Hi,

I need some feedback on what size of breaker I need. The conductor size is 10mm2 with 4 copper cores 6 strands each. Application is 220V 3 phase AC with Neutral.

According to the wire specifications, the maximum load for a 4 core 10mm2 cable when enclosed in the wall within a conduit is 46A at 30C (86F) ambient, conductor operating temperature being 70C (158F).

The circuit breaker I selected was a 4 pole 40A breaker, which I thought would be sufficient and would give me a 6A buffer.

But I had not compensated for high ambient temperature. So I re-did the calculations to compensate for high ambient temperature. The wire rating factor is 0.82 at a temperature of 40C (104F). This is the maximum ambient temperature that I am forecasting. So the maximum current the wire can carry is now recalculated as 46 x 0.82 = 37.72A.

So my question is should I go for a 30/35A breaker instead of 40A? Please note that I will never be pulling 46A through the wire. The wire will be under continuous load, the maximum continuous current being 35A. Should I stick to the 40A breaker or should I select a 30/35A one. I am also concerned about the wire temperature. I wouldn't mind decreasing the load a bit to keep the wire a bit cooler.

Would appreciate your opinion on this. Thanks.

 

Then how do you plan to have a 30/35A breaker protect the circuit without tripping ?

You're right. A 30A nor 35A nor 40A breaker would hold.

Our electrical codes are not the same as those in Pakistan.
It sounds like you are stuck using a 40A breaker at this time without increasing wire size.

 

I don't have the option of increasing wire size because the wiring is already done but I do have the option of reducing the load. The wire is going to be connected to a 3 phase solar inverter, so I can divide the load among three phases.

For a continuous maximum load of 35A would I not be ok with a 30A breaker or do I need to go lower?

 

You can't size a breaker under the load. It will trip as soon as it gets hot.

 

From what I gather, if the maximum load the conductor can handle is 37A, then to be on the safe side, the breaker I should use is 30A and my maximum continuous load should be less than 30A.

Would that be ok?

 

That would be much better.

 

Under the NEC motor and compressor wiring uses different sizing rules for the breakers and conductors . You size them from the nameplate data.

 

Ok. Thanks for your help.