How to Select a Cable

The answer is in your local electrical regulations handbook.

Depending on your local standards and regulations you will likely need to run cable rated for the rating of the socket, not just the two fluorescent lights you are connecting to it. Hence the load(kw) value is the rating of the socket (V*I).

As a rough guide (consult your local standards and regulations), if the two sockets are connected in parallel then your circuit is probably protected by a circuit breaker or fuse with a rating of 13A, hence your load(kw) value is roughly P=V*13. This means that the cable you are running must also be rated for 13A (taking into account voltage drop and de-rating factors).

This is only a rough informative guide. Consult your local electrical regulations handbook and only perform the above work if your are qualified to do so or if your local regulations allow you to perform the above mentioned work if it is inspected and signed off by an electrical inspector.

In addition to what Jack says, you are in control as to what the load will be. If you do not add anything but the lights, then the load is very small. If you add a large motor then the load has increased. You must determine what is going to be in the garage and then get the amperage.

British Standard 7671 is needed to calculate the protection needed for the feeder cable. If it is 4mm² SWA cable then protection from the main dis board at around 20A is needed at a first stab though it needs to be checked via the standard. 2.5mm² might be struggling at 20A, which is why the check calculation needs to take place.

Because it is outside the house, the garage new dis board must have 30mA RCD protection these days, and it falls under Part P of the Building Regulations, which means notifying the local authority and documenting the test results performed by a qualified person ready for local authority sign-off on the works (Part P doesn't stop DIY installation by a competent person, it is only about documenting the final tests appropriately). Inspection and test companies may be found in Yellow Pages and carry out the necessary for a modest fee.

At the board:

• 2x13A socket can be fed generally from a 2.5mm² T+E spur if the spur is protected at no more than 20A (who uses more than 20A continuously at a garage, what?).
• 1x lighting circuit is fed from the same board at 5 or 6A protection. These days 1.0mm² is frowned upon if there is any insulation present where it passes. Generally, 1.5mm² T+E is used for lighting.

Garage dis board assemblies with the right combination of breakers are available over-the-counter at places like Screwfix and B&Q (usual disclaimer).

Thanks for your prompt replies guys. I've checked the regs and of course correct me if i'm wrong but if I am only having two 13a sockets in the garage it is basically 230X13=2990W plus 2X58W lights= 3106W.

I have calculated that that a 4mm cable will allow for up to 7kw so that will easily cover the demand plus allow for any additional circuits in the future. I don't think there is any correction factors to be added.

Just one more thing at the risk of bending towards a new thread but if I put an electric door on my garage how will the motor be fed? Would it be from a fcu fed from the existing 13a socket?