2500 amp service

Status
Not open for further replies.

cripple

Senior Member
2500 amp service

If the service conductors or in a duct bank none the Tables in 310 will apply.
Table 310.16 Allowable Ampacities of Insulated Conductors Rated 0 Through 2000 Volts, 60?C Through 90?C (140?F Through 194?F), Not More Than Three Current-Carrying Conductors in Raceway, Cable, or Earth (Directly Buried), Based on Ambient Temperature of 30?C (86?F)
Table 310.17 Allowable Ampacities of Single-Insulated Conductors Rated 0 Through 2000 Volts in Free Air, Based on Ambient Air Temperature of 30?C (86?F)
Table 310.17 Allowable Ampacities of Single-Insulated Conductors Rated 0 Through 2000 Volts in Free Air, Based on Ambient Air Temperature of 30?C (86?F)
Table 310.18 Allowable Ampacities of Insulated Conductors Rated 0 Through 2000 Volts, 150?C Through 250?C (302?F Through 482?F). Not More Than Three Current-Carrying Conductors in Raceway or Cable, Based on Ambient Air Temperature of 40?C (104?F)
Table 310.19 Allowable Ampacities of Single-Insulated Conductors, Rated 0 Through 2000 Volts, 150?C Through 250?C (302?F Through 482?F), in Free Air, Based on Ambient Air Temperature of 40?C (104?F)
Table 310.20 Ampacities of Not More Than Three Single Insulated Conductors, Rated 0 Through 2000 Volts, Supported on a Messenger, Based on Ambient Air Temperature of 40?C (104?F)
Section 310.15(C) would have to be applied.
Which be better understand by reading the follow I find on the web.
Where Did Table 310-16 Come From?
History
Since 1889, many individuals and organizations have attempted to find the correct ampacity for conductors so they would not overheat and ruin the insulations. In 1889 Kennelly published one of the first tables listing 46 amperes as the ampacity of a number 10 conductor. In 1890 Fisher listed 19.1 amperes, and in 1894 the insurance industry listed 20 amperes as the ampacity for the same conductor. But that was not the end of it. By 1937 there were 16 ampacities discovered for the same size conductor. In 1938 Samuel J. Rosch, an associate member of the American Institute of Electrical Engineers and the manager of insulated products development for the Anaconda Wire and Cable Company, conducted a thorough investigation to find the correct ampacities for all the standard size conductors used at that time. To establish the maximum prolonged operating temperature for insulations, he performed aging and elongation tests in environmental ovens. He built a structure, wired it, embedded thermocouples in the conductors, and applied voltages and measured the ampacities and temperatures. He published his findings in a paper titled, "The Current-Carrying Capacity of Rubber-Insulated Conductors" delineating the results of his experiments. His work resulted in a table XI that became Table 310-16 of the National Electrical Code. Rosch's original table was based on an ambient temperature of 30 degrees centigrade and a conductor temperature of 50 degrees centigrade for code grade rubber, the type of insulation used in those days. If we convert the ampacities in table XI to 60 degrees centigrade using the formula given in note 1 to tables 310-69 through 310-84, setting delta TD equal to 0 (delta TD is for high voltages: we are only concerned with 600 volts and under), and rounding off to the nearest 5 amperes, we can calculate the ampacities for 60 degree insulations as found in the first column of table 310-16. Likewise, the same calculation can determine the the ampacities for the 75 degree and 90 degree columns in table 310- 16.
Faults with Table 310-16
There are three very important deficiencies in Rosch's paper. First, he did not investigate the effects of proximal heating from adjacent conduits, ducts, and duct banks. Secondly, his experiments were only for above ground installations. Thirdly, the heat produced by high voltages was not investigated. But for most applications when load calculations are performed according to Article 220, there is enough safety margin built in to preclude any problems. To explain this, a fine print note was added to section 310-15(a) in the 1990 NEC stating that Tables 310-16 through 310-19 are application tables that are for use in determining conductor sizes on loads calculated in accordance with Article 220. When calculating loads per article 220 a substantial safety margin is included as opposed to some engineering calculations that calculate the "actual" load.
The deficiencies to Table 310- 16 became a problem in the 1950's when Americans began installing very large air conditioning systems in the larger buildings, using underground service laterals run in massive underground duct banks. In cases where engineers performed load calculations using engineering methods in place of Article 220, and used Table 310-16 to determine the size of conductors, conductors overheated and burned open, especially the conductors located near the center of the duct banks. Rosch used a basic heat transfer equation with the addition of a term "n" for the number of conductors in the same cable or raceway. But there were no terms in his equation to adjust the ampacity for heat that came from adjacent ducts and duct banks, or for the differences for heat dissipation in an underground installation. Later calculations using the Neher-McGrath equation found in 310-15(b) of the NEC would determine that the center conductors in a 3 by 3 duct banks must be derated to almost 60 percent because of the proximal heating effect from adjacent ducts and duct banks.
To develope a more accurate method of finding the ampacity of conductors in underground installations two cable engineers, in 1957, developed the Neher-McGrath equation found in 310-15(c) of the 1999 NEC.
 

inspector141

Senior Member
Location
Westminster, MD
I think some AHJ's would allow as an exception, 8 sets of 350's (2480 amps) for a single 2500 amp service as long as the calculated load does not exceed 2480 amps. It's so close and there are worse scenarios with sizing conductors that fall under the 800 amps or less(240.4(B) rule or the 2-6 rule, than this one.
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
inspector141 said:
I think some AHJ's would allow as an exception, 8 sets of 350's (2480 amps) for a single 2500 amp service as long as the calculated load does not exceed 2480 amps. It's so close and there are worse scenarios with sizing conductors that fall under the 800 amps or less(240.4(B) rule or the 2-6 rule, than this one.

For 20 amps I don't see a real world problem, but what part of the NEC would allow the inspector to permit the installation of a violation?
 
dbuckley said:
Unlerss there is some reason for the neutral to be counted as a CCC, then it wont be.

Good reasons include mainly non-linear loads. If that service is in a mall, then you'll be fine, no neutral CCC. If the service is in a data centre, then neutral is definitely a CCC.

Judgement call required as to percentage of non-linear loads...

"(4) Neutral Conductor.
(a) A neutral conductor that carries only the unbalanced
current from other conductors of the same circuit
shall not be required to be counted when applying the provisions
of 310.15(B)(2)(a).
(b) In a 3-wire circuit consisting of two phase conductors
and the neutral conductor of a 4-wire, 3-phase, wyeconnected
system, a common conductor carries approximately
the same current as the line-to-neutral load currents
of the other conductors and shall be counted when applying
the provisions of 310.15(B)(2)(a).
(c) On a 4-wire, 3-phase wye circuit where the major
portion of the load consists of nonlinear loads, harmonic currents
are present in the neutral conductor; the neutral conductor
shall therefore be considered a current-carrying conductor."

Unfortunately the Code gets sloppy again as 'major portion' is not an exact data, nor are all nonlinear loads have the same heat producing quantities.

The temperature column you use depends on the conductor terminating device's temperature rating on each end. It could be 90C*.

Lastly you need to adjust for ambient temperature.​
 

inspector141

Senior Member
Location
Westminster, MD
infinity said:
For 20 amps I don't see a real world problem, but what part of the NEC would allow the inspector to permit the installation of a violation?

As a plans examiner and a code instructor the answer to an installation that consists of 8 sets of 350kcmil for a single service is that the overcurrent device cannott exceed 2480 amps according to the code.
However, there are times when exceptions are warranted. Often times NEC article 90.4 will allow an AHJ to grant an exception. Sometimes, local amendments allow exceptions. One of the problems with government is that they are so far removed from the real world that they know longer recognize it. It's us versus them. If you can't or won't grant warranted exceptions, then some counties and jurisdictions might opt out of the NEC or create amendments that will encourage granting unwarranted exceptions. Be careful what you wish for.

-Would I allow 8 sets of 350kcmil for a single 2500 amp service? Yes!
-Would I answer this as yes on a masters test? No!
-would I be responsible if the service burns up? No, because that would never happen.
 

mthead

Senior Member
Location
Long Beach,NY
2500 amp service

141--While your "opinion" that this 2500a serv fed as presented would never fail may be valid as far as what we might all know to Likely be true,-remember-if it does go bad you have nothing to back you up except your instinctual feeling/your general annecdotal knowledge.
In a court of law annecdotal evidence does not become valid untill numerous, documented similar annecdotes are presented.
In other words -no body wants to take responsibility for any decision until the information that prompted that decision can be shown to be as old as the "10 Commandments".
Wether or not you know in your gut that something may be ok and not a problem in real world applications if it doesn't meet code -you cannot take it upon yourself in your infinite wisdom[and I can be guilty of being infinitely wise myself]to let it go-not unless you are "ABSOLUTELY" the AHJ.And if you go against a published code and something happens you might not remain that AHJ for long]
By this same reasoning we as inspectors cannot choose to interpret code in a manner that would be in what we thought was the intent/we know best mode.
Code is a double edged sword that is rarely perfectly clear-at least not when you get to the good stuff if you know what I mean.
Here's a prime example.
Code say's the AHJ has ultimate authority.
In NY state the AHJ theoretically is some wizard up in albany but he writes in the state code that enforcement lies with the various localities [cities,towns,counties,villages]to implement as they feel is warranted.
The ball is allways in somebody elses court.
As an inspector anywhere you should be sure that you can not only live with what you see and report but be able to back it up completely because when push comes to shove we're really on our own very often.
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
I have to agree with mthead. Over the years I have seen some amazing things happen in court. More than once I have been questioned as to the presence of a NEC violation on the job that had nothing (in my mind) to do with the reasons we were in court. The point being, if there are Code violations in one area there sure might be in others (sounds good in court).
Every time I'm tempted to use logic over Code, I recall some of the cases.
You better have a heck of a case of credentials and firm footing as an AHJ if you are going to take those chances.. IMHO.
 

terille

Member
2500 Amp Service

2500 Amp Service

You should check and see if you have variable speedcontrols or soft starts. These are non-linear loads and if more than 50% you will need to count your neutral
 

inspector141

Senior Member
Location
Westminster, MD
mthead said:
141--While your "opinion" that this 2500a serv fed as presented would never fail may be valid as far as what we might all know to Likely be true,-remember-if it does go bad you have nothing to back you up except your instinctual feeling/your general annecdotal knowledge.
In a court of law annecdotal evidence does not become valid untill numerous, documented similar annecdotes are presented.
In other words -no body wants to take responsibility for any decision until the information that prompted that decision can be shown to be as old as the "10 Commandments".
Wether or not you know in your gut that something may be ok and not a problem in real world applications if it doesn't meet code -you cannot take it upon yourself in your infinite wisdom[and I can be guilty of being infinitely wise myself]to let it go-not unless you are "ABSOLUTELY" the AHJ.And if you go against a published code and something happens you might not remain that AHJ for long]
By this same reasoning we as inspectors cannot choose to interpret code in a manner that would be in what we thought was the intent/we know best mode.
Code is a double edged sword that is rarely perfectly clear-at least not when you get to the good stuff if you know what I mean.
Here's a prime example.
Code say's the AHJ has ultimate authority.
In NY state the AHJ theoretically is some wizard up in albany but he writes in the state code that enforcement lies with the various localities [cities,towns,counties,villages]to implement as they feel is warranted.
The ball is allways in somebody elses court.
As an inspector anywhere you should be sure that you can not only live with what you see and report but be able to back it up completely because when push comes to shove we're really on our own very often.


My response was not based on my "gut feeling" or "instinct", but on rational, thought out logic. I think there is some"you don't know what the code says" responses here. I stated very clearly what the code is on sizing the conductors. Consider this example:Two sets of 500 kcmil at 75 degrees has an ampacity of 720 amps. These conductors under most circumstances can be protected by a 800 amp OD. As we know, the load on the conductors cannot exceed 720 amps, but the OD exceeds the ampacity of the conductors by 80 amps, which is allowed under the code. If the code can allow conductors to be sized 80 amps below the OD device(240.4(b), then the AHJ, if they so choose, can easily justify allowing a 2500 amp OD on 2480 amp conductors. A mere 20 amp differential. Of course, the AHJ can interpret intent if they have solid proof to back it up. Does the example I gave above give solid proof? Sure, we must be very careful when allowing AHJ exceptions. They must be rare and when granted, must be fullproof. My point, once again, is not to be over the top here, but simply point out that this COULD be warranted and WOULD NOT end up in court. And if it did end up in court (which we all know it would not) then it could be justified very easily. Unfortunately, the courts have most of us working scared, but that does not mean we can't grant exceptions when they are completely justified. If you choose to work scared, that's your choice, but when I'm ABSOLUTELY sure, I will grant an exception.
 

benaround

Senior Member
Location
Arizona
Marty,

Were you absolutely sure that all the bus bars were spec tight. How about

terminating all those service conductors, sometimes the lugs can be very

difficult to get an allen wrench in, hope that guy felt strong that day and had

some altered tools to get tight connections. I can go on, but I'm not going to

until I'm absolutely sure I should.

The point is that going out on the limb for someone 'could' backfire no matter

how foolproof it looks, on the other hand, it's nice to know that some guys

are willing to do so. :)
 

inspector141

Senior Member
Location
Westminster, MD
benaround said:
Marty,

Were you absolutely sure that all the bus bars were spec tight. How about

terminating all those service conductors, sometimes the lugs can be very

difficult to get an allen wrench in, hope that guy felt strong that day and had

some altered tools to get tight connections. I can go on, but I'm not going to

until I'm absolutely sure I should.

The point is that going out on the limb for someone 'could' backfire no matter

how foolproof it looks, on the other hand, it's nice to know that some guys

are willing to do so. :)

What is it with some of you guys! This has NOTHING to do with OTHER violations. Are you saying that if you give an exception to one piece of the electrical installation that the inspector therefore will let the rest of the entire inspection process fall into the toilet. Of course not! Why are some (just a few) so bent on proving someone wrong that they go outside of the scope of the discussion to prove a point that is really not relevant to the discussion. The conductors will have to be terminated and torqued properly regardless of how many sets you use. If you are suggesting that somehow, someway, if the incorrect termination will, somehow, someway lead to the exception being found out and somehow, someway the exception will be blamed for incorrect torquing, well then my friend, some inspectors should never, ever grant an exception. Never! It's just too risky.
 
Status
Not open for further replies.
Top