Disclaimer: I'm in Brazil. Although we do have electrical standards, building code is almost non-existant.
According to our standard (and some other sources since I've found it) the following apply to 2 circuits using 2.5 mm² monocore wire in the same conduit in a non insulated wall:
- Two charged conductors 2.5mm² (per circuit) in a B1 method (not insulated wall) can carry 24 amps.
- Since there are 2 circuits together, so apply the grouping factor of 0.8, now each conductor can carry 19.2 amps.
I want those circuits to power 20 Amp sockets (this a standard socket in Brazil). I know that it is almost impossible for it to reach 20A since by law 20 amps is the maximum allowed for electrical appliances plugs. So my 19.2 amps should suffice and work fine.
What would happen should I run the full 20A in both circuits. Will it get too hot? How hot? The PVC insulation is rated for 70°C and I think it must stand up to 90°C without being damaged, I doubt it could come close to that.
Also, I suppose those factors consider that both circuits are constantly on, which is often not the case in a home setting. Being on the safe side the surest thing to do would be to use 4mm² wire, but this is more expansive, harder to route and just annoying in general to use since most of the conectors and sockets work better (fitting and instalation) with 2.5mm² wire.
So back to the tile, what will happen if I run a circuit close or just above its calculated maximum/recommended current?
Best Answer
When a wire is used close to its maximum current it gets hot.
When a wire carries any current it heats up but when used within its design limits it will not get too hot. As the current increases the wire gets hotter. A wire that carries too much current can get hot enough to start a fire. Many cables in houses and other buildings are buried in the walls and so are surrounded by insulation and wood which might not be fire-resistant. The consequences of an overloaded (and very hot) cable in such a wall can be a house fire.
The electrical system should have fuses (breakers) that are set to trip at currents well below the maximum current of the cables. I am sure that the various countries have regulations about the fuses to be used for different cable types in different installations. The goal is that the fuse breaks the circuit, which is a minor inconvenience, long before the house might catch fire.
Given the disclaimer text in you question I would recommend that you choose cables that can carry significantly more than the expected current plus fuses for the expected maximum current. For exact choices I recommend that you talk to a local electrician or to an expert in the shop where you buy the cables and fuses. If your shopkeeper does not know then find a better shop.