Electrical Load Calculations

elecrical load calculation based on network ? What does that mean? What type of load? Utility, commercial, residentail, industrial?

Wareagle, most of my applications are onsight power, mostly shipboard. However, I think that a Load Flow Analysis such as sold as a seperate module from SKM, ETAP, etc. would probably be better suited for Plants, High Rises, etc. but I'm not an expert on those applications as my 45 years in the marine industry never required it.

I think I see where you guys are headed but NEC has guide lines as ya'll well know for Load Analysis's and has gone as far as watts per square foot by some engineers as a guesstimate.

Electrical load calculations are based on "duty factors", i.e., What percentage of the load (individually), represented in kilowatts, operate in a 24 hour period. I've attempted to find software as I perform these calculations frequently in order to determin the size of generators or transformers. Couldn't find any! The idea behind it is not to oversize your power supply weither it be a generator/s or transformer/s. Here is an example: Suppose you have a refrigerator, it's rated at say, 1 hp which equals 746 watts. It runs about half of the day. Then your duty factor would be 0.5 times 746 watts. Calculating all connected loads and assigning duty factors to them will determine your power requirements. Then add about 1.15% as a cusion.

Jraubsr Quote "Suppose you have a refrigerator, it's rated at say, 1 hp which equals 746 watts. It runs about half of the day. Then your duty factor would be 0.5 times 746 watts.

Suppose everything only runs half of the day, but it all comes on at 8:00 AM. Then what happens?

Demand factors are based on years of recording eqpt. for specific applications. Common sense plays and important role as well. For example, using the refrigerator analogie, could be if the refrigerator was sitting outside in the summer time it would certainly run more than it would if it were in a controlled enviourment. A crane or gantry could operated only while unloading ships. If the dock worked 24 hours a day doesn't mean that the eqpt. is operating at full capacity. Subsequently you must have knowlege of your application. Basically it's a guestimate! Here is one formula used under NAVIC 2-89 Shipboard regulations.

LOAD FACTOR=OPERATING KW/RATED KW X NO. HRS OPERATION/24 HOURS or you can substitue Rated KW with Rated bhp.

Hello,

For the purpose of load calculation in a particular system, you need to know the following factors :

Demand factor is the ratio of the sum of the maximum demand of a system (or part of a system) to the total connected load on the system (or part of the system) under consideration. Demand factor is always less than one.

Diversity factor is the ratio of the sum of the individual maximum demands of the various subdivisions of a system (or part of a system) to the maximum demand of the whole system (or part of the system) under consideration. Diversity is usually more than one.

As an example :

Suppose a main feeder from substation switchgear that is supplying five feeders with connected loads of 400, 350, 300, 250 and 200 kVA with demand factors of 75, 80, 85, 90 and 95 percent respectively. What shall be Transformer rating feeding the switchgear if the diversity factor is 1.5?

Solution

Calculate demand for each feeder:

- 400 kVA × 75% = 300 kVA

- 350 kVA × 80% = 2805 kVA

- 300 kVA × 85% = 255 kVA

- 250 kVA × 90% = 225 kVA

- 200 kVA × 95% = 100 kVA

The sum of the individual demands is equal to 1250 kVA

Using the diversity factor of 1.5, the required Transformer rating shall be 1250/1.5

= 833 kVA. Considering safety factor of 20% the transformer rating shall be 1000 kVA

The base is

100% of Continuous Load + 50% Intermittent Load + 0% Standby Load

The calculation will be easy if the load is lighting and small office appliance only.If central air conditioning is used you should consult its supplier.If large motors are used you should consider the sequence of starting ,starting kVA etc.If a pf correcting capacitor bank is used ,for generator sizing you should omit it.If the installation is already energised the kVA reading of the utility company would guide you but as it indicates kVA in a 15min window starting current of large motors will influence the rating of the generator

KKK,

Can I use this method to determine Generator rating? And is it okay to use diversity factor & demand factor per small load (such as lighting point,socket,cooker unit,A/C unit,etc) when calculating ? How can I get latest diversity & load factor Table?

The diversity factor,load factor etc should be applied by experience only, depending generally on the type of occupancy(home,office,super market,industry,roads etc) and sequence of operation of equipment in a particular situation.Some diversity factors are given in BS 7671 to send you I do not have a soft copy. Do you watch " myth busters"?

KKK, thats exactly what I use it for. I have typical tables as mandated by regulatry bodies involved with the marine industry. Many times my job is to size generators and transformers. Again, the application is everything when it comes to heavy eqpt. But general usage as you've referred to are pretty standard unless your kitchen is open 24 hours/day.