M&E calculators
16 M&E calculators: cable sizing, voltage drop, circuit load and breaker, derated current capacity, lighting, water demand, pipe sizing, drainage falls, pump head, heat load, duct area and supports.
◌Water Demand CalculatorPeak day and peak hour demand, with pipe and tank sizing.⌇Cable Size CalculatorDesign current from load, then the conductor size to carry it.↯Voltage Drop CalculatorPercentage voltage drop with a 5% compliance check.◫Circuit Load and Breaker CalculatorDesign current, breaker loading and suggested ratings.≋Cable Ampacity Derating CalculatorTemperature, installation and grouping derating in one step.☀Lighting Level and Fixture CalculatorLumen method fixture count, achieved lux and power density.⊘Water Pipe Sizing CalculatorBore from flow and velocity, with actual velocity and friction loss.⟍Drainage Pipe Slope CalculatorStandard slope by diameter, fall over the run and velocity check.⇡Pump Head CalculatorStatic lift plus losses, giving total head and shaft power.♨Heating Load CalculatorLoad, radiator sections, fuel use and running cost.▭Duct Area CalculatorDeveloped area for rectangular or round duct, plus insulation and steel weight.⌐Duct Support CalculatorBracket count, steel weight, rod weight and anchors in one go.⊟Cable Tray CalculatorTray, bracket and rod weights together with the bracket count.⌇Conduit and Wiring CalculatorConduit, wire and copper weight per circuit, all at once.⌶Pipe Support CalculatorRun length and spacing give bracket count and steel weight.◍Fire Water Tank CalculatorIndoor plus outdoor plus sprinkler, converted to tank volume.
What this group solves
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.
M&E comes in discrete steps: conductor size, pipe diameter, duct size and drain slope are all picked from standard ranges, not calculated continuously. So the job is not precision, it is picking the right step and then checking it — ampacity passes, then check voltage drop; diameter fixed, check velocity; slope set, check the velocity ceiling. Skip a check and it comes back at inspection.
Which one for your case
| Sizing a conductor | Cable Size Calculator |
| Long runs — voltage drop check | Voltage Drop Calculator |
| Circuit load and breaker setting | Circuit Load and Breaker Calculator |
| Cable ampacity with installation derating | Cable Ampacity Derating Calculator |
| Fitting count and illuminance for a room | Lighting Level and Fixture Calculator |
| Supply pipe diameter from fixture units | Water Pipe Sizing Calculator |
| Drain diameter and slope | Drainage Pipe Slope Calculator |
| Pump head | Pump Head Calculator |
| Duct surface area and insulation | Duct Area Calculator |
Where this group goes wrong
- Sizing conductors on ampacity alone with no voltage-drop check. Beyond about 50 m the drop limit bites before the ampacity limit, terminal voltage falls below 95% of nominal, and motors struggle to start while lamps flicker.
- Reading ampacity straight from the table with no derating. Table values assume a single cable in air at 30 °C; in conduit ×0.8, multi-layer tray ×0.7, at 40 °C ×0.87. Multiply them together and you may have half the nominal figure.
- Believing a steeper drain slope always drains better. Above about 4 m/s the solids outrun the water and the pipe blocks more, not less — DN100 should sit between 12‰ and 20‰.
- Computing duct area and forgetting insulation. Insulated area is measured on the outside of the insulation; 30 mm on a 1000×320 duct adds roughly 15%, and the insulation itself is a separate line item.
FAQ
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.
What drain slope should be used?
DN50 at 25‰, DN75 at 15‰, DN100 at 12‰ and DN150 at 7‰ are the minimums. Using 1.5-2 times those is safer, but velocity must stay under about 4 m/s — faster separates solids from the water and blocks more readily. Long horizontal runs also need rodding eyes.
Why do cable ampacities need derating factors?
Tables are calibrated for a single cable in air at 30 °C. In conduit ×0.8, several layers in a tray ×0.7, ambient 40 °C ×0.87, and direct burial in high thermal resistivity soil takes a further cut. Multiply them together and the effective ampacity can be half the tabulated value, so a direct look-up undersizes by one or two steps.
What illuminance applies to each room?
Dwellings: living room 100-150 lx, bedroom 75 lx, study 300 lx, kitchen 300 lx, bathroom 100 lx. Offices: general 300 lx, design studio 500 lx. Classrooms 300 lx, corridors 50-100 lx, car parks 30-50 lx. These are maintained values; divide by a maintenance factor of 0.7-0.8 to get initial illuminance when choosing fittings.
How is supply pipe diameter selected?
Add up fixture units, look up the design flow, then check velocity: 1.0-1.5 m/s for domestic supply and not over 2.5 m/s for fire. A one-bathroom dwelling takes DN20-DN25; one kitchen and two bathrooms DN25-DN32. Too small and terminal pressure is poor; too large and velocity is so low the water stagnates.
Related data tables
Other groups
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.