A 208Y/120-volt three-phase service calculation for a shop totals 84,000 volt-amperes with every motor already counted at 100 percent of its full-load current. The largest motor on the service draws a full-load current of 27 amperes at 208 volts three phase, and the code requires 25 percent of the largest motor load to be added to the total. What is the final calculated service load?
- A93,727 VA
This adds the entire largest motor a second time instead of 25 percent of it. The motor is already included at full value in the 84,000 volt-ampere total.
- 86,432 VA
- C85,404 VA
This omits the square root of three when converting the three-phase motor current to volt-amperes, which understates the motor by about 42 percent and therefore understates the addition.
- D105,000 VA
This multiplies the entire service total by 125 percent. The 25 percent addition applies only to the largest motor, not to every load on the service.
Why B is correct
The three-phase volt-ampere formula requires the square root of three, so the 27 ampere motor represents 1.732 x 208 x 27 = 9,727 VA. One quarter of that, 2,432 VA, is the additional amount the code requires for the largest motor, giving a final total of 86,432 VA. The other motors are carried at 100 percent with no addition.
What this question is testing
Tests three things at once: converting a three-phase current to volt-amperes with the correct square root of three factor, recognizing that only the single largest motor receives the additional 25 percent, and understanding that the addition supplements a total in which the motors already appear at full value.
On the job
Shops and light industrial buildings live or die on this line. Add the whole motor twice and you sell the customer a service one size too big; forget the square root of three and you undersize the service and the utility transformer request along with it. When several motors are close in size, identify the largest by full-load current from the motor tables, not by horsepower nameplate alone, because voltage changes the current.
Memory technique
Everybody counts once, but the biggest motor counts once and a quarter.
Exam tip
Find the largest motor by full-load current, convert it to volt-amperes with 1.732, then add one quarter of that number to your total.
Where to look it up
Article 220 motor load reference, which sends you to Article 430 for the actual sizing rules. Keep a tab on the three-phase motor full-load current table because you will need it in the same problem.