Why do Level 2 home charging points use 7 kW?

Question

As I understand it in both the UK and the US (and I assume other places) have a Level 2 car charging standard that uses 7 kW and charges a standard car in about 9.5 hours. Why was this level chosen?

Domestic electric cookers typically use up to 5 kW, and most people have wiring to take this power to an isolation switch on the wall of the kitchen, which will be within the reach of a cable for many people who have access to off road parking. Had 5kW been chosen the costs for installation would have been lower for people who could have used their electric cooker wiring to supply power.

Most people would like to be able to charge a car quicker. For example many people may want to charge their car over an 8 hour work or sleep period, which they could have done had 8.3 kW been chosen. Given fitting these systems cost $1,350 - $2,500 there seems no obvious limit to their capacity below the mains supply to a house, which is frequently much more than this.

If one was motivated to encourage uptake of electric cars one would choose either the slightly slower but much cheaper option of 5kW or a significantly higher value to allow people to charge their car quicker. One would expect both governments and electric car manufacturers to want to encourage uptake of electric cars.

Why was 7 kW chosen for level 2 car charging?

Answer 1

It wasn't. 22kW was choosen. It's because 22kW @ 400V 3-Phase is a very common maximum for the wall box. This equals 32A, a common MCCB size.

Single phase - N at 230V this becomes 7kW (230V*32A=7360W).

If the wiring in your building can't support 32A, it's trivial to electronically limit the charging to a lower value. In Norway that's permitted installation; limit the charger to what the circuit can supply.

Most people would like to be able to charge a car quicker. For example many people may want to charge their car over an 8 hour work or sleep period, which they could have done had 8.3 kW been chosen.

No, most people don't care. Few people drive enough a day to empty their batteries daily. My typical driving requires charging twice a week, and if I charge daily even 2kW would be enough to cover daily driving with an 8-hour charge.

Second, as I mentioned many wall boxes support 22kW. It's the cars that doesn't support more. This is probably because of demand; 7kW is enough. The added cost and weight isn't something a lot of people want.

Had 5kW been chosen the costs for installation would have been lower for people who could have used their electric cooker wiring to supply power.

Almost all EVSE wall boxes can be configured for a max current, usually from ~5-32A. The actual charger is in the car, but the wall box sends a PWM signal to the car to tell it the allowed current. The adaptation of change is fast enough that it can actually be used for load regulation...

But using this signal you can wire your wall box to a 10A fuse if you want. I have a wall box rated for 22kW (32A @ 400V) wired for 16A @ 230V, and configured for 16A. The car ends up charging at 3.6kW.

Answer 2

I've read the US linked site and a number of factors are somewhat inaccurate. I've connected to a 3 kW EVSE and to a 5 kW EVSE as well as the 10 kW EVSE installed in my house.

Who chose 7 kW? I chose 10 kW and I have no choice afield at those sites equipped with 3 kW EVSE units.

The other aspect of this question regarding the expression "standard car" at 9.5 hours. There is no standard car. Battery pack capacities are quite wide ranging, which will determine charge duration from an undefined battery-low level.

Our 10 kW EVSE charges at 30 miles per hour while the 3 kW EVSE in the wild provides far slower charging speed, but the missing information is at what level of the battery pack does the charge begin? If I've traveled 100 miles, I can expect a bit more than a three hour charge at home and far longer in the field.

If one requires shorter duration charges, one simply charges more frequently, specifically and especially when the EVSE is located at home. One does not often run one's fuel tank in an ICE (Infernal Combustion Engine) vehicle to the very bottom, but one also does not often have the convenience of a fueling station at one's home.

Expense certainly plays a part for one who intends to purchase EVSE units, but the differences between the lower and higher power units are not too severe, nor does the wiring cost present an unusually large difference. Of course, some houses are unable to accept the current load required for the high end equipment.

I believe the source information is inaccurately and too-generally presented.

Answer 3

Below are the three levels of EV charging. Homes can have level 1 or 2. Businesses can have level 3 for fast charging