A/C wire size.

[wwhitney]

trip curve

Two questions about trip curves:

1) Is the width of the region shown purely attributable to variation or calibration differences between different units? I.e. if we could repeatedly test a single given circuit breaker at the specified cold start ambient 40C temperature, would we give a single curve on the graph within that region?

2) Can anyone point me towards an annotated trip curve where an algebraic formula is given for the shape of the curve in each region? For a thermal magnetic breaker, my first thought is that there are only two regions, the thermal region where trip time t is proportional to 1/I², and a magnetic region, where trip time is constant. I imagine that's a bit too simplistic, though.

Cheers, Wayne

 

[Eddie702]

It also shows just how complicated answering a seemingly simple question can become.

It shows that the code is not written properly if all the smart minds on this forum can't come to a consensus, then how is the average contractor or inspector supposed to come to the correct answer?

The way I look at it (which is probably wrong) is that wires are selected for HVAC equipment in 440 and standard motors in 430 allow for 125% of the load on a standard motor or in the case of HVAC equipment based on 125% of the largest motor (usually the compressor) in multi motor equipment. The equipment ground is sized based on the overcurrent protection which in most cases exceeds the "normal" current rating of the wire so motors and HVAC equipment are the only equipment that I am aware of that in a normal installation the over current protection does not match the conductor size. Although I am sure there are other cases

In any case the equipment ground must be sized to a minimum as in table 250:122 That table has not changed in decades and #10 copper has been allowed for a 60 amp breaker forever. If a 60 amp breaker has been determined to be able to carry any fault that a 60 amp breaker will let through then oversizing the circuit conductors both hot and equipment ground should be allowed as ling as 250:122 is complied with;

How about Tap conductors? In the case if tap conductors the equipment ground may be undersized in relation to the over current protection ahead of the tap and in that case the equipment ground does not have to be sized larger that the ungrounded conductors of the tap.

 

[Elect117]

Reading and Configuring Time-Current Curves « Knowledge Base – Design Master Software

1) Is the width of the region shown purely attributable to variation or calibration differences between different units? I.e. if we could repeatedly test a single given circuit breaker at the specified cold start ambient 40C temperature, would we give a single curve on the graph within that region?

That is how I understand it.

2) Can anyone point me towards an annotated trip curve where an algebraic formula is given for the shape of the curve in each region? For a thermal magnetic breaker, my first thought is that there are only two regions, the thermal region where trip time t is proportional to 1/I², and a magnetic region, where trip time is constant. I imagine that's a bit too simplistic, though.

I can only find where they write it like piecewise functions or define each section of protection with their limit.

https://library.e.abb.com/public/114371fcc8e0456096db42d614bead67/2CDC400002D0201_view.pdf