Breaker Design

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siva

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Hi
Here we have some circuits having 30A breaker with 12 awg(4mm2) wires.Load current is only 16A to 17A.My doubt is that since max current carrying capacity of 4 mm2 wire is 20 A,how 30A breaker will protect the wire.When I asked about this to our designer,he answered that breaker is designed for the load.Is this OK....???

Regards / Siva
 
Re: Breaker Design

the mention of metric size wire makes me wonder if the OP is overseas.

wire sizes are chosen based on two factors - one is how much current flows through it, the second is how hot it gets because of that current flow. The wire's insulation has to be able to stand up to the heat generated.

the NEC had compiled 100+ years of experience and calculations into a series of tables for determining appropriate wire size and insulation type that work for virtually all cases for premises wiring.
 
Re: Breaker Design

Hi
I am working with US Navy in Middle East.Inside base we are following NEC and outside they follows BS standards.Due to the non-availabilty of NEC standard wires we allows the contractor to use BS Standard wire.I got this 28A(sorry..its not 20A) for 4 mm2 wire from BS table.The wire size is OK with me.But in some case if the circuit draws more than 28A,how the 30A breaker protect the wire?Is there any rule in NEC like ...current rating of circuit breaker shall not exceed the current rating of the conductor to be protected...???

Regards / Siva
 
Re: Breaker Design

If the current rating of the wire does not fall on a standard size of breaker or fuse the next larger standard size can be used.
 
Re: Breaker Design

Originally posted by laidman:
If the current rating of the wire does not fall on a standard size of breaker or fuse the next larger standard size can be used.
Click to expand...
That would depend on the type of circuit served. 240.4(B)(1)

The NEC answer to this question is it is a violation unless it falls under one of the items listed in 240.4(G)

Bob
 
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