# M&E

Building services take off along two chains. Electrical is a three-stage check — current capacity, then protection, then voltage drop; picking a cable size is not the end of it, because long runs usually fail on the 5% drop limit rather than on current. Plumbing and HVAC are flow, diameter, then resistance: supply pipe size from fixture units and diversity, drainage from discharge units subject to minimum falls.

The bulk of installation labour hides in supports and accessories: duct hangers spaced by duct width, tray brackets by span, pipe brackets by diameter and temperature. These invisible items routinely account for more than 30% of installation labour, so the tools here return bracket counts, steel weight and anchor counts instead of leaving it to guesswork.

- Category page: https://jianzhujisuan.com/en/c/me
- Data checked: 2026-10-25

## Tools in this group

- [Cable Size Calculator](https://jianzhujisuan.com/en/wire-size): Design current from load, then the conductor size to carry it.
- [Voltage Drop Calculator](https://jianzhujisuan.com/en/voltage-drop): Percentage voltage drop with a 5% compliance check.
- [Circuit Load and Breaker Calculator](https://jianzhujisuan.com/en/circuit-load): Design current, breaker loading and suggested ratings.
- [Cable Ampacity Derating Calculator](https://jianzhujisuan.com/en/cable-ampacity): Temperature, installation and grouping derating in one step.
- [Lighting Level and Fixture Calculator](https://jianzhujisuan.com/en/lighting-lux): Lumen method fixture count, achieved lux and power density.
- [Water Demand Calculator](https://jianzhujisuan.com/en/water-demand): Peak day and peak hour demand, with pipe and tank sizing.
- [Water Pipe Sizing Calculator](https://jianzhujisuan.com/en/pipe-diameter): Bore from flow and velocity, with actual velocity and friction loss.
- [Drainage Pipe Slope Calculator](https://jianzhujisuan.com/en/drain-slope): Standard slope by diameter, fall over the run and velocity check.
- [Pump Head Calculator](https://jianzhujisuan.com/en/pump-head): Static lift plus losses, giving total head and shaft power.
- [Heating Load Calculator](https://jianzhujisuan.com/en/heating-load): Load, radiator sections, fuel use and running cost.
- [Duct Area Calculator](https://jianzhujisuan.com/en/duct-area): Developed area for rectangular or round duct, plus insulation and steel weight.
- [Duct Support Calculator](https://jianzhujisuan.com/en/duct-support): Bracket count, steel weight, rod weight and anchors in one go.
- [Cable Tray Calculator](https://jianzhujisuan.com/en/cable-tray-support): Tray, bracket and rod weights together with the bracket count.
- [Conduit and Wiring Calculator](https://jianzhujisuan.com/en/conduit-quantity): Conduit, wire and copper weight per circuit, all at once.
- [Pipe Support Calculator](https://jianzhujisuan.com/en/pipe-support): Run length and spacing give bracket count and steel weight.
- [Fire Water Tank Calculator](https://jianzhujisuan.com/en/fire-pool): Indoor plus outdoor plus sprinkler, converted to tank volume.

## Questions

### How do I choose a cable size?

Calculate design current — I = P / (√3 × U × cosφ) three-phase or I = P / (U × cosφ) single-phase — select a size from the current-carrying table with 1.2-1.5 times margin, then check voltage drop against 5%. On long runs the drop check governs, not the current rating.

### How is supply pipe diameter sized?

Convert fixture units to a design flow rate, then size for economic velocity: 0.8-1.5 m/s up to DN50, 1.0-2.0 m/s above. Too fast and you get noise and water hammer, too slow and you waste pipe. Then confirm the pressure at the most unfavourable outlet still meets the minimum working pressure of the fixture.

### How is duct surface area calculated?

For rectangular duct, 2 × (long side + short side) × length; for circular duct, π × diameter × length. That is the bare duct development, excluding flanges, stiffeners and insulation. Insulation area is measured separately on the insulated outside perimeter.