Underground cable derating

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faresos

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I have a situation where we have a 3000A overcurrent protection (main service) and the calculated load at 2100A. I'm running the conduit in a duct bank, do I need to derate the cables based on the calculated load or the overcurrent device? In another word, when I do the calculations, should the total ampacity of the cables in the ductbank be at minimum of 2100A or 3000A?

Thanks for thoughts in advance
 
My opinion.

You size the cable per the OC device that's protecting the cable at it's supply end (240.21), unless it's a tap and complies with 240.21(B).

steve
 
Derate means you are going to install a larger cable to provide the same ampacity. You are hoping to under size your cables. Your overcurrent protection must protect your wire and equipment, so you would need a minumum wire ampacity of 3000A after any derating. Sounds like this one might be over you head if you are doing this install yourself. I would recommend haven't someone design the service for you.
 
See 230.90(A)- "Such protection shall be required by an overcurrent device in series with each ungrounded service conductor that has a rating or setting not higher than the allowable ampacity of the conductor..."

Now look at exception number 2. This exception allows the service conductors to be smaller than the ampacity of two to six service disconnects provided that the service conductors are large enough for the total calculated load. In short, when there is only one service disconnect, the service conductors must be fully sized based on the service overcurrent device.

Marty
 
I don?t understand your use of the word ?derate.? What makes you think that the ampacity of the conductors might have been one value, but now must be given a lower rating because of something about the installation? If you put more than three current-carrying conductors in a conduit, then you reduce (derate) the ampacity of the conductors. If the ambient temperature is over 86F, then you reduce (derate) the ampacity of the conductors. Does one of these situations apply?

As to making selections of conductors and overcurrent settings, the process goes as follows:

? Step 1, calculate the load. You have done that, with the result of 2100 amps.
? Step 2, select a conductor that has an ampacity, under the installed conditions, of at least 2100 amps.
? Step 3, provide an overcurrent protection device that will protect the selected conductors.

So if you select conductors with an ampacity of only 2100 (e.g., 5 sets of 600 MCM THHN), then you must set the OCPD to no higher than 2100 amps. If you wish to use an OCPD setting of 3000 amps, then you must select conductors that have an ampacity of at least 3000 (e.g., 8 sets of 500 MCM THHN).
 
Since this installation is underground, table 310-16 won't apply. The IEEE "brown book" section 13.4 discusses this. Appendix B in the NEC can provide you with some helpful information. Earth is a good insulator and will retain the heat generated by the current through the cable. The ampacity of the cable in an underground installation MUST be reduced
 
bsh said:
Since this installation is underground, table 310-16 won't apply. The IEEE "brown book" section 13.4 discusses this. Appendix B in the NEC can provide you with some helpful information. Earth is a good insulator and will retain the heat generated by the current through the cable. The ampacity of the cable in an underground installation MUST be reduced
Click to expand...

??? Are you sure ???
 
The description for table 310-16 says "not more than 3 current carrying conductors" so it won't apply to your installation.
 
bsh said:
The description for table 310-16 says "not more than 3 current carrying conductors" so it won't apply to your installation.
Click to expand...

That table shows the ampacity based on up to 3 conductors, at ambient temperatures. This is the baseline condition.

You must use this table. If there are more then 3 current carrying conductors then you have to derate based on Table 310.15(B)(2)(a). If your temperatures are other then ambient the you must derate based on correction factors at the bottom of table 310.16.

You can't disregard the NEC. I don't know about the brown book, but I don't believe it has any regulation of this service. Sounds like it is a design reference, but at the end of the day you must meet NEC requirements. Somebody correct me if wrong.
 
Years ago (1986) table 310-16 included "in free air". At that time Appendix B (now called Annex B) gave guidlines on the ampacity of underground installations of cable. This annex is not an NEC requirement and is included only as information.
Logically, if the ampacity of a cable in conduit is "X" then when that same cable is put in the ground the current carrying capacity is reduced because the earth acts as a barrier to heat dissipation. Annex "B" refers to Neher and McGrath (as does IEEE "Brown Book")and discusses underground cable ampacity.
Table 310-16 is an instance where the NEC is not perfectly clear and could be applied for underground installations
I believe that most systems may not load the service continuously and therefore few problems may be encountered for underground installations. If the actual load is not known then the worst case must be used (3000 amps in this case) and the feeder must be sized to carry that load continuosly.
 
This is not an easy situation to pin down, and the NEC wording is no help. Let me offer a few comments.

First of all, Table 310.15(B)(2)(a) does not apply to a situation in which you have three current-carrying conductors in one conduit, and three more in another conduit, and three more in another conduit, with these three conduits being in close proximity in an underground run, and with the three conduits sharing a concrete encasement. That table only applies when you have more than three current-carrying conductors in the same conduit. Not the same ductbank, the same conduit.

Next, the top of Table 310.16 does say that it applies to ?not more than three current carrying conductors in raceway. . . .? So if you have three conductors in one raceway, and three more in a nearby raceway, are you no longer matching the conditions of the table? That has been a matter of some debate. Looking at the physics of the situation, the presence of a nearby conduit full of conductors will impede the ability of another conduit full of conductors to reject its heat to the surrounding environment. This is particularly true if the surrounding environment is a concrete encasement that is itself surrounded by a bunch of dirt. All true, but none of it is relevant.

Several years ago I posed this question to the Chief Inspector of the State of Washington. The timing of my query was fortunate, as the state inspectors were having a group meeting the following week. The reply I received was that they were going to allow the use of Table 310.16 as the basis for assigning ampacity values for underground installations. This was restricted to the case in which the value of ?required ampacity? was determined through an Article 220 load calculation. The reasoning was that the load calculation process has a great deal of inherent conservatism. The real value of load that you can expect to see will be far lower than the calculated value. This will tend to offset the adverse impact of having conduits in close proximity on their respective ability to reject heat.
 
Thanks Charlie. My concern is the design of a service for an industrial applications. During the initial design the loads are usualy fairly well defined and the service is sized accordingly. My experience has been that MOST of the time additional loads are added to the point that the service is at capacity. For commercial, retail, etc. applications the use of 310-16 is probably fine.
 
bsh said:
Since this installation is underground, table 310-16 won't apply. The IEEE "brown book" section 13.4 discusses this. Appendix B in the NEC can provide you with some helpful information. Earth is a good insulator and will retain the heat generated by the current through the cable. The ampacity of the cable in an underground installation MUST be reduced
Click to expand...


It doesn't sound like derating applies here. An ambient temp of 86F would not require any correction so I do not see a requirement to derate 3 or less conductors in separate underground conduits that maintain adequate spacing. Annex B is for specific installations electrical engineers can use.

It goes right back to what most of us have been saying- 3000 amp service disconnect needs 3000 amps of wire or 8 sets of 500 kcmil. Now if there were two 1600 amp service disconnects, then the conductors can be sized based on the calculated load of 2100 amps or 6 sets of 500 kcmil. (NEC 230.90(A) ex 2). Seems a little crazy, but that's what the code requires/allows.
 
bsh said:
The description for table 310-16 says "not more than 3 current carrying conductors" so it won't apply to your installation.
Click to expand...

If you look at the definition of Raceway you will find conduit listed. If the neutral only carries the unbalanced load, there will only be 3 conductors in the conduit.

The description for table 310-16 says "not more than 3 current carrying conductors" so it won't apply to your installation.
Click to expand...
See above

Logically, if the ampacity of a cable in conduit is "X" then when that same cable is put in the ground the current carrying capacity is reduced because the earth acts as a barrier to heat dissipation. Annex "B" refers to Neher and McGrath (as does IEEE "Brown Book")and discusses underground cable ampacity.
Click to expand...

Unfortunately or fortunately the installation is required to meet the NEC.
This becomes a design issue.

Table 310-16 is an instance where the NEC is not perfectly clear and could be applied for underground installations
I believe that most systems may not load the service continuously and therefore few problems may be encountered for underground installations. If the actual load is not known then the worst case must be used (3000 amps in this case) and the feeder must be sized to carry that load continuosly.
Click to expand...

Table 310.16 does not rule out the use of cable in underground conduits and as you said Article 220 adds enough leeway for loading the cable. I do not know anywhere the the code requires that the feeder must be designed for
continuous loads as you stated.
 
Arguably the temperature of the material surrounding the raceway is the 'ambient temperature' for purposes of the adjustment factors in 310.16. IMHO this is not the temperature that would be present if there were no conductors adding heat to the situation, but instead the actual temperature of the material as used.

IMHO if you have an underground ductbank, you might be forced to do a certain amount of engineering in order to determine the 'ambient temperature' in order to apply 310.16...in which case you might as well go whole hog and use the 'engineering supervision' to determine the conductor ampacity.

-Jon
 
bsh said:
Thanks Charlie. My concern is the design of a service for an industrial applications. During the initial design the loads are usualy fairly well defined and the service is sized accordingly. My experience has been that MOST of the time additional loads are added to the point that the service is at capacity. For commercial, retail, etc. applications the use of 310-16 is probably fine.
Click to expand...
That is exactly correct..the use of 310.16 ampacities for underground duct banks only works when the loads have been calculated per Article 220. If a realistic calculation is used, which is common for industrial work, an underground duct bank installed using 310.16 ampacities will often burn up because of the excessive heat. There was some extensive debate on this issue for two code cycles back in the 80s (I think). The tables that are now in annex B were in the code book for once cycle, but with an effective date of 3 years later. In the ROP/ROC process for the next cycle these tables were moved to the annex.
 
Well first i agree with all the post so dont get me wrong but what if the conductors were underground or overhead and yearly temp of cold weather lets say 50 degrees or you lived in northern Greenland or the artic circle ?

Just wondering what if
 
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