Electrical Demand Factor: From Connected Load to Real Demand

If you added up the ratings of every appliance, socket, light, and machine in a building, you would get a large number called the connected load. In practice, not everything runs at full power at the same time. The oven, the washing machine, the air conditioner, and every socket outlet are rarely all at maximum load in the same minute. Demand factor is the tool that bridges that gap: it is the ratio of maximum demand to connected load, and it lets designers size supplies, feeders, and boards for what will actually happen, rather than for a theoretical worst case that never occurs. The Electrical Demand Factor Calculator applies that ratio to your connected load, but the result is guidance only. Local electrical codes and a licensed electrician or qualified designer govern real installations, because demand allowances are set by code and by the type of building.

This guide explains demand factor in plain terms, distinguishes it from two related ideas that are often confused with it, and shows a worked example with checked arithmetic. It is general information for understanding load estimation, not a design for any particular installation.

What demand factor means

Demand factor equals maximum demand divided by connected load. Maximum demand is the highest average load over a specified interval, often fifteen or thirty minutes, that the system actually experiences. Connected load is the sum of the ratings of all connected equipment. Because maximum demand is almost always less than connected load, demand factor is almost always less than one. A demand factor of zero point six means the system is expected to peak at sixty percent of its total connected rating. That single number captures a lot of real-world behaviour: intermittent use, thermostats cycling on and off, people using different rooms at different times, and equipment that is installed for occasional rather than continuous use.

Demand factor is not a guess about one appliance; it is a property of a group of loads. A single motor that runs continuously at full load has a demand factor close to one, because its maximum demand equals its rating. A group of twenty socket outlets in an office has a much lower demand factor, because only a fraction are heavily loaded at any one time. That is why codes give demand factors or demand allowances by load type and building type, rather than one universal figure. Lighting in a home, cooking appliances in a restaurant, and socket outlets in a school all have different usage patterns, and their demand factors reflect those patterns.

Demand factor, load factor, and diversity: three different ideas

Demand factor is often confused with load factor and diversity factor, so it is worth separating them clearly. Load factor equals average load divided by maximum demand over a period. It describes how steady the load is: a load factor close to one means the load runs near its peak most of the time, while a low load factor means short peaks with long quiet periods. Diversity factor equals the sum of the individual maximum demands divided by the maximum demand of the group. Because individual peaks occur at different times, the group peak is less than the sum of individual peaks, so diversity factor is greater than one. Demand factor, by contrast, compares group peak with total connected rating, and is less than one.

An example makes the distinction concrete. Suppose three workshops each have a connected load of one hundred kilowatts, and each peaks at eighty kilowatts, but at different times of day. The sum of individual maximum demands is two hundred and forty kilowatts. Suppose the combined peak, when all three are considered together, is one hundred and eighty kilowatts. The diversity factor is two hundred and forty divided by one hundred and eighty, which equals one point three three. The connected load of the group is three hundred kilowatts, so the demand factor is one hundred and eighty divided by three hundred, which equals zero point six. Both numbers describe the same reality from different angles: diversity says peaks do not coincide, demand says the group never reaches its full connected rating. Confusing them leads to errors in both directions, either oversizing by ignoring diversity or undersizing by applying the wrong ratio.

How demand factor is used in design

In design, the process usually runs in stages. First, list the connected loads by category: lighting, sockets, cooking, heating and cooling, motors, and special loads. Second, apply the demand factor or demand allowance that the applicable code gives for each category, or a justified figure from measured data for a similar building. Third, add the resulting demands to get the estimated maximum demand for the board, feeder, or supply. Fourth, size the supply and protective equipment for that demand, with appropriate margins and with the code's rules for the largest motor or the largest load, where starting currents or special conditions apply. Fifth, check voltage drop and fault performance at that demand, because a supply sized only for demand but not for voltage drop or fault clearing is incomplete.

It is important to understand what demand factor does not do. It does not reduce the rating of an individual circuit: a socket circuit or a cooker circuit is still sized for its own load and its own protective device. Demand factor applies at the level of a group, a board, or a supply, where diversity exists. Applying a group demand factor to a single circuit would undersize that circuit. It also does not excuse a proper load schedule: you still need to list what is connected, where, and on which phase, because phase balance matters in three-phase systems. An unbalanced board can overload one phase even when the total demand looks fine, so the load schedule and the demand calculation go together.

Worked example with sample values

Take a sample small building with the following connected loads: lighting twenty kilowatts, socket outlets thirty kilowatts, cooking fifteen kilowatts, and heating and cooling twenty-five kilowatts. Total connected load is twenty plus thirty plus fifteen plus twenty-five, which equals ninety kilowatts. Now apply sample demand factors, clearly labelled as samples for illustration, not code values: lighting zero point eight, sockets zero point five, cooking zero point seven, and heating and cooling zero point eight. The estimated demands are: lighting twenty multiplied by zero point eight, which equals sixteen kilowatts; sockets thirty multiplied by zero point five, which equals fifteen kilowatts; cooking fifteen multiplied by zero point seven, which equals ten point five kilowatts; heating and cooling twenty-five multiplied by zero point eight, which equals twenty kilowatts. Add them: sixteen plus fifteen plus ten point five plus twenty equals sixty-one point five kilowatts. The overall demand factor is sixty-one point five divided by ninety, which equals about zero point six eight. The supply and main feeder would be planned around sixty-one point five kilowatts, not ninety, with margins and code rules applied on top.

To see the saving, compare the currents. At a sample three-phase voltage of four hundred volts, and assuming a power factor of one for simplicity in this illustration, the current for ninety kilowatts is ninety thousand divided by the square root of three multiplied by four hundred. The square root of three is about one point seven three two, so the denominator is about six hundred and ninety-two point eight. Ninety thousand divided by six hundred and ninety-two point eight equals about one hundred and twenty-nine point nine amperes. For sixty-one point five kilowatts, the current is sixty-one thousand five hundred divided by six hundred and ninety-two point eight, which equals about eighty-eight point eight amperes. The difference is about forty-one amperes, which can change the feeder size, the board rating, and the supply capacity required. That is why demand factor matters commercially as well as technically: sizing everything for connected load wastes money and space, while sizing without any diversity risks overload.

Now test sensitivity. If the socket demand factor were zero point seven instead of zero point five, socket demand would be thirty multiplied by zero point seven, which equals twenty-one kilowatts instead of fifteen, a rise of six kilowatts. Total demand would become sixty-seven point five kilowatts, and the overall demand factor would be sixty-seven point five divided by ninety, which equals zero point seven five. A single assumption change moves the answer by about ten percent, which shows why demand factors should come from code tables or measured data for similar buildings, not from optimism. When in doubt, use the more conservative figure and discuss it with the designer, because the cost of a slightly larger supply is usually far less than the cost of a supply that trips or overheats at peak times.

Common mistakes to avoid

The first mistake is applying one demand factor to everything, including continuous industrial loads that genuinely run together. The second is double-counting diversity: applying a demand factor and then a diversity factor to the same group, which understates demand twice. The third is forgetting the largest load rule: some codes require the largest motor or load to be counted at full value plus a percentage of the rest, because starting currents and coincident peaks matter. The fourth is ignoring growth: a building that gains equipment over time will outgrow a demand estimate that assumed today's load list is final. The fifth is using a residential demand factor for a commercial building, or vice versa, where usage patterns are completely different. The sixth is forgetting phase balance: a demand total that looks fine can still overload one phase if loads are unevenly distributed.

Another mistake is treating estimated demand as measured demand. An estimate is a planning figure; a maximum demand meter or a monitoring period gives real data. For an existing building being extended, measured demand from bills or metering is often more reliable than a fresh estimate from connected loads, because it captures actual behaviour. Combine both: estimate for the new loads, measure for the existing loads, and reconcile any large difference before sizing the upgrade.

Using the calculator well

List your loads honestly and completely, including loads that are easy to forget, such as water heaters, pumps, lifts, and outdoor equipment. Apply demand factors by category, not as a single blanket figure, and note the source of each factor, whether it is a code table or an assumption to be confirmed. Record the connected load, the demand factor, and the resulting demand for each category, so the calculation can be reviewed. Then test sensitivity: what happens if one category's factor rises by zero point one or zero point two? If the supply size changes, that category deserves more attention and better data. Finally, hand the calculation to a qualified designer to check against the applicable code, the load schedule, phase balance, voltage drop, and fault performance, because demand is only one part of a complete design.

For related calculations, the Three Phase Power Calculator helps convert between power and current, and the Single Phase Power Calculator does the same for single-phase loads. You can also read the related guide on air conditioner sizing at /blog/air-conditioner-tonnage-calculator-guide/ for how one large load category is estimated. Remember throughout: demand factor results here are guidance only, and local codes and a licensed electrician or qualified designer govern real installations.