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Old 125kVA, New 2000A, and NESC for 208V Equipment
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Author:  EEAZ11 [ Thu Sep 20, 2018 9:39 am ]
Post subject:  Old 125kVA, New 2000A, and NESC for 208V Equipment
Hello All.

It appears that there are somewhat conflicting information regarding the NESC Table 410-1 and the IEEE 2002 and 2018 (Draft) for 3-phase 208V equipment.

1584 2002 provided the 125kVA statement implying that three-phase 208V equipment fed from less than 125kVA transformer could be labeled at less than 1.2cal/cm2.

Next came the NESC Table 410-1 which stated three-phase 208V equipment could be labeled with 4.0cal/cm2, with no indication on a transformer or fault current limit.

It appears 1584 2018 has changed the 125kVA value to a 2000A or less fault current.

I am curious what others will be doing for 208V equipment going forward?
Author:  Jim Phillips (brainfiller) [ Thu Sep 20, 2018 3:17 pm ]
Post subject:  Re: Old 125kVA, New 2000A, and NESC for 208V Equipment
Yes it is a bit confusing right now.

The 2002 Edition of IEEE had the 125 kVA "exception" but it was discovered that it might have been a bit optimistic. Under specific conditions - although not common, we had arcs sustain down to 2500A so we took 80% to arrive at 2000A However, this is an empty test box with the exception of the electrodes terminating in a barrier.

The NESC data is based on testing actual equipment and is very specific based on the accompanying notes. I know the people involved with that research very well - work with them on various committees.
Author:  EEAZ11 [ Thu Sep 20, 2018 7:38 pm ]
Post subject:  Re: Old 125kVA, New 2000A, and NESC for 208V Equipment
Jim Phillips (brainfiller) wrote:
Yes it is a bit confusing right now.

The 2002 Edition of IEEE had the 125 kVA "exception" but it was discovered that it might have been a bit optimistic. Under specific conditions - although not common, we had arcs sustain down to 2500A so we took 80% to arrive at 2000A However, this is an empty test box with the exception of the electrodes terminating in a barrier.

The NESC data is based on testing actual equipment and is very specific based on the accompanying notes. I know the people involved with that research very well - work with them on various committees.


Hi Jim.

I guess I am a bit confused. When is the NESC applicable since it doesn’t give too much detail? Any 208V panelboard regardless of Current is 4cal or less? When would the IEEE come into play?

It seems to me that they are conflicting. 2000A is very low compared to the blanket 4cal or less from NESC.
Author:  stevenal [ Fri Sep 21, 2018 8:55 am ]
Post subject:  Re: Old 125kVA, New 2000A, and NESC for 208V Equipment
1) The scope of the NESC is fairly narrow. I would make sure you are in scope before adopting NESC tables.
2) OSHA has expressed doubts regarding some of the Table's footnotes.
Author:  Flash [ Mon Sep 24, 2018 9:04 am ]
Post subject:  Re: Old 125kVA, New 2000A, and NESC for 208V Equipment
Attachment:
OSHA Letter Titus Diamond_20150716184438 - OSHA LoR on 410-1.pdf [121.91 KiB]
Downloaded 408 times


stevenal wrote:
1) The scope of the NESC is fairly narrow. I would make sure you are in scope before adopting NESC tables.
2) OSHA has expressed doubts regarding some of the Table's footnotes.


OSHA has also endorsed some of the NESC work - see attached
Author:  David Castor [ Mon Nov 26, 2018 10:41 am ]
Post subject:  Re: Old 125kVA, New 2000A, and NESC for 208V Equipment
No question the standards are conflicting at present. We just have to deal with it. If the Categories Method in NFPA 70E-2018 is used, Category 1 (4 cal/cm2) is the required PPE for working on panels < 250 V (within specified fault current and clearing time maximums).

So 4 cal/cm2 seems like a reasonable level for <250 V, basically consistent with NESC. It will be interesting to see how calculations using IEEE 1584-2018 equations at 208V end up - especially when the additional variables of electrode configuration and enclosure size are factored in.
Author:  PaulEngr [ Tue Nov 27, 2018 10:09 pm ]
Post subject:  Re: Old 125kVA, New 2000A, and NESC for 208V Equipment
The sources of information in NESC are described only in vague terms but it is my understanding that the data comes from the EPRI testing. The testing went up to 20 kA available fault current and measured incident energy as high as 3.2 cal/cm2...hence rounding up to 4 cal/cm2.

As far as reconciling the two, first let me ask a simple question: is it better to use a theoretical calculation or an empirical equation? And based on that decision where does actual equipment testing fit into the framework? And I'll just say this...engineers tend to be biased strongly towards calculations over table-based results irrespective of the underlying basis behind it, even when it is inappropriate to do so.

There are really 3 cases to consider:
1. You don't have enough information to use IEEE 1584 in the first place. In this case the table based approach (NESC or NFPA 70E) is really your only practical option. This is generally the case when doing an arc flash study for the first time.
2. The conditions are known and don't match those specified in the tables (70E or NESC). Note that NESC is kind of low on detail so this is a little more work to determine. Then the fall back obviously should be IEEE 1584.
3. Conditions are known to the point where IEEE 1584 empirical calculation could be used, AND the conditions match known values in the NESC or NFPA 70E table based approach. In this case the NFPA 70E table shouldn't be used...since it will be essentially the IEEE 1584 calculation at discrete points, using the calculated method gives lower (and tighter) values. However NESC table based approach will often produce lower estimates because it is based on actual testing on equipment. This would be appropriate if your conditions match. If not, IEEE 1584 produces more accurate results. Note that NESC is pretty vague on details about the sources so it takes some sleuthing to find it.
Author:  Flash [ Wed Jan 02, 2019 2:11 pm ]
Post subject:  Re: Old 125kVA, New 2000A, and NESC for 208V Equipment
More food for thought - take a look at the tables, in some cases they can be applied at 480 volts. As an example, the LV (480V) side of a pad mount transformer is a max of 4 cal/sq cm. To further substantiate that number, OSHA has issued a letter confirming the NESC value of 4 cal/sq cm can be used on pad mount transformers.
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