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This happens with electrical calculations, too. These calculations are typically done in software, so errors are due to inputting the wrong thing or not checking a box.
Nobody likes recalculating everything on a drawing. It also seems silly to redo by hand in the field what was done so efficiently in the office. Not to mention it’s costly.
Fortunately, there’s an easy way to check without grinding through the calculations again. The trick is to calculate with the constants that apply to electrical calculations, but round them to a single digit. You can find these constants in any electrical formulas book.
Here’s an example. Suppose you want to know the amperage of a 120V AC circuit supplying two 800W space heaters. You could divide the kW by the volts, but it’s easier to multiply by the 120V constant. That constant is 8.33. You can multiply 1.6W times 10 by adding a zero. The amperage is less than 16A. If you subtract 3 (1.6 times 2), 13A is close to the actual value. This assumes power factor is unity, so if values differ greatly take a look there first.
If you’re installing 277V heaters, the constant for a similar calculation is 2.77. If you round both numbers a bit, you end up multiplying 1.5 times 3, resulting in 4.5A. That’s only 0.07 less than actual.
Source: Mark Lamendola | Mindconnection