Calculating Power Supply Capacity

Size it for peak loads, or it will trip out on overload during them.

Hi, Thanks a lot for your reply.

As PWSlack rightly says size it for the peak load. But keep in mind that while taking the 10% of the standby loads we should match it with the largest rating of the standby load and in case the 10% calculated value is lower then the largest standby load then we should take the value of the largest machine in our calculation.

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So we have to take into consideration the value of whichever is high, either 10% of the standby loads or largest machine.

Please check whether my understanding is right.

Thanks again.

Your understanding is correct

10% of the standby loads or the biggest standby load

Bulls eye.

10% of the total standby loads or the biggest standby load

There is no definitive rule to calculate the electrical loads. I worked in various projects and each of the projects follows different type of calculation. The most common method I found is:

(1) Find the rating of the connected loads (preferably name plate rating) including the future loads that are known.

(2) Estimate the duty factor (the duty factor of a motor refers to the on-time of the motor versus the total time. For continuous load, the duty factor is 1; for standby load, the duty factor is 0, for intermittent load, the duty factor may be considered 0.5 or in between 0 to 1. If an intermittent load is likely to be run during the peak period, consider it 1).

(3) Estimate the loading factor for each of the loads (loading factor is the average load divided by the peak load, over a period of time. If a 50 kw motor's average load is 40 kw, the loading factor is 0.8).

(4) Multiply each connected load (from 1) with duty factor (from 2) and loading factor (from 3)

(5) Sum up all the loads (found in 4) to obtain the total load

(6) Multiply the total load (found in 5) with 1.2 (considering 20% reserve capacity) to find the estimated calculated total capacity.

It will be helpful to prepare a spread sheet using the formula with this logic to calculate the total capacity.

Go to the recent discussion http://cr4.globalspec.com/thread/43104/Electrical-Load-Calculations and it will also help.

- MS

Peak loads are the key. This URL leads to a very good application note on measuring peak power losses on Switched Mode Power Supplies (SMPS).

http://www2.tek.com/cmsreplive/tirep/15774/54W_23880_0_2009.07.23.13.09.39_15774_EN.pdf

It also covers magnetic losses.

Hi, Thanks all of you.