There is a fundamental principle of engineering that says a calculation?s final answer cannot be expressed to a higher degree of precision than the input information. In this context, ?precision? has to do with rounding to the nearest whole number of units. For example, suppose you need to measure the distance across a building, in order to estimate the amount of wire you will need for a home run. One way might be to walk across the building, and count your steps. Another might be to break out a long tape measure that has markings every sixteenth of an inch. In the first case, you might count out 116 steps, then say that one of your steps is about 2.5 feet, and conclude the distance is about 290 feet. In the second case, you might measure a distance of 284 feet, 14 inches, plus 3 sixteenths of an inch. This equates to a total of 54,755 sixteenths of an inch. The second reading is more ?precise,? because it is using units of measure that are smaller (i.e., you are rounding to the nearest sixteenth of an inch, as opposed to rounding to the nearest number of 2.5 foot-long steps).
You would be outside the rules of good engineering practice, if you were to take your first measurement, multiply 290 times 12 and times 16, and say the distance is 55,680 sixteenths of an inch. This would be invalid because you cannot tell whether any given step was exactly 2.5 feet, and not 2.49 feet, and not 2.51 feet. Your degree of precision, when you made the measurement, was no better than the nearest half foot or so. Your degree of precision, when you made the second measurement, was as good as the nearest sixteenth of an inch.
I bring this up because I cringed when I saw the two numbers, 150 amps, and 27879VA. The first number is precise to about the nearest 5 amps. The second number is precise to the nearest single VA, or the nearest 0.0042 amps (assuming a 240V single phase system). So I don?t buy the situation at all.
Bob?s statement was simpler, but similar. How did you arrive at the number 150 amps?
To the above, I will add one more question: Are we talking single phase or three phase?
