CALCULATION FOR A SINGLE-PHASE EV CHARGER

First we must find our design current (Ib)Most single-phase EV chargers are 7.4kW so let’s use a single-phase 7.4kW charger in this example.

We can find Ib by using the formula Power/ Voltage. Therefore 7400/230 = 32.17A

The next step is to select a protective device (In) equal to or greater than the design current (Ib).

For this example we will say we are using a BSEN60898 MCB (type B), therefore the next size up is a 40A MCB (Max Zs = 1.09)

The rated current of the protective device (In) must then be divided by any applicable rating factors such as ambient temperature (Ca) , cables buried in the ground (Cc) etc.

Let's assume our cable is buried in the ground at a depth of 0.5m, therefore Cc=0.9. Let’s assume this is the only factor applicable to keep it simple. Therefore: 40/0.9 = 44.44A

We must then select a cable with a tabulated current carrying capacity equal to or greater than the value above for the circuit installed under its defined conditions. It ≥ 44.44.

Selecting a multicore armoured 90⁰ thermosetting cable from table 4E4A , column 6 (reference method D) of BS7671, we need to select a 6mm2 cable to satisfy this. 6mm2 has a tabulated CCC of 53A when installed to this reference method.

Volt drop must then be verified to check the requirements of BS7671 are satisfied. The R1+R2 and Zs of the circuit must then be verified against the max permissible Zs. The minimum CPC size should also be verified using the adiabatic equation, as should the tripping time using time/current curves from BS7671.

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