Conductor size and network cost
The key idea
A cheaper conductor can reduce the bill and still fail voltage or thermal requirements. Compare cost and electrical results on the same route and demand basis.
Separate the decisions
Conductor choice changes resistance, current capability and price. Route choice changes lengths and connections. If you change both at once, a lower total cost does not tell you which change caused it.
A fixed-route comparison holds geometry still. It is useful when you want to isolate the conductor effect. A re-route asks the designer to search again under the new conductor choice and may produce a different layout.
Try a fixed-route alternative
The baseline has two 150 m spans: one small and one large. Select a conductor and an override rule.
Every trunk span applies that conductor even where it downsizes a larger span. Only smaller spans sets a minimum size and preserves larger spans. Neither rule changes service conductors in this teaching comparison.
Baseline: 150 m of 35 mm² + 150 m of 70 mm². Route, poles, service conductors and transformer stay fixed.
- Installed total
- $5,770
- Change from baseline
- −$600
- Current / rating
- 116.6%
Voltage screen fails (6.82% drop). Thermal screen fails. Cost is reported even when a screen fails.
Read the values and assumptions
| Cost item | Quantity / basis | USD |
|---|---|---|
| Span 1 · 35 mm² | 0.15 km | 1275 |
| Span 2 · 35 mm² | 0.15 km | 1275 |
| Connections | 6 × 120 USD | 720 |
| Transformer | 1 × 2,500 USD | 2500 |
| Total | No separate pole charge | 5770 |
Model note · Illustrative 2026 USD, no geographic market or vendor quotation. Line rates: 35 mm² $8,500/km; 70 mm² $12,500/km, including poles, conductor, structures and labour. Six service connections at $120 each and one transformer at $2,500 are separate. Fixed 300 m route, 40 kW endpoint demand, 400 V, pf 0.9. Ratings are assumptions; the linear voltage/current screen is not PF3. No tax, land or financing cost.
Select 35 mm² for every span. Its total is lower, but both teaching electrical screens fail. Select 70 mm² for every span. The price rises while resistance and loading percentage fall.
Now select the minimum-size rule with 35 mm². The baseline large span stays large. A floor is not the same as making all spans identical.
Read the price basis
The installed per-km rate in this example already includes conductor, poles, pole-top structures and labour. Connections and the transformer are priced separately.
Do not add the poles again simply because the bill also lists their count. A count can support construction planning while its cost is already inside another rate.
A rate needs a currency, date, region or project source, and a statement of what it covers. These teaching rates use illustrative 2026 USD with no geographic market. They are not Phasor catalog defaults or supplier quotations.
The math, if you want itOptional — the page reads completely without it
The installed line cost is each span's length times its per-kilometre rate, plus the items that are priced separately:
installed total
C = Σ Li · ri + nconn · cconn + ctransformer
the baseline, two spans
C = 0.15 × 8,500 + 0.15 × 12,500 + 6 × 120 + 2,500 = 6,370 USD
The small conductor throughout gives 0.30 × 8,500 + 720 + 2,500 = 5,770 USD; the large conductor throughout gives 6,970 USD. Poles are already inside the per-kilometre rate.
resistive loss at the same current
Ploss = 3 · I² · R
thermal loading
loading = IIrating × 100%
Resistive loss scales with the total phase resistance; the voltage drop also carries the reactance and power-factor terms from the previous lesson.
What a rerun does not establish
A saved alternative with a violation is still useful evidence: it shows the cost of a choice that does not meet the selected limits. Saving it does not approve it.
The thermal ratings here are deliberately stated assumptions. Real cable capability depends on installation, temperature, grouping, material and construction. Use appropriate equipment data and design conditions.
The 5% voltage screen is also a teaching threshold. Select project limits from the applicable design basis; do not infer a universal requirement from this explorer.
See it in Phasor
Use “Rerun with conductor” on a finished design. Choose “Every LV span” or “Only smaller spans,” then “Keep poles and routes” or “Re-route.” Compare the saved alternative’s findings and bill on its own cost basis. MV spans, transformer zones and service conductors retain their separate scope.
Continue
Keep the comparison traceable with design versions and assumptions. Read distribution network design for the route context.