# What does happen when a wire is run close to its maximum "calculated" current?

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. Sep 15, 2016 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. Sep 15, 2016 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. Sep 15, 2016 at 12:22
• 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?" Sep 15, 2016 at 16:22