Help in understanding current ratings of wires from datasheet

[Wrichik Basu]

Need some help in understanding the current ratings of wires. I had asked about this in another thread, and learnt that power ratings of wires depend on numerous factors. To simplify things for me, I could successfully dig out the product datasheet from one of the notable companies in my country in this business, and whose wires we have installed in our house recently.

Tables from one such datasheet is shown below:

I am looking at the "In conduit" ampere ratings. Do these ratings hold true for any voltage ≤ 1100V RMS?

 

[DaveE]

Do these ratings hold true for any voltage ≤ 1100V RMS?

Yes, anything below the voltage rating of the wire insulation. The ampacity is about heating and the temperature rating of the insulation material, not voltage. A higher voltage rating may mean thicker insulation which will lower the ampacity. That's why the tables should specify which insulation type they apply to. In the USA this will appear as THHN, THHW, UF, FEP, etc. above the appropriate column. Like this:

 

[Baluncore]

I am looking at the "In conduit" ampere ratings. Do these ratings hold true for any voltage ≤ 1100V RMS?

Yes. The manufacturer's table is correct for those insulation thicknesses. It seems they assume 20°C, so if the conduit is exposed to the sun, you will need a different table.

The heat, dissipated by the resistance of a conductor, is a function of current. W = I²R. The thickness of insulation electrically insulates the wire, but it also thermally insulates the wire, so the wire is hotter with thicker insulation. That higher temperature causes higher resistance, which increases the heat generation, and increases the voltage drop.

You must follow the manufacturer's advice, and not exceed those ratings, because electrical insulation is damaged by higher temperatures.

 

[sophiecentaur]

You must follow the manufacturer's advice, and not exceed those ratings, because electrical insulation is damaged by higher temperatures.

Ambient conditions make a big difference to the temperature of the conductor. To be certain that you have 'behaved yourself' in your circuit design you have to use the manufacturer's information. The industry has decades worth of experimental measurements to justify their figures. Don't be optimistic about what you can get away with. Insurance will not pay out if you flout the recommendations.

 

[Guineafowl]

From your circuit diagram, it looks like you have lots of violations of the ampacity table above.


Main fuse (63A?) to meter would typically be 16 or 25 mm²
Ditto meter to distribution board.
You have 31A 6 mm² cables on the 40A MCB.
The 4 mm² branches to MCBs A-G are similarly undersized.
MCBs B and C are sailing close to the wind at 20A through “<=2 mm²”.

(Might be a quirk of the diagram, but you seem to have double-pole fusing on the incomer. This is obsolete and dangerous, if it’s the case).

C curve MCBs are rare in domestic circuits over here. If present, they would be installed for something specific, eg large aircon units, often with thicker wiring to ensure timely tripping. It’s unlikely your circuits would achieve the loop values needed.

Aforementioned comments on adding some sort of earth fault protection.

Lastly, nitpicky, you are using the 40A DP MCB as the means of whole-house isolation. This would be considered bad practice unless it has active-break contacts. Otherwise, the live side contacts could weld shut, leaving only the neutral open and the circuit apparently, but not actually, dead. Why not have all the MCBs A-G on the 40A DP isolator? These won’t click to ‘OFF’ unless both contacts positively open.

What’s been installed there is pretty unsafe. Sorry to be a misery-guts, but your own IS 3961 tables do seem to say so.

 

[Wrichik Basu]

From your circuit diagram, it looks like you have lots of violations of the ampacity table above.
...
What’s been installed there is pretty unsafe. Sorry to be a misery-guts, but your own IS 3961 tables do seem to say so.

Abridged by me

Yeah, I know. The entire circuit is full of violations. None of the 25A or 16A rated circuits will ever be able to carry their full capacity. Fixing all the violations requires two things: money, to buy good quality higher capacity wires, and an electrician who will listen to me/advice I have received here. Both of these are unavailable at the moment. Even if the former becomes available in the upcoming years, the latter is impossible to find, to be honest.

 

[Guineafowl]

Abridged by me

Yeah, I know. The entire circuit is full of violations. None of the 25A or 16A rated circuits will ever be able to carry their full capacity. Fixing all the violations requires two things: money, to buy good quality higher capacity wires, and an electrician who will listen to me/advice I have received here. Both of these are unavailable at the moment. Even if the former becomes available in the upcoming years, the latter is impossible to find, to be honest.

If it’s any consolation, on an inspection over here, the undersized wires would probably be coded as the lesser C2 (requiring urgent attention), rather than C1 (immediate action required - fix or disable circuit).