Egc 250-122b

[Dvanosdall]

The Project Engineer, using a engineered supervised calculation, has determined using 3 sets of 750 kcmil XHHW-2 copper with 1-1/0 copper XHHW-2 ground for a 800 amp 480 volt 3 phase 3 wire branch circuit to a MCC. Mainly due to being in a duct bank with heating issues.

It is my determination that the EGC needs to increased in size to a 250 kcmil, to be in accordance to NEC 250-122(B).

The Engineering firm is objecting stating this is not correct, because they are not increasing the size of the ungrounded conductors, but found the size needed to carry by using the calulations to determine the ampacity as NEC 310.15(A)(1) allow.

I am asking for some input on this.

 

[kingpb]

Would need a copy of the calc to see what they have done.

Best way to ease your mind, is have the AHJ stop by and go over it with him, and get a ruling ahead of the install.

Other option is to put your blinders on, follow the engineer's direction, when the AHJ comes out, and if he turns it down, you will have the ammo needed to get a big fat extra. The engineer will have to spend his time arguing with him about it, and will probably lose, and you still get the extra.

BTW: you will need a copy of the calc to show to the AHJ.

 

[jumper]

Are you the Inspector on this job?

 

[Dvanosdall]

Yes I am and I am also the AHJ

 

[steve66]

What year NEC are you under??

If you are under the 2008, I basically agree except I come up with 4/0. I got that by assuming (2) sets of 600's would normally be ran for a 800A breaker.

Steve

 

[Dvanosdall]

Egc 250-122b

NEC 2005 250-122(B) Increased in size

 

[Dennis Alwon]

This is without doubt a gray area in the NEC. If the engineer determines the wires must be a certain size, I would like to know how they came about this. The Nec doesn't really state anything more then if you increase the conductor size then the egc must be proportionately increased. The problem is where is the starting point.

I tend to agree with you. My limited understanding makes me think a larger egc is necessary to clear any ground fault that may occur given the larger conductors.

If the engineer signs off on it, then I am not sure I would lose sleep over it.

 

[charlie b]

I agree with the engineering firm. The conductors have been "sized," not "increased in size."

But I also recognize that the code language is in serious need of improvement. It appears that the CMP has accepted, at least in principle, my proposal to revise the wording of 250.122(B), for the 2011 issue. The whole problem with the notion of "increased in size" is that we don't know the starting point. Specifically, nothing tells us that, "If you use a conductor larger than what table 310.16 would have allowed you to use, then you shall be deemed to have 'increased' the size of the ungrounded conductors." Why should we start with that table?

For those who have a copy of the Report on Comments (ROC) for the 2011 edition, I refer you to item 5-181, Log # 2235. The present version of the wording backs up the engineering firm's viewpoint. It says that you first find out the minimum size wire that has sufficient ampacity for the installed configuration, without yet having dealt with adjustment or correction factors. Then if you use a larger wire for any reason, including temperature corrections, or adjustments for more than three current carrying conductors in a conduit, or just because that is what you had in the truck, you shall be deemed to have "increased" the size of the ungrounded conductors.

In this case, the engineer determined the minimum size conductor, using a method that the NEC permits them to use. They did not use a conductor that is larger than that value. So the ungrounded conductors were not "increased in size," and the EGC need not be increased either.

 

[charlie b]

Mainly due to being in a duct bank with heating issues.

Without having seen the calculation, I interpret this statement as having nothing to do with the temperature corrections at the bottom of table 310.16. Every time I have done an amacity calculation for an underground ductbank, the results have been lower than the ampacity values in 310.16. That is because of the mutual heating effects of cable runs in close proximity. Even if there are only three conductors in each conduit, and even if the ambient temperature is below 30C, and even if the run is too short to worry about voltage drop, the presence of other conduits within the same concrete encasement will cause the calculated ampacity to be lower than the values shown in 310.16.

 

[jumper]

Dear lord,

I thank you that I am not the installer caught between an engineer and an AHJ.

in nomine Patris, et Filii, et Spiritus Sancti

Amen!!!

 

[chris kennedy]

It is my determination that the EGC needs to increased in size to a 250 kcmil, to be in accordance to NEC 250-122(B).

Yes I am and I am also the AHJ

I agree with Charlie that we are looking at a poorly worded section. That said, as an installer I would increase the size of the EGC.

As an AHJ you may be interested in this bit of HB commentary.

Equipment grounding conductors on the load side of the service disconnecting means and overcurrent devices are sized based on the size of the feeder or branch-circuit overcurrent devices ahead of them. Where the ungrounded circuit conductors are increased in size to compensate for voltage drop or for any other reason related to proper circuit operation, the equipment grounding conductors must be increased proportionately.

 

[iwire]

The conductors have been "sized," not "increased in size."

Can you give me a legitimate application where I could not use that same argument?

I agree the current wording is murky.

 

[steve66]

I have to disagree with Charlie. I think it is very clear that "increased in size" means any increase above what would normally be expected. For a 800 amp breaker, Table 310.16 clearly shows that two sets of 600 KCM would be a reasonably sized feeder. To say that three sets of 750's are not increased in size, well, I just don't buy it.

And I haven't read the ROC, but I don't see how that supports the engineers argument at all.

If you say that three sets of 750's are needed for 800 amps, you have used either adjustment or correction factors. I think its that simple.

That said, I don't think it is a real safety issue. If the run isn't really long, the three sets of 750's will only make the breaker trip faster (and easier) than two sets of 600's.

 

[charlie b]

For a 800 amp breaker, Table 310.16 clearly shows that . . . .

You bring us to my point exactly, Steve. Why did you cite that table? Who cares what that table has to say about ampacity? The ampacity of the conductors was not determined using that table, but rather by an engineered calculation. Take a look at 310.15(A)(1). It says that ampacity can be determined from the tables or from a calculation.

 

[steve66]

Can you give me a legitimate application where I could not use that same argument?

I agree the current wording is murky.

If I remember right, the previous wording was even worse. The equipment grounding conductor only had to be increased when the phase conductors were increased for voltage drop.

The problem was engineers (and electricians) would never admit voltage drop was the reason why the phase conductors were being increased in size.

Steve

 

[steve66]

You bring us to my point exactly, Steve. Why did you cite that table? Who cares what that table has to say about ampacity? The ampacity of the conductors was not determined using that table, but rather by an engineered calculation. Take a look at 310.15(A)(1). It says that ampacity can be determined from the tables or from a calculation.

I used it because it allows me to use a smaller wire. If the calculation gave me an even smaller wire size, I would have used that as a starting point.

If you aren't using whatever allows the smallest size wire, then, IMO you have increased the wire size.

Steve

 

[charlie b]

Can you give me a legitimate application where I could not use that same argument?

I think I can. Indeed, it is one that I previously presented to the CMP, and they did not buy it.

Let me start with a reminder that the definition of ?ampacity? includes the notion of ?under the conditions of use.? Let us then consider a calculated load of 50 amps, a conduit that will have a single phase 120 volt circuit, a total circuit length of 20 feet, a conductor type THWN copper, and an ambient temperature of 33C. Table 310.16 tells me that a #8 has sufficient ampacity for this load. But when I take the ambient temperature into account, I have to move up to #6.

I submit the argument that the ?conditions of use? include the 33C ambient, that a #8 does not have sufficient ampacity under those conditions, that the minimum allowable size is a #6, and that use of a #6 is not an increase above the minimum allowable. Therefore, I don?t have to upsize the EGC, because I didn?t upsize the ungrounded conductors. The CMP did not accept this argument.

 

[charlie b]

I used it because it allows me to use a smaller wire.

Ah, but it doesn't! Consider, if you would, the wording at the top of that table. It says that the table applies to not more than three current carrying conductors in raceway. But in an underground ductbank, you have not only three conductors in a raceway, you also have three more in a nearby raceway, and three others, and three others and . . . . The presence of the others will adversely impact the ability of each conductor to reject its heat to the surrounding world. Therefore, if you use table 310.16 to select a smaller wire than that calculated in accordance with accepted engineering methods, you are artificially, and inappropriately, undersizing the circuit. It is the calculation, and not the table, that determines the correct, minimum wire size.

 

[infinity]

I used it because it allows me to use a smaller wire. If the calculation gave me an even smaller wire size, I would have used that as a starting point.

If you aren't using whatever allows the smallest size wire, then, IMO you have increased the wire size.

Steve

Steve, I think that I agree with you.

So in your opinion you would start with the smallest permitted conductors and any larger deviation from those conductors would make 250.122(B) applicable? So in the case of this scenario the EGC could be the same size?

 

[yired29]

The Project Engineer, using a engineered supervised calculation, has determined using 3 sets of 750 kcmil XHHW-2 copper with 1-1/0 copper XHHW-2 ground for a 800 amp 480 volt 3 phase 3 wire branch circuit to a MCC. Mainly due to being in a duct bank with heating issues.

It is my determination that the EGC needs to increased in size to a 250 kcmil, to be in accordance to NEC 250-122(B).

The Engineering firm is objecting stating this is not correct, because they are not increasing the size of the ungrounded conductors, but found the size needed to carry by using the calulations to determine the ampacity as NEC 310.15(A)(1) allow.

I am asking for some input on this.

Does the MCC instructions state a minimum circuit ampacity? (110.3 (B))
If so then that would be the starting point IMO.