The Joe Bloggs guide to the maximum element size.

Elements are rated (quoted power) at a nominal voltage, for the example below we will use 230Vac. It should be noted that the voltage should be stamped on the element beside the power rating.  Some examples, "3200kW @ 230V", "3.6kW @ 230V", "5.0kW @ 230V". Voltage supply variations in EU are allowed to be 230V +/-10%, 207V to 253V.

3200W (3.2kW) Element

Step 1, work out the resistance (R) from the equation, Nominal Power (P) = Nominal Voltage (V) Squared divided by Resistance.  P=( V^2)/R, which rearranges to R=(V^2)/P.  For a 3.2kW@230v element this becomes, R = (230*230)/3200, R=16.5 Ohms.

Step 2, work out the highest delivered voltage to your premises, in Europe this is 230V + 10%, 230V+23V, = 253V.

Step 3, work out if you have an element with 16.5 Ohms resistance and 253V, what will the current (i) be, V=iR, or i=V/r,…. So, i=253/16.5, i maximum is 15.3A. Can your circuit and controller cope with 15.3A + pumps?

3600W (3.6kW) Element

Step 1, work out the resistance (R) from the equation, Nominal Power (P) = Nominal Voltage (V) Squared divided by Resistance.  P=( V^2)/R, which rearranges to R=(V^2)/P.  For a 3.6kW@230v element this becomes, R = (230*230)/3600, R=14.7 Ohms.

Step 2, work out the highest delivered voltage to your premises, in Europe this is 230V + 10%, 230V+23V, = 253V.

Step 3, work out if you have an element with 14.7 Ohms resistance and 253V, what will the current (i) be, V=iR, or i=V/r,…. So, i=253/14.7, i maximum is 17.2A. Can your circuit and controller cope with 17.2A + pumps?

5000W (5.0kW) Element

Step 1, work out the resistance (R) from the equation, Nominal Power (P) = Nominal Voltage (V) Squared divided by Resistance.  P=( V^2)/R, which rearranges to R=(V^2)/P.  For a 5.0kW@230v element this becomes, R = (230*230)/5000, R=10.5 Ohms.

Step 2, work out the highest delivered voltage to your premises, in Europe this is 230V + 10%, 230V+23V, = 253V.

Step 3, work out if you have an element with 10.5 Ohms resistance and 253V, what will the current (i) be, V=iR, or i=V/r,…. So, i=253/10.5, i maximum is 24A. Can your circuit and controller cope with 24A + pumps?

Now let's look at reducing the element power.

2900W (2.9kW) Element

Step 1, work out the resistance from the equation, Nominal Power (P) = Nominal Voltage (V) Squared divided by Resistance.  P=( V^2)/R, which rearranges to R=(V^2)/P.  For a 2.9kW@230v element this becomes, R = (230*230)/2900, R=18.2 Ohms.

Step 2, work out the highest delivered voltage to your premises, in Europe this is 230V + 10%, 230V+23V, = 253V.

Step 3, work out if you have an element with 18.2 Ohms resistance and 253V, what will the current (i) be, V=iR, or i=V/r,…. So, i=253/18.2, i maximum is 13.9A. Can your circuit and controller cope with 13.9A + pumps?

2500W (2.5kW) Element

Step 1, work out the resistance from the equation, Nominal Power (P) = Nominal Voltage (V) Squared divided by Resistance.  P=( V^2)/R, which rearranges to R=(V^2)/P.  For a 2.5kW@230v element this becomes, R = (230*230)/2500, R=21.16 Ohms.

Step 2, work out the highest delivered voltage to your premises, in Europe this is 230V + 10%, 230V+23V, = 253V.

Step 3, work out if you have an element with 21.16 Ohms resistance and 253V, what will the current (i) be, V=iR, or i=V/r,…. So, i=253/21.16, i maximum is 11.9A. Can your circuit and controller cope with 11.9A + pumps?

In this article 230V + 10%, 253V was used as the maximum voltage. If you are confident (have actual knowledge) that the supplied voltage never goes above 245V for example, then you can substitute 245V for 253V in the above calculations.

Conclusions, many people oversize their element, beyond what the circuit and controller will take. Don't do it. If you don't understand the above logic, then consult someone that does.