What the Electrical Demand Factor Calculator does
This calculator turns a list of connected loads into a maximum demand figure. Each group carries its own demand factor (how much of it runs at once) and diversity factor (how little its peak coincides with the rest), and the totals are added as vectors so the kVA figure is honest rather than the arithmetic sum of the group kVAs.
It matters because connected load and maximum demand are rarely within a factor of two of each other. A building with 350 kW connected may never draw more than 180 kW, and sizing the supply, the transformer and the main cable for 350 kW wastes a great deal of money. Getting it wrong the other way trips the incomer on the first busy Monday.
How to use it
- List the load groups. Group things that behave alike: lighting, socket outlets, HVAC, kitchen, motors, IT, EV charging. A group of one is fine for a single large item.
- Enter each group's connected kW - the sum of the nameplate ratings in that group.
- Set the demand factor: the fraction of that connected load you expect to be running at the peak. It is a multiplier and it is usually below 1, though a largest-motor allowance can be above it.
- Set the diversity factor if you use one. It is a divisor, not a multiplier - a diversity factor of 1.4 means the group's parts peak at different times and its contribution is divided by 1.4. Leave it at 1 if you have already allowed for that in the demand factor.
- Add an overall coincidence factor and a spare-capacity allowance, then read the kVA the supply has to provide.
- If you have metered energy over a period, enter it with the period length to get the load factor, which is the best available check on whether your demand factors are realistic.
Reading the results
Maximum demand in kW is what the loads actually draw at the peak. Maximum demand in kVA is what the supply, the transformer and the main cable have to carry, and it is the bigger number whenever the power factor is below one.
The overall demand factor is maximum demand divided by connected load. For an office it is commonly 0.4 to 0.6; for a data centre, close to 0.9; for a house with everything connected, often below 0.3.
Load factor is average demand over a period divided by the peak in the same period. A low load factor - under 0.3 - means the peak is brief and expensive, which is what makes demand charges and load shifting worth looking at.
The supply capacity figure includes your spare allowance. Ask the supplier for that figure, not for the connected load.
Worked example: a small office building
Lighting 18 kW at demand factor 1.0 and pf 0.95; socket outlets 40 kW at 0.5 and 0.9; HVAC 55 kW at 0.9 and 0.88; a server room 25 kW at 0.9 and 0.95; lifts 22 kW at 0.5 and 0.8. Connected load is 160 kW.
Group demands are 18, 20, 49.5, 22.5 and 11 kW - a total of 121 kW. The reactive parts are 5.92, 9.69, 26.71, 7.39 and 8.25 kVAr, totalling 57.96 kVAr.
Maximum demand in kVA is therefore the square root of 121 squared plus 57.96 squared, which is 134.2 kVA - not the 137.9 kVA you would get by adding the group kVA figures arithmetically. The overall power factor is 121 / 134.2 = 0.902, and the overall demand factor is 121 / 160 = 0.756.
With a 20% spare allowance the supply needs about 161 kVA, which at 400 V three phase is 232 A. That points at a 200 A or 250 A supply and a 160 kVA or 200 kVA transformer, depending on how much of the spare allowance you believe in.
If the meter shows 1,450 kWh over a 12-hour working day, the load factor is 1450 / (121 x 12) = 0.999 - which would be suspicious, and a strong hint that the demand factors above are too low for this building.
Formulas and scoring rules
- Group demand
demand kW = connected kW x demand factor / diversity factorDemand factor multiplies; diversity factor divides. Using both on the same group double-counts unless you mean them to.- Group reactive power
kVAr = demand kW x tan(arccos pf)- Total apparent power
kVA = sqrt((sum kW)^2 + (sum kVAr)^2)kVA figures must never be added arithmetically unless every group has the same power factor.- Maximum demand
peak = total x coincidence factor- Overall demand factor
maximum demand / connected load- Load factor
energy in the period / (peak kW x hours in the period)Always between 0 and 1; equal to 1 only for a perfectly flat load.
Demand factor, diversity factor, coincidence factor
The three terms are used loosely and sometimes interchangeably, which causes real errors. Demand factor is maximum demand divided by connected load for one group - a fraction, at most 1 for most groups. Diversity factor, in the classical definition, is the sum of individual maximum demands divided by the coincident maximum demand of the group, so it is a number of 1 or more and it divides. Coincidence factor is its reciprocal and multiplies.
This calculator keeps them apart deliberately: the demand factor column multiplies, the diversity column divides, and the overall coincidence factor multiplies the whole total. Use whichever you have figures for and leave the others at 1 rather than applying two allowances for the same effect.
Where the numbers should come from
The indicative factors shown on this page are planning conventions, not requirements. NEC Article 220 sets out prescriptive demand factors for dwellings, receptacles, kitchen equipment, motors and other categories, and the result of that calculation is what an inspector will check against in a NEC jurisdiction. IEC-derived practice tends to leave more to the designer's judgement, with guidance rather than tables.
The best source is measurement: half-hourly metered data from a comparable building, or from the same building before a refurbishment. Second best is the equipment schedule with each item's duty cycle. Assumed factors are a starting point for a feasibility study, not a design.
Limitations: what the result does not prove
- Demand factors are judgement, not physics. The calculator does the arithmetic honestly, but the answer is only as good as the factors you enter, and the page does not know your building.
- It does not apply any code's prescriptive method. If your jurisdiction works to NEC Article 220, IEC 60364 or a national annex, the calculation those documents specify is the one that counts.
- Future load is not predicted. Spare capacity is an allowance you choose, not a forecast.
- The result sizes nothing on its own. Transformer, cable, busbar and protection all need their own calculations, and the whole design must be verified by a qualified engineer against the applicable standard and local regulations.
Privacy: where your data goes
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Standards and sources
- IEC 61439-1 - Low-voltage switchgear and controlgear assemblies - checked 19 Sep 2026
- NFPA 70 - National Electrical Code, Article 220 (branch-circuit, feeder and service load calculations)
- IEC 60364-1 - Low-voltage electrical installations: fundamental principles and assessment of general characteristics
Frequently asked questions
What is the difference between connected load and maximum demand?
Connected load is the sum of every nameplate rating in the installation, as though everything ran flat out at the same moment. Maximum demand is the largest total the installation actually draws, over the averaging window the supplier uses - usually 15 or 30 minutes. The ratio between them is the overall demand factor.
Is a diversity factor the same as a demand factor?
No, and treating them as the same is the commonest error here. A demand factor is a fraction that multiplies a group's connected load. A diversity factor, in its classical definition, is the sum of individual peaks divided by the combined peak, so it is 1 or greater and it divides. Applying both to the same group without meaning to halves your answer twice.
What demand factor should I use for socket outlets?
In an office, 0.3 to 0.5 of the connected socket load is common, because most outlets sit idle. NEC 220.44 has specific rules for receptacle loads in non-dwelling occupancies, and a jurisdiction working to it expects that calculation rather than a judgement. Where laptops and monitors dominate, measured data from a similar floor beats any table.
Why is the total kVA less than the sum of the group kVAs?
Because apparent power is a vector quantity. Two groups of 50 kVA at different power factors do not make 100 kVA; their kW components add, their kVAr components add, and the total kVA is the hypotenuse of those two sums. Adding kVA arithmetically always overstates the answer, sometimes by several per cent.
How much spare capacity should I allow?
It depends on how likely the building is to change. 10 to 20% is common for a completed building with a known tenant; 25 to 50% for a speculative fit-out, a building that may add EV charging, or a plant that expects a production line. Allow it explicitly rather than by inflating the demand factors, so the assumption stays visible.
What is a good load factor?
Higher is cheaper. Above 0.6 means a fairly flat load that makes efficient use of the supply capacity you pay for. Below 0.3 means a short peak dominating the bill, which is where demand-charge management, load shifting or storage start to pay. It is a measure of how you use the supply, not of how efficient your equipment is.
Does the largest motor need a special allowance?
Usually yes. NEC 430.24 requires a feeder to carry 125% of the largest motor's full-load current plus the full-load current of the others, because the largest motor's starting current and continuous duty dominate. That is why the industrial example on this page carries a demand factor above 1 for the largest motor.
Can I use this figure to apply for a supply?
Use it to prepare the application, then have it checked. A distribution network operator wants a maximum demand in kVA or a main-fuse rating, and will usually ask how it was derived. A figure with the groups, factors and assumptions listed - which is what the CSV export gives you - is far easier to agree than a bare number.
Last reviewed by the A2Z.Tools team against the sources listed above.
