampacity | not less than 100 A | Below the design current the conductor would overheat, regardless of how small its voltage drop is. |
| Terminal size | not more than 95 mm2 | Above the terminal limit the cable cannot be landed on the supplied switchgear without a transition. |
Any standard size that holds every stated limit is defensible. Larger sizes buy margin but cost more and may not fit the terminals.
Your call
Your answer
Hints
- Voltage drop and ampacity are separate checks. A size can pass one and fail the other.
- Compute the minimum size for ampacity, then check the drop for the candidate sizes around it.
- Do not forget the terminal limit. A very large conductor can pass both electrical checks and still be unbuildable.
Your answer Engine result
Where that comes from
Each limit, checked
Why
- Ampacity alone requires at least 35 mm2, because 25 mm2 is rated 90 A and 35 mm2 is rated 110 A.
- Voltage drop at 35 mm2 is about 4.08 %, just outside the 4 % limit. The next standard size, 50 mm2, gives about 3.02 % and passes.
- So 50 mm2, 70 mm2 and 95 mm2 all satisfy both voltage drop and ampacity. Each is a defensible choice: 50 mm2 is the minimum, while 70 and 95 mm2 buy margin against future load or rerouting.
- The 95 mm2 terminal limit excludes 120 mm2 and above, even though those sizes would pass the electrical checks.
- The trade-off is real: a larger conductor reduces losses and leaves growth margin, but it costs more and is harder to terminate. The constraints, not a single invented optimum, define the acceptable set.
What this challenge does not establish
- Ampacity is the tabulated free-air value; installation method, ambient temperature, grouping and derating are not applied.
- Reactance, skin effect and harmonics are neglected.
- The terminal limit is a project assumption, not a manufacturer’s verified data sheet.
- Protective-device coordination, short-circuit withstand and earth-fault loop impedance are separate checks.