Selecting the right breaker size

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Old 04-17-17, 10:37 PM
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Selecting the right breaker size

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.
 
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Old 04-17-17, 11:33 PM
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The wire will be under continuous load, the maximum continuous current being 35A.
Then how do you plan to have a 30/35A breaker protect the circuit without tripping ?

Please note that I will never be pulling 46A through the wire.
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.
 
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Old 04-17-17, 11:52 PM
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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?
 
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Old 04-18-17, 12:12 AM
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You can't size a breaker under the load. It will trip as soon as it gets hot.
 
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Old 04-18-17, 12:22 AM
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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?
 
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Old 04-18-17, 01:07 AM
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That would be much better.
 
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Old 04-18-17, 02:03 AM
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Under the NEC motor and compressor wiring uses different sizing rules for the breakers and conductors . You size them from the nameplate data.
 
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Old 04-18-17, 02:13 AM
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Ok. Thanks for your help.
 
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