Conduit and Wiring Calculator
Conduit, wire and copper weight per circuit, all at once.
Quick answer
With the default inputs: Circuits 10 circuits · Average run per circuit 25 · Wires per conduit 3 pcs etc.Total wire length 825 m
Edit the inputs below to recalculate.Inputs
Result
Total wire length
825m
| Conduit net length | 250.0m |
| Conduit with waste | 262.5m |
| Wire net length | 750m |
| Wire with allowance | 825m |
| Copper per metre | 0.0222kg/m |
| Total copper weight | 18.4kg |
| Coils at 100 m | 9coils |
| Wire per circuit | 82.5m |
This is a circuits × average-run estimate for early design and procurement; final quantities must be measured from drawings. Conductor size affects weight only — ampacity and voltage drop are not checked here and must be calculated separately from design current, installation method, ambient temperature and allowable drop. 1.5 mm² lighting and 2.5 mm² socket circuits are conventional but not transferable to heavy loads. Copper is taken at 8.9 g/cm³ excluding insulation, which adds about 10-15% for BV cable.
Reverse: known result, work out one input
Enter a target result; the other inputs stay fixed and this one is worked out.
Formula
| Conduit net length | L = circuits × average run |
| Conduit total | L = net × (1 + conduit waste) |
| Wire net length | L = conduit net × wires per conduit |
| Wire total | L = net × (1 + allowance) |
| Copper weight | W = total length × area(mm²) × 8.9 ÷ 1000 |
10 circuits at 25 m each, 3 × 2.5 mm² per conduit: 250 m net conduit, 262.5 m with 5% waste; 750 m net wire, 825 m with 10% allowance; 18.4 kg of copper, nine 100 m coils.
How to work it out
- Conduit net length = circuits × average run per circuit. Measure the average along the actual route — walls, floor, around beams — never use the room diagonal.
- Conduit total = net × (1 + waste). Concealed work wastes 5-8% in bends, couplings and cuts; surface work 3-5%.
- Wire net length = conduit net × conductors per conduit. A single-phase circuit is L + N + PE, three conductors; three-phase is four or five. Omitting the earth is the standard mistake.
- Wire total = net × (1 + allowance). Half the box perimeter inside each distribution board, 0.3 m at each lighting point, 0.15 m at each socket — 10-15% overall.
- Copper weight = total length × cross-section (mm²) × 8.9 ÷ 1000 kg. 2.5 mm² is about 22 g per metre, useful for scrap valuation or checking delivered weight.
- Check the conduit fill: total conductor area must not exceed 40% of the conduit bore. Three 2.5 mm² conductors total 7.5 mm² against roughly 220 mm² inside PVC20 — only 3.4%, comfortable. More conductors or larger sizes need recalculating.
Where it goes wrong
- Counting two conductors per conduit. A single-phase circuit has L, N and PE — omitting one understates by 33%. Three-phase circuits carry four or five, so the error is worse.
- Forgetting the allowance inside distribution boards. Half the perimeter per board is 1-1.5 m, so ten boards quietly lose 15 m.
- Skipping the fill check. Six 2.5 mm² conductors in PVC20 exceed 40%, and the pull simply will not go through — the wall gets opened up.
- Multiplying conduit length by conductor count without an allowance. The tails then come up short and joints get made inside the conduit, which is expressly forbidden because they cannot be inspected.
Material and cost
Flame-retardant PVC20 runs 1.5-2.5 CNY/m, JDG steel 20 about 6-10, KBG about 5-8. BV2.5 cable is 180-260 CNY per 100 m coil. Chasing for concealed work is 8-15 CNY/m extra. Conduit and cable only — chasing and pulling labour excluded.
On site
- Joints inside conduit are prohibited — connections belong in boxes only. This is a mandatory rule and it is spot-checked at handover, with rework if found.
- Conductors of different voltage levels or different circuits must not share a conduit (fire-linked circuits have their own rules), otherwise a fault in one circuit propagates into another.
- Photograph concealed conduit in walls with dimensions before closing up. It is the single best defence against drilling through a conduit when hanging pictures or cabinets later.
- Keep the bend radius at least 6D (D being the outside diameter). More than three bends or a run over 30 m needs a draw box, or the cable will not pull.
Basis
- GB 50303-2015 Code for acceptance of construction quality of building electrical installation, conduit and cable installation
- GB 50054-2011 Code for design of low voltage distribution, erection of distribution circuits
- JGJ 16-2008 Code for electrical design of civil buildings, wiring systems
FAQ
How many wires fit in one conduit?
Total conductor cross-section including insulation must stay under 40% of the conduit bore — that keeps pulling feasible and heat dissipation adequate. Practically, PVC20 has roughly a 16 mm bore and 201 mm² area, so 80 mm² is the limit; BV2.5 is about 4.2 mm overall and 13.9 mm², allowing five at most and three to four in practice. Overfilling makes pulling impossible and re-wiring later hopeless.
Why and how much wire allowance?
Terminations need slack. Each conductor in a panel is left half the panel perimeter for dressing and terminating, boxes take 100-150 mm for joints, and expansion joints need compensation. The rule is half-panel-perimeter in cabinets and 150 mm in boxes, totalling 8-15% of run length — take the upper end for short runs with many enclosures. Without allowance the wire sits under tension and loosens with heat or vibration.
Why 1.5 mm² for lighting and 2.5 mm² for sockets?
Because the loads differ. A lighting circuit is small — often 10-20 luminaires under 1 kW — and 1.5 mm² copper in conduit carries about 14-16 A, sufficient and economical. Socket circuits feed kettles, hairdryers and air conditioners, where 2.5 mm² carries 19-22 A with a C16 or C20 breaker. Kitchens, air conditioning and instant water heaters need their own 4 or 6 mm² circuits. These sizes are a safety floor, not a place to economise.
Cite this page
Use this format in reports, method statements or coursework. The link is permanent.
Building Calculator. Conduit and Wiring Calculator. 2026-10-25. https://jianzhujisuan.com/en/conduit-quantity
Results are estimating aids only and are not a design basis. Construction and final accounts must follow the drawings, the applicable standards and an approved method statement. Anything affecting structural safety, shoring or falsework must be checked by a suitably qualified engineer.