Voltage Drop and the EGC

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The footnote to table 250.122 has been changed in the 1999 and 2002 NEC
It used to say may need to be
Now it states shall be
There are times when 250.122 results in an undersized EGC. We all know to upsize the EGC for voltage drop.
There is a free software program "GEMI" available at Mikes website, that allows one to calculate the max length of rigid for a circuit and when a supplemental EGC is required. Its based on studies done at the University of Georgia.
Based on my use of the program, rigid conduit typically needs an EGC starting when its over a 300 ft run.
 
code is minimum

code is minimum

NEC are minimun standards. They don't always work. We had voltage monitors that were shutting a feeder down because of the fault current flowing on a mimimum sized EG. The distance to the fault was over 1300 feet. The fuses never blew. Monitor detected the low voltage, shut things down, a few minutes latter it would attempt to restart again. Each attempt lasted several seconds. The total impedance of the circuit was to great. Live and learn, hopefully.
 
ptonsparky said:
NEC are mini mun standards. They don't always work. We had voltage monitors that were shutting a feeder down because of the fault current flowing on a minimum sized EGA. The distance to the fault was over 1300 feet. The fuses never blew. Monitor detected the low voltage, shut things down, a few minutes latter it would attempt to restart again. Each attempt lasted several seconds. The total impedance of the circuit was to great. Live and learn, hopefully.
Click to expand...

I am sort of astounded to learn that anyone would run a feeder 1300 feet with minimum size conductors.
 
I appreciate all the replies. I've been searching through some old threads to add to the discussion.

In my mind, the single most important function of 250.122(B) is to compensate for the increased available fault current available when larger conductors are used. I had a hard time describing the principle that the current available to the Equipment Grounding Conductor increases if the circuit conductors increase, hence the need for the requirement.

Some oldies and goodies I've found (posting full links for printability):
http://www.mikeholt.com/code_forum/showthread.php?t=65108
http://www.mikeholt.com/code_forum/showpost.php?p=618011&postcount=3

I've thought of a plumbing analogy that might work well. I might use that, if I hone it a bit. :)

Thanks for all the help. ;)
 
tom baker said:
The footnote to table 250.122 has been changed in the 1999 and 2002 NEC
It used to say may need to be
Now it states shall be
There are times when 250.122 results in an undersized EGC. We all know to upsize the EGC for voltage drop.
Click to expand...

That's a good point - I just now looked at the table to see what it says:
Note: Where necessary to comply with 250.4(A)(5) or (B)(4), the equipment grounding conductor shall be sized larger than given in this table.
Click to expand...
As you said, the note must be applied when using the table. (For better examples of this, see Table 220.55 - that is clearly enforceable code.)
 
George, the point about the available fault current increasing is a very valid one. If you increase the size of the phase conductors for any reason, the resistance of the circuit goes down, which is good for voltage drop. However, when you decrease the resistance of the circuit, you increase the amount of current that will flow under fault conditions (Ohms law). For example, if I have a 20 amp circuit that I pull 4/0 AWG for my phase conductors and I install 12 AWG for my equipment ground, the 12 AWG conductor is going to vaporize if there were a fault between the phase and the EGC. This would be a violation of 110.10. Simply making the wires as big as possible to compensate for voltage drop (both under regular situations and in fault conditions) is only half of the issue. Take a look at the 2008 ROP, and read Mike Johnston's proposal to make an exception to 250.122(B). At this point it is being accepted, but after comments I don't think it will. Anyway, read the panel member's negative comment that basically says the same thing I just said, if you want to hear (or read) it from multiple sources.
 
ryan_618 said:
George, the point about the available fault current increasing is a very valid one. If you increase the size of the phase conductors for any reason, the resistance of the circuit goes down, which is good for voltage drop. However, when you decrease the resistance of the circuit, you increase the amount of current that will flow under fault conditions (Ohms law). For example, if I have a 20 amp circuit that I pull 4/0 AWG for my phase conductors and I install 12 AWG for my equipment ground, the 12 AWG conductor is going to vaporize if there were a fault between the phase and the EGC.
Click to expand...

I have not done a detailed analysis, but this really sounds like a red-herring to me.

If a breaker will protect 50 feet of 12AWG each way in a bolted fault, then it should do just fine with 500 feet of 4/0 going out and 500 feet of 12AWG coming back. Even if the phase conductors had _zero_ voltage drop the EGC has loads.

-Jon
 
Jon,
If a breaker will protect 50 feet of 12AWG each way in a bolted fault, then it should do just fine with 500 feet of 4/0 going out and 500 feet of 12AWG coming back. Even if the phase conductors had _zero_ voltage drop the EGC has loads.
Click to expand...
There are two problems with this circuit. First the impedance of the #12 will limit the current flow and it is likely that the OCPD will never see enough current to cause it to open the circuit. The second problem is the voltage drop on the EGC will appear as voltage to earth on any metal parts of the faulted equipment and this voltage will be there until the OCPD operates. In this case maybe never...a very dangerous condition.
Don
 
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