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?

  • You would hope that the breaker would open if it went above the maximum recommended current. However, if as you say the building codes are ignored there, that might be a luxury. – Francis Sep 15 '16 at 4:30
  • If the breaker is rated for 20 amps it won't break in my scenario. Anyway, I want to know what happen to the wire installation. – Luiz Borges Sep 15 '16 at 4:32
  • I think that the wire should be fine if the current it carries is at or below its rated capacity. I'm not sure what you are saying about the two circuits in the same conduit, but if as I think, you are saying the code says that the limit is 19.2 amps then a 20 amp breaker is not providing sufficient protection to the wires. It would be fine for the socket, but for safety's sake you have to protect the weakest link in the circuit. You would have to use a 15 amp breaker. Maybe it would be safe to use 20 amp breakers in a subpanel protected with a 35 amp main. – Francis Sep 15 '16 at 12:22
  • 1
    See How do I calculate the temperature rise in a copper conductor? - " if I have a 7.2kW load powered by 240VAC, the current will be 30A. If I transmit this power to the load via a 2.5mm² copper conductor, how do I calculate how hot this conductor will get?" – RedGrittyBrick Sep 15 '16 at 16:22

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.

  • I understand all of that, my question is more on an engineering/physics point of view. What I mean to known, is how hot (as in some approximation delta temperature for example) it gets in that situation and his that is worrisome with more that one circuit. I think that the voltage drop would mean the heat loss, if so with 3% voltage drop in 10 m using 2.5mm² wire, it is losing 100W when carrying 20 amps. How hot is that in practical terms? How much it affects the neighboring cables? – Luiz Borges Sep 15 '16 at 12:51
  • If you are pushing both the current max and voltage drop max Bite the bullet and install larger conductors. This will be more expensive and a pain in the back side as you noted. The lower the voltage can cause some loads to "brown out" and draw higher current than normal this is true with inductive motor loads, but not restive heat loads. Although the wire is rated for 20 amps the nominal load should only be 80% or that is the guideline here in the U.S. – Ed Beal Sep 15 '16 at 13:10

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