How do you calculate electrical needed for building?

Answer 1

for USA, Canada and countries running a 60 Hertz supply service.

This is a complex question that requires you to do some reading, as it confuses even electricians. The applicable reference is: NEC 220.40 Feeder and Service Calculations.

Typically if you are asking this question you should call an electrician to help you. However if you read the applicable NEC article you will be able to calculate your own service.

This can be done by using a clampmeter, this is a instrument that clamps over the main incoming cable ( hot wire ) where it enters the distribution board. You set the meter to read amps and this gives you the total amps being used at that time.

I always ask the customer to find their electrical useage for the last twelve months. Use the highest month in KWHr, divide that by days in month, then hours in day, then supply voltage such as 240-volts. This will show the highest average ampere demand for which your service conductors are supplying. Don't forget to convert to watts at the end.

Surprisingly, most homes average about 10 amps continuous or less. Minimum service rating is 100 amps.

If you are paying $.1 per KWHr and the bill is $150 over 30 days, then you used 1500 KWHr, or 8.68 amps per hour continuously on average.

You could simply install socket power readers on all the outlets that have electrical appliances attached to them, and that would give you an indication of your power usage. If you wanted to know amps, then that's something else all together.

In residential installations, this is usually not a necessary factor to calculate. The utilities companies usually only offer 100 amp, 200 amp or 400 amp service. The typical new home in today's market will use the standard 200 amp service.

If you have concerns like having a garage that you will do work in using electrical equipment, then you would want an electrician to calculate the additional load. 200 amp service is quite adequate for most purposes unless you have the aforementioned scenario. My son has a welding shop and business and operates the entire shop with 200amp residential service including a 3 phase generator.

Before you do any work yourself,

on electrical circuits, equipment or appliances,

always use a test meter to ensure the circuit is, in fact, de-energized

IF YOU ARE NOT REALLY SURE YOU CAN DO THIS JOB

SAFELY AND COMPETENTLY

REFER THIS WORK TO QUALIFIED PROFESSIONALS.

Answer 2

You will need to do a load calculation. Find an NEC 2008 codebook. Article 220 has a load calcualtion you can use. I recommend you contact an electrician, who will be able to do this for you. You'll need to know what size of a service entrance you will need and this will be based on what appliances you will be using in the home. The use of central air, electric heat, and other appliances will be determining factors. You don't want to get this wrong and wind up with an inadequate electrical service!

Answer 3

I'm assuming what you really want to know is "how big of a backup generator should I buy?", so I somewhat geared this answer to that end.

The most straightforward way would be to find the consumption of your major power-consuming appliances (microwave, water heater, oven, a/c unit). They should all be rated on the units themselves in the same way a light bulb has a wattage rating (i.e. 60 watt light bulb). Then add them up, so that you could run all of them at once if needed. Most require more watts to start than to run, but it's assumed that they won't all turn on at once, and most generators have a rating called "surge wattage' which gives a little extra oomph for starting heavy-draw appliances. Make sure your RMS wattage (or the amount of sustainable power available by the generator) is rated for more than all your major appliances combined.

EX: stove = 2500 watts, water heater = 2000 watts, a/c unit 3,500 watts, microwave = 1200 watts, tv = 1000 watts, lights and phone chargers, etc @ 60 watts each or less.

Total power need = between 10 kW and 13 kW.

Usually when running off of a generator you would use power more conservatively, thus you could just make sure to not bake and use the microwave at the same time, or turn the tv off when taking a shower, and if you are conservative such a house could run off of a smaller generator such as 8 kW. Lights are easy to figure, as a typical 3 bedroom house may have 10 fixtures at an average of 2 bulbs each. With 60 watt bulbs, all of the lights in your house could be on at the same time and use the same wattage as a 1200 watt microwave.

Another thought: anything that creates or removes heat energy consumes GOBBS of energy. If you have several girls in the house that like to blow dry and straighten or curl their hair, each one of those can also use significant wattage. Second freezers or fridges should be taken into account as well.

I prefer extra power so I don't have to worry about the generator being bogged down, so I added them all up together, added about 10%, and ended up with a 15kW generator for my 2-bedroom home. It's about double what I could comfortably live off of, but I can take 3-hour showers while my wife bakes cookies and dries her hair with the tv on in the other room and the a/c and every light in the house on and still be at a fraction of the potential of a 15kW beast.

Hope I helped.