What this calculator works out
This calculator applies a headroom margin to an equipment load, converts it to current at the supply voltage and phase configuration, and gives the heat output.
The heat figure matters as much as the current. Everything drawn as electricity leaves as heat, and cooling capacity is a separate constraint that frequently binds first.
Enter your details
Before you size a circuit
- Equipment load should be measured or taken from nameplate figures. Nameplate ratings are maximums and real draw is usually well below them.
- Headroom covers growth and the difference between typical and peak draw. 25% is a reasonable minimum.
- UK single phase is 230 V; three phase is 400 V between lines, and the calculation uses the line-to-line figure with the √3 factor.
- Circuits should not be loaded continuously above 80% of their rating, which the headroom figure broadly accounts for.
Computing equipment converts essentially all the electricity it draws into heat — none of it leaves as useful work in the physical sense. A rack drawing 4 kW needs 4 kW of cooling removed from the room. That is why power and cooling are planned together, and why a rack can be within its electrical limit and beyond what the room can cool.
How this calculator works
Load plus headroom, converted to current:
Design load = equipment watts × (1 + headroom ÷ 100)
Single phase current = watts ÷ volts
Three phase current = watts ÷ (√3 × volts)
Heat output = watts × 3.412 BTU/hThe √3 factor for three phase arises because the three line currents are 120 degrees out of phase, so the same power is delivered at a lower current per line. This is why three phase is used for larger loads.
Worked example: a 3.5 kW rack
Using the default figures — 3,500 W of equipment, 230 V single phase, 25% headroom:
- Design load: 4.375 kW
- Estimated current: 19.0 A
- Heat output: 14,928 BTU/h
19 A sits comfortably within a 32 A circuit and above a 16 A one. The heat figure is the one worth pausing on: nearly 15,000 BTU/h is roughly the output of a 4.4 kW air conditioning unit, which is what the room needs to remove continuously just to stay level. A rack that is electrically fine and thermally impossible is a common outcome.
Common mistakes
- Sizing from nameplate ratings. They are maximums, not typical draw.
- Loading a circuit above 80% continuously. It is outside normal design practice.
- Planning power without cooling. Every watt drawn becomes a watt of heat.
- Using 400 V without the √3 factor. The current per line differs.
- Treating this as a design. Electrical installation is notifiable work for a qualified person.
Frequently asked questions
Do I need a qualified electrician?
For anything beyond plugging equipment into an existing socket, yes. Installing or altering circuits is notifiable work under the Building Regulations in England and Wales and must be carried out or certified by a competent person. This calculator is for planning conversations, not for specifying an installation.
What is power factor?
The ratio between real power in watts and apparent power in volt-amperes. Equipment with a poor power factor draws more current than its wattage suggests, which matters for circuit and UPS sizing. Modern IT power supplies commonly have power factor correction giving figures near 1, so the distinction has narrowed — but UPS ratings in VA rather than watts exist precisely because of it.
How much cooling does a rack need?
Roughly the same in kilowatts as it draws electrically. A 4 kW rack needs about 4 kW of cooling removed continuously, plus a margin. Room cooling capacity, airflow management and hot and cold aisle separation all affect whether that capacity actually reaches the equipment.
Should I use single or three phase?
Single phase suffices for most small installations. Three phase distributes higher loads at lower current per line and is standard for larger racks and rooms. The decision is usually made by what supply is available, and changing it is an electrical infrastructure project rather than a rack one.
Is what I enter stored?
No. Figures are processed in your browser and never transmitted or retained.
Related tools
References
- GOV.UK — Approved Document P and notifiable electrical work
- Health and Safety Executive — electrical safety at work and safe systems for electrical installations
Sources are checked at publication and can change — how I choose and check references.