Building Load Calculations

Sir,

I gather from the 220v, 3ph, 60Hz power supply that your project is not in the USA. From your location, I would assume the project is in your own country, Saudi Arabia. I am not familiar with the electrical codes in your country or specific location. However, they generally are aimed to provide you with safety from fires and safety to people, and are a minimum standard instead of an ideal standard.

Do I assume that you are dealing with the service to each of 100 buildings (or units) as well as the total service for all of the buildings? Does each building have the loads you described (8 A/Cs 18000 BTUH each, 4 pcs water heaters 1500 watt each , 50 fluorescent lighting fixtures 80 watt each)? Your post didn't include any other loads, such as receptacle outlets, cooking, fans, etc.

In the National Electrical Code (USA), the lighting and receptacle outlet loads are usually included in a single allowance of # watts per square foot, with the # ranging from 0.25 in corridors, to 3 in homes, to 3.5 office buildings). I will be happy to share with you how these calculations would work, but I would need to know: 1) area of each building in sq.ft. or sq.meters, 2) specific loads such as cooking appliances or dedicated circuits, 3) other motors.

You are correct to allow for a demand factor when you say "some times I multiply the total by 0.9 or 0.8 because all units will not run one time". However, you have to be cautious because cooling loads might all be running at once while heating may be partially in use.

Regarding the use of a 60A CB to feed each building: If your total load is the 42A per phase you mention, yes I see no problem. This will allow the feeders to be sized for the 60A upstream protection. The design rules I must follow require derating the CB to 80% for any load that is "continuous"--on for 3 hours or more. Thus, the 60A CB can feed a continuous 48A load, or a non-continuous 60A load, or some combination of the two between these two numbers. If, in the future, you anticipate that the loads can increase (and they often do!), I suggest using a larger-sized cable so the CB can be changed without the trouble of installing new cable.

I hope you are also allowing for voltage drop on the cables (by increasing the cable size as necessary).

A complete design will include: The service load and size for each unit (or building). The number of individual units on each distribution panel. The calculated load on the distribution panel, with demand factors chosen for the types of loads. The phase distribution of the loads so there is a balance between the phases. The number of distribution panels on each transformer, and the allowed demand factor for each transformer. The diversity factor when adding up the load for the total facility. The total calculated load.

--JMueller

thank you jmueller for this detailed answer , please correct me if I am wrong , what I understand from your answer is that if the total ampere for the appliances ( in general any appliances) , is 90A , so when the panel board is 3 phase , so I will use 30A CB not 90A !!! ( of course not exactly 30 , ( 30 * 1.35 ).am I wright??

please not that the load is single phase ( 110v or 220 v / single pole CB or double pole Cb)

If 90A is the total single phase loads and these are fed by a 3 phase supply then the computed 3 phase line current is 30 times root 3. Only then can you apply other factors for anticipated loads etc...

regards

sorry i did not understand from where you brought 30 ?is it 90/3=30.

more over in the three phase connections LV load star connection is there so the line current will be equal to phase current and line voltage =s.root 3 times the phase voltage .

now the quastion is in case i have machine 20kw(TP Load) .the cb rate will be arround 40A TP .if you measure by clamp meter the current passing in each phase you will not read 40A .why?

normally when we tell 40A TP CB means each phase can handle up to 40 A .but i never got reading near to 40A by clamp meter.

kindly reply on email abdmsall@yahoo.com .if you can send me some supportive documents i will appreciated.

os708090,

The answer is "it depends". If all appliances and other loads are connected between line and neutral and are equally balanced between the incoming lines, then yes the load for each line is the total connected to that line only (as you have described). Often, however, appliances which use more electrical power are connected line to line (at a higher voltage), so the answer is no because at least some of their load is on both lines. Where you have a single-pole CB, it is connecting the load between the line and neutral. But where you have a double-pole CB it is connecting the load from line-to-line. Yet, in both of these cases (SP or DP CB's, the load is a single-phase load, even though it may be supplied from a three-phase electrical panel.

On single-phase systems, with the neutral midpoint between two lines, balancing loads and related calculations are pretty simple to do. However, with three-phase systems, this becomes more difficult because there are at least 4 different ways that three-phase is done, and you have to use the correct one.

The one you probably have is a called a 3-pole, 4-wire "wye", with 220 volts between any two phase conductors and 127 volts (2/√3*110≈127) from each one to the neutral. If you were to draw this on paper, make a triangle with equal sides, and label the bottom left corner "A", the top center corner "B" and the bottom right corner "C". Then put an "N" in the very center of the triangle and draw lines from it to each corner. Each line, from N-A or N-B or N-C is the single-phase low voltage of 127 volts, and each line from A-B or B-C or C-A is the single-phase high voltage of 220 volts. As you can see from it, the low voltage loads draw power from only one of the A, B, or C lines and balance themselves on the Neutral "N". The high-voltage loads draw their power from a pair of the A, B, and C lines, and would need to be balanced with themselves and the low-voltage loads.

A less-common one that you might have is a 3-pole, 4-wire high-leg "delta", with 220 volts between any two phase conductors and the neutral on the midpoint between the A and C phases. You would draw this as a triangle like before, but put the N in the middle of the line between A and C, then draw a dashed line straight up to B. The voltage on the dashed line between B and N is significantly higher (√3*110≈190 volts), and cannot be used for any loads. Since this high-leg B-N cannot be used, the B phase is only able to supply the 220-volt loads between A-B or B-C. In this case, B will always have a lower load than A and C. This type of connection gives many difficulties and chances for errors, so it is usually avoided.

The other two common types are 3-pole, 3-wire "delta" systems, which you won't have because they are both single-voltage (no neutral), and all CB's will be 2-pole for single-phase loads.

Remember from my earlier post, that if a load is "continuous", you should not load the CB to greater than 80% of its rating, so your multiplier in your question is not ( 30 * 1.35 ), but 30*1.25 and is then sized to the nearest larger standard size of CB. Thus a "continuous" load of 18-amps becomes 18*1.25 = 22.25-amps and would require at least a 25A CB (but probably a 30A CB would be used because 25A is uncommon).

Many contributors to CR4 may complain about my use of 127 and 220 volts for the "wye" system because in the USA the standard is 120 and 208 volts. I have used the first set of numbers because I understand your system to be outside the USA and therefore to operate under probably different standards.

I hope this helps and doesn't confuse your understanding.

--John Mueller

my dear ,

actually i am facing same problem in load calculations and cable size accordingly .if you put 60 A it is ok but be sure what type of load you are using i mean resistive load or inductive load .i mean AC load is different from light loads since we have starting current for machines and motors you have to consider when you decide the CB rating thats why we have to encrease the rating even though the clamp meter reading will show 42 A only .i agree with you we can decrease the rating from 125 A but withen limits .

moreover for me as per the country rules we have to take approval from electrical department related to the city you are living ,so they put standards which force us to use 125 A not 60A .

keep in touch .

REGARDS .

ABDULKADER