250.4 (A) 5

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mbrooke

mbrooke

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What is the established maximum impedance for each OCPD rating and its associated circuit while still being complaint with 250.4 (A) 5?
 
While that is a performance requirement, in general the NEC is a prescriptive code and not a performance code.

It is assumed that systems installed in compliance with the prescriptive rules in the code meet the performance requirements of 250.4.
 
Mike Holt has a graphic that shows the fault current must be 6 to 10 times the OCPD rating to allow the OCPD to trip in the instantaneous region. There is more information on your question in the IEEE green book. But as Don states we follow the prescriptive rules (how to) that will then comply with 250.4 A 5.
 
don_resqcapt19 said:
While that is a performance requirement, in general the NEC is a prescriptive code and not a performance code.

It is assumed that systems installed in compliance with the prescriptive rules in the code meet the performance requirements of 250.4.
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How is basic shock and fire protection considered performance?
 
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tom baker said:
Mike Holt has a graphic that shows the fault current must be 6 to 10 times the OCPD rating to allow the OCPD to trip in the instantaneous region. There is more information on your question in the IEEE green book. But as Don states we follow the prescriptive rules (how to) that will then comply with 250.4 A 5.
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Thanks. 6-10 times sounds more reasonable.

What rule besides 250.4 A 5 would be violated if the code was followed to the letter but could not trip a breaker or do so safely?
 
mbrooke said:
How is basic shock and fire protection considered performance?
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Any code rule that does not specify exactly how to comply is a performance rule. It tells you what needs to be accomplished buy does not tell you how to accomplish it. A prescriptive rule tells you exactly what to do. In general all of the rules in the NEC are prescriptive rules.
 
don_resqcapt19 said:
Any code rule that does not specify exactly how to comply is a performance rule. It tells you what needs to be accomplished buy does not tell you how to accomplish it. A prescriptive rule tells you exactly what to do. In general all of the rules in the NEC are prescriptive rules.
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Or discretionary?

I get that a breaker needs to open, but the code doesn't say in what amount time. Beyond a certain time a hazard begins to exist despite the legality of said hazard.
 
From the green book:

The basic objectives of an equipment grounding system are the following:

1) To reduce electric shock hazard to personnel.

2) To provide adequate current-carrying capability, both in magnitude and duration,
to accept the ground-fault current permitted by the overcurrent protection system
without creating a fire or explosive hazard to building or contents.

3) To provide a low-impedance return path for ground-fault current necessary for the
timely operation of the overcurrent protection system.
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While I fully agree with this statement in full, it is still vague. For example, "timely operation" is not defined. The IEEE or CMPs could consider 3 cycles as timely, while if left to discretion I could consider 60 seconds as timely operation. No guidance, equations, definitions, conditions, or tables are given as to what the authors' had in mind.


The impedance of the grounding conductor must be low enough to accept the available
line-to-ground-fault current without creating a hazardous impedance (IZ) voltage drop.
The available ground-fault current of the supply system will have a direct bearing on the
EGC requirements.
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This statement is not practical as I see it in the real world. And of course the conditions, time and voltages which are hazardous in the authors eyes are not explained or defined.

For example- assuming the EGC is the same size as the phase conductor the voltage will divide by half. So a 277 volt circuit would give 138 volt to remote earth during a ground fault.

138 volts in of itself is hazardous when allowed to persist beyond a certain time, but that time is not stated. Of course one could oversize the EGC to the point no more than 30 volts is present to remote earth during a fault- but I don't see that in the real world.


I want to think the CMP and IEEE have something very specific in mind?
 
tom baker said:
You will find answers to your questions in the IEEE Green Book
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What is interesting is that the equation in 2.7.4.2 brings about trip times as to prevent smaller sized EGC from over heating or fusing, even latter going as far as indirectly saying table 250.122 can be inadequate in some installations.

My only gripe is that the equation is incomplete in that the resistance of the conductor goes up as the fault is happening which lowers finishing current. A conservative approach is to use 150*C R in calculating I2, but nothing is listed for 150*C.


However we are still left with 15, 20 and 30 amp circuits where thermal damage can not occur from slow tripping or minimum fault currents.
 
mbrooke said:
Or discretionary?

I get that a breaker needs to open, but the code doesn't say in what amount time. Beyond a certain time a hazard begins to exist despite the legality of said hazard.
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You have plenty of time to work out PIs and technical substantiations for changes that will appear in the 2026 code. The will be due in early September of 2023.

Looking forward to seeing your Public Inputs on these issues.
 
The IEEE green book is copyrighted. I no longer have a copy, but in reading there was information on how the size of the EGC was determined.
 
tom baker said:
The IEEE green book is copyrighted. I no longer have a copy, but in reading there was information on how the size of the EGC was determined.
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Do you remember the details?

The book I found online hints that 250.122 may not be safe in all reasonably common systems/applications.
 
don_resqcapt19 said:
You have plenty of time to work out PIs and technical substantiations for changes that will appear in the 2026 code. The will be due in early September of 2023.

Looking forward to seeing your Public Inputs on these issues.
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Right- but I'm sure the code has a number in mind. I don't know they are so hush-hush about it. I mean I can guess, but conjecture is just that, conjecture.
 
mbrooke said:
Right- but I'm sure the code has a number in mind. I don't know they are so hush-hush about it. I mean I can guess, but conjecture is just that, conjecture.
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The are not hush hush about things....However you have to know when the rule was first put into the code to research the proposal, substantiation, and panel statements.
 
don_resqcapt19 said:
The are not hush hush about things....However you have to know when the rule was first put into the code to research the proposal, substantiation, and panel statements.
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Good point, I haven't seen it all.

Hopefully an impedance was mentioned.

In so far I've found nothing, but I'd like someone to beat me to it.
 
mbrooke said:
Good point, I haven't seen it all.

Hopefully an impedance was mentioned.

In so far I've found nothing, but I'd like someone to beat me to it.
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Digging in to the old ones takes a lot of time, and not all of the original documents have been scanned and put online. Like I side the very first thing you need to do is find the first edition of the NEC that the rule appeared in. That in itself takes time, and then you have to find the revision documents associated with that rule.
 
don_resqcapt19 said:
Digging in to the old ones takes a lot of time, and not all of the original documents have been scanned and put online. Like I side the very first thing you need to do is find the first edition of the NEC that the rule appeared in. That in itself takes time, and then you have to find the revision documents associated with that rule.
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Agree.


I have a feeling it was more along the lines that everyone realized that ground rods do not open breakers. Rather than an actual ohm value per handle rating.
 
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