Conductor Ampacity

British Standard 7671.

Could you be more specific, threshold of what? The conductor getting so hot it destroys the insulation in a cable or the conductor blows like a fuse?

Stick to the recommended ratings given, it's safer!

Thanks for your response JRaef.

You seem to be very familiar with the code, so I hope you can answer another question, for non-motor loads, is it ok to base the size of wire on the breaker size, say, I have a 100A breaker, I will size the wire for 101A (for example)? What for motor loads?. Motors loads would have higher breaker capacity for the spike currents but is it ok for the wires to be rated less than the circuit breaker?

Basically all equipments have a thermal withstand characteristics. This is usually defined by an equation of the format

[I²] x[ t] = K

Normally this is an inverse characteristic. with infinite time at currents less than continous rating , which reduces inversly when current exceeds rating.

This thermal inverse characteristic should be well above the protection characteristics. 1 Ampere margin may not be sufficient. I would keep about 20 to 25% margin.

The decision of this margin may vary from engineer to engineer depending on their personal experience.

This is much enlightening. Thank you Radhunath7!

You need to be more specific about your application and intent.

Chap. 3, Table 310.16 of the NEC contains the informaton you need if your application is for 600V or less.

There is a process you must apply to select the current (amps) that is needed and from that process, you can source the conductor from the table.

You never want to know when a conductor is going to give out. You always select overcurrent protection devices to elimate, limit or remove that possibility.

If unsure how to digest this information and the advise given by others, you really need to consult a qualified electrician

Cable withstand current depends on tripping time when overloaded(Amp) or short circuited(kA),ambient temperature,derating factors,maximum operating temperature of conductor and insulation etc. Consult the cable manufacturer.

The current carrying capacity of a conductor depends on

1. Allowable temperature rise [based on thermal withstand capability, ambient temperature etc]

2. Permisible voltage drop in case of long conductors

3. Stability of interconnected systems, in case of very long lines [EHV transmission lines for example]

Whichever the above 3 is the least, will determine the current carrying capacity. Most likely this value will be less than the figures provided in the tables for a safe design.

With a view of copper saving, loading to maximum possible CCC may be a welcome move.

Hi PNABAN. I think we shouldn't risk it.

I was leaning more on devicing a set-up which can demontrate, SAFELY, conductor failure. A future project maybe.

You need to define "threshold values" and "tolerance" better. Generally speaking, 1 amp for 5 seconds for every 42.25 circular mils of copper will not cause permanent damage from overcurrent, annealing etc. You should check your termination torques, as you would after any major fault. The NEC is a rule book for the safe use of electricity. It is not intended for use as a design guide and does not contain basis information for the rules that it provides.

Thanks for your response Goodho!

What I meant by threshold and tolerance was, how high a continuous current can you apply to a conductor without any adverse effect and for which a higher value would degrade or cause the conductor to fail. But your response would answer this.

Just wondering, where did you get such value (1A / 42.25CM)?

If you are a member of NFPA, The NFPA offers online webinars that speak directly to applications for your queries. I recommend that you take these online webinars. Knowing where to find ampacities for cables in the NEC is good knowledge. However there are other things to take into account such as in-rush current, voltage drop, distance of cable, types of cable, number of conductors in the raceway, type of raceway, temperature factors, and other design factors that are not really covered from an intergrated design system standpoint in the NEC 2011. These seminars allow you to "put the entire picture together", as a member of NFPA you can take these online webinars at a discount and recieve daily updates of changes in NEC code. Membership is not expensive. The webinars are inexpensive and keep you up to date with industry developements. Mike Holt also offers good courses in these areas and they come in CD sets.