Series / parallel circuit rounding

[YoungSinatra]

When calculating a series or parallel circuit, is there a specific number of places after the decimal that you should round to? Like would you round 76.72489v to 76.72v or 76.725v?

 

[JoeStillman]

In my daily work, I use 3 figures of accuracy. If in doubt, ask your professor. My daughter's physics teacher gigged her on a question because she gave him three figures of accuracy in an answer, but the question had stipulated only 2.

 

[LarryFine]

Welcome to the forum.

When calculating a series or parallel circuit, is there a specific number of places after the decimal that you should round to? Like would you round 76.72489v to 76.72v or 76.725v?

I was taught that two decimal places were plenty adequate for electrical calculations, so 76.72v.

 

[tom baker]

See 220.5 B for fractions of an ampere. I learned from Mike Holt to round once during a calculation.

 

[JoeStillman]

See 220.5 B for fractions of an ampere. I learned from Mike Holt to round once during a calculation.

Right! And the voltages in part (A) are all given in 3 significant figures. Note that significant figures are not the same thing as decimal places. 277V is 3 significant figures but 277.000V is six.

 

[jim dungar]

If in doubt, ask your professor.

Always check with the person in charge of approving the calculation.
I once had a professor that wanted answer out to 7 digits after the decimal point. One time, his 20 question test had all the answers being less than 1.0000000.

In my real life, I have rarely needed to be more than 3 significant figures, as mentioned by Joe. For example what are the actual, versus rounded, L-N values when the L-L voltages are 208 and 480 on a wye system.

 

[synchro]

Joe's comment about knowing what "significant figures" are and using them when showing the results of your calculations would be helpful.
As an example, 0.0458 amperes is equivalent to 45.8 milliamps and both expressions have 3 significant figures. The zero after the decimal point when the result is in amperes does not add any more precision to the measurement, it's just the units that have changed. But 1.00458 amperes can also be expressed as 1004.58 milliamperes and both imply there are 6 significant figures. More information about significant figures is at the links below, if you're interested.

As was mentioned, usually 3 significant figures is adequate for most electrical power calculations. That's not to say that more digits can't be used during the calculation process if available, it's just that the final result should not reflect any more precision (in other words, number of significant figures) than the numbers that you entered into the calculation. Otherwise it may be misleading to someone looking at the result who would think it's more accurate than it really is.

Significant Figures

Intro to significant figures (video) | Khan Academy

Significant figures are the number of digits in a value, often a measurement, that contribute to the degree of accuracy of the value. We start counting significant figures at the first non-zero digit.

www.khanacademy.org

 

[GeorgeB]

My post-calculator education was to carry as many significant digits as are available in the CALCULATIONS, then round to the number of the least significant measurement. So if given 2.885 ohms, 462 volts, 3 amps, 4.00:1 transformer ratio, and a sq root of 3 involved, I'd maintain 5 or 6 digits (all that were available really, maybe 8 or 9) until the answer, then round to 1 (at most 2) based on the "3 amps". With a slide rule, I used the 3 digits until the end, then rounded the same way.