Overview of EV charging equipment — EV Charging Installation Practice

Sample questions

1. 01A customer calls the new 7 kW wall unit on their drive "the charger". Which of these is one of the jobs that unit actually does on AC?

   • AIt turns the AC supply into DC before it reaches the car
   • BIt stores energy so the car can charge faster at peak times
   • CIt raises the supply voltage so the battery fills more quickly
   • DIt tells the car the most current it is allowed to draw

   Why: In Mode 3 the control pilot function extends to control equipment in the charge point (BS 7671 Part 2, Mode 3 charging), and that is how the unit tells the car the current it may draw. On AC the conversion to DC happens in the car; only Mode 4, which uses an off-board charger, converts outside the car. A wall unit does not store energy or raise the voltage.

2. 02A balanced three-phase charge point draws 32 A in each line on a 400 V supply. A colleague works out its power as 400 × 32 and gets 12.8 kW. Roughly what is the real figure?

   • A22 kW
   • B38 kW
   • C12.8 kW
   • D7.4 kW

   Why: For a balanced three-phase load P = √3 × U × I, so 1.732 × 400 × 32 = 22,170 W, about 22 kW, the same as 3 × 230 V × 32 A. The colleague’s 12.8 kW leaves out √3. 38 kW comes from multiplying 400 V by three, and 7.4 kW is one phase at 32 A.

3. 03A 22 kW three-phase charge point is fed at 400 V. What line current should the design use for it?

   • AAbout 55 A
   • BAbout 32 A
   • CAbout 18 A
   • DAbout 96 A

   Why: I = P ÷ (√3 × U) = 22,000 ÷ (1.732 × 400) = 31.8 A, so about 32 A in each line. 55 A comes from dividing by 400 V alone, 96 A from putting the whole 22 kW on one 230 V phase, and 18 A from dividing by 3 × 400 V.