Calculator
Example data table
| Scenario | Run | Install type | Bends | ΔT | Sag/span | Loops | Contingency | Total required |
|---|---|---|---|---|---|---|---|---|
| Tray in plant corridor | 120 m | Tray | 6 | 20 °C | 25 mm | 0.5 m × 2 | 2% | ~124.9 m |
| Conduit to panel room | 75 m | Conduit | 4 | 10 °C | 0 mm | 0.4 m × 2 | 1% | ~77.0 m |
| Suspended service drop | 45 m | Overhead | 2 | 30 °C | 40 mm | 0.6 m × 2 | 3% | ~48.2 m |
Formula used
- Base allowance: L × base% where base% depends on installation type.
- Bend allowance: L × (bends × per-bend%) for routing flexibility.
- Service loops: loop_each × ends for terminations and future rework.
- Thermal allowance: |α × L × ΔT| using a material coefficient α.
- Sag allowance: per span extra length ≈ (8 f²)/(3 S); multiplied by number of spans.
- Contingency: subtotal × contingency% after all planned allowances.
- Total required: L + allowances + contingency; slack = total − L.
How to use
- Enter the route run length based on drawings or site walkdown.
- Select installation type and conductor material for planning factors.
- Add bends, service loops, and expected temperature change.
- If the run is suspended, enable sag and set spacing and sag.
- Click Calculate to view totals and the allowance breakdown.
- Download CSV or PDF for submittals and procurement records.
Route takeoff inputs that drive accuracy
Start with a measured centerline run length from drawings or a site walkdown. For tray and ladder routes, include vertical rises, drops, and offsets at equipment elevations. If the route has multiple segments, sum them and enter one total. On tight projects, re-measure any segment that changes more than 2% after coordination to avoid reordering cable. Capture spare pathway notes, pull directions, and access constraints for installation sequencing early.
Thermal movement allowance for real operating conditions
The calculator applies linear expansion using |α·L·ΔT|, where α depends on material. For copper, α≈16.5×10⁻⁶/°C; for aluminum, α≈23×10⁻⁶/°C. A 120 m run with a 20°C rise adds about 0.04 m for copper and 0.06 m for aluminum. In outdoor or rooftop runs, use the full seasonal swing and document assumptions in the PDF.
Sag effects across spans and long tray runs
When “Include sag allowance” is enabled, extra length is estimated per span with a small-sag parabola approximation: (8f²)/(3S). Use average support spacing S and midspan sag f. Example: S=12 m and f=25 mm yields roughly 0.0014 m extra per span, multiplying across the route’s spans. If your route length already follows the sagged cable profile, disable sag to prevent double counting.
Bends, service loops, and field-friendly slack
Routing changes and terminations often consume more cable than pure geometry. Add bends as notable direction changes and apply a per-bend allowance (commonly 0.10–0.30%). Service loops should reflect termination standards; 0.40–0.60 m per end is common for control and instrumentation terminations, while larger power terminations may require more. The tool separates each allowance so crews can validate against method statements.
Contingency and procurement documentation
Contingency is applied to the subtotal after planned allowances, helping cover minor offsets, damaged ends, and pull setup waste. Typical contingency ranges from 1–3% for controlled tray work and 3–5% for congested retrofits. After calculation, export CSV for takeoff logs and the PDF for approval packages, ensuring every assumption is traceable during material control and closeout.
FAQs
1) What does “planned slack” represent?
Planned slack is the extra cable length added beyond the measured route. It covers bends, service loops, thermal movement, sag (if enabled), and contingency so the installed cable can be terminated without overstress or rework.
2) Should I always enable sag allowance?
Enable sag when the cable is suspended or has long support spacing where midspan sag is expected. Disable it if your measured route length already follows the sagged cable profile or if the cable is fully supported.
3) How do I choose a per-bend allowance?
Use 0.10–0.30% per bend for planning, then align with your installation standard and pulling method. Higher values may suit congested trays, frequent offsets, or routes requiring extra dressing and tie-down adjustments.
4) Why is thermal allowance taken as an absolute value?
Slack planning typically reserves length for movement in either direction. Using an absolute value avoids negative slack when ΔT is negative, while still reflecting the magnitude of expected expansion or contraction.
5) What service loop length is typical at terminations?
Many construction teams use 0.40–0.60 m per end for control and instrumentation terminations, and more for large power terminations. Match your project specifications, cabinet depth, and termination hardware requirements.
6) What contingency percentage is reasonable?
For controlled new-build tray work, 1–3% is common. For retrofit work with unknown obstructions or frequent routing changes, 3–5% can be more realistic. Use your historical waste data when available.