# EV Charging tripping MCB

We have found that our two EVs are tripping the 32A MCBs in our Consumer Unit, but from my basic knowledge the 2*6.6kW chargers on UK 3-phase should only draw about 19A.

I've measured the power consumption by timing the meter disk1 and each car draws about 6kW (the two at the same time on two different circuits draw a total of 13kW).

$I = \frac {P} {3 \times V_{LN}} = \frac {P} {\sqrt{3} \times V_{LL}} = \frac {13kW} {3 \times 230V} = \frac {13kW} {\sqrt{3} \times 400V} = 18.8A$

I can't believe that EV charging has a power factor of 0.59, but another question - MCB of desired rating - alluded to MCBs having low tolerances (20A MCB tripping between 18A to 25A). Is this correct?

The charging posts we have are two 2x22kW Rolec Autocharge posts. Our director likes to buy big (futureproofing being his motto)... but because of lack of total capacity into our building our electricians limited the supply to the posts to 22kW each (using a 32A MCB). Still, our current 13kW usage falls well short.

1 Meter is 60 revs / kWh. Measure time for 5 revolutions with no EV charging load, and use that as our base usage (5 rev in 20s = 15kW). Then attach one EV, wait for load to settle (5 rev in 14s = 21kW). Difference is 6kW.

RapidTables Kilowatts to amps calculator

• What is your actual line voltage? Aug 1, 2018 at 9:35
• @RDrast Using our ChargeCheck in the EV pedestal, and then testing the voltage on the UK socket, I get 235V. I have no [safe/legal] way of measuring the 3-phase line-line voltage Aug 1, 2018 at 10:02
• Then you need to recheck your formulas. 13000 / (sqrt(3) * 235) is 31.9 Amps. Aug 1, 2018 at 10:15
• @RDrast But if 235 is the Line-Neutral voltage, then the Line-Line voltage is 407? Using the rapid tables calculator (link above) I get the same Amps result using either. If I flip the voltages then I get two vastly differing values... Aug 1, 2018 at 10:47
• @RDrast That is probably a few minutes until thermal trip with the current heatwave in europe. Breaker will be warm to the touch. Aug 1, 2018 at 11:08