EGC Derating

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lumenpro

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I am working on the relighting of a large prison yard & fence, and have multiple circuits with long conductor runs which I have sized for voltage drop. I have also derated the conductors per number in the raceways. These are 480 volt single phase lighting circuits with a maximum OCPD of 50A. The voltage drop calculations and quantity calculations independently indicate the same size conductor - #2CU.

I am going to use a single EGC for multiple circuits per 250.122(C), and resize according to 250.122(B). I'll increase the #10 EGC size interpreted from Table 250.122 by 640% according to the difference in circular mils between a #10 & #2 conductor.
But my question is - after derating for voltage drop... then derating for the number of conductors in a raceway... would you have to recalculate the voltage drop for the quantity derated conductors? And if so, how?
And then size the EGC by this final calculation?

Thanks for any help!
 

bob

Senior Member
Location
Alabama
lumenpro said:
These are 480 volt single phase lighting circuits with a maximum OCPD of 50A. The voltage drop calculations and quantity calculations independently indicate the same size conductor - #2CU.

I am going to use a single EGC for multiple circuits per 250.122(C), and resize according to 250.122(B). I'll increase the #10 EGC size interpreted from Table 250.122 by 640% according to the difference in circular mils between a #10 & #2 conductor.
But my question is - after derating for voltage drop... then derating for the number of conductors in a raceway... would you have to recalculate the voltage drop for the quantity derated conductors? And if so, how?
And then size the EGC by this final calculation?

Thanks for any help!
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For a 50 amp ckt you need a #8 conductor. You calculated the VD on #10 and found it to be too small. You re-calculated and found that #2 was adequate. You did not "derate the conductor". You derate the conductor for temperature and # of conductors in conduit. When doing so, you use the 90C rating if your conductor insulation is rated for 90C. The process of derating a conductor is for conductor ampacity. The voltage drop would not be changed.
 

charlie b

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You didn't give enough information for us to perform a calculation of our own, to check your numbers. But perhaps it will be enough for me to say that you should do two specific things in a spedific order. First, determine the size of wire needed to supply the load, without any consideration of voltage drop, but with taking into consideration a derating due to multiple conductors in a raceway. That is your "base case," the wire size you start with, when you calculate the new EGC size. Take the size of the wire you choose, after you compensate for voltage drop, and compare it to the size of the wire you could have used, if VD were not an issue. This is the context in which the phase wires will have been "increased in size." This is what you use to determine the EGC size.
 

lumenpro

Member
Thanks to everyone for the input...Bob, you're right - I should have said "resized the conductor" for the VD calculation. But the process outlined by charlie b was what I suspected to be true, but wanted to verify that I wasn't overlooking any steps.

There will be 60 pole locations with pole heights varying from 35' to 50' at distances of up to 2000' from the contactors controlling the circuits. Because of the pole locations around the prison campus we are reluctant to place remote transformers or other equipment where inmates would have access to them. So, we will bore and/or bury multiple conduits underground and accommodate for distance, load and quantity of conductors per conduit.

Thanks again for your help, I very much appreciate it!
 

augie47

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Tennessee
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State Electrical Inspector (Retired)
Egc

Egc

charlie b said:
You didn't give enough information for us to perform a calculation of our own, to check your numbers. But perhaps it will be enough for me to say that you should do two specific things in a spedific order. First, determine the size of wire needed to supply the load, without any consideration of voltage drop, but with taking into consideration a derating due to multiple conductors in a raceway. That is your "base case," the wire size you start with, when you calculate the new EGC size. Take the size of the wire you choose, after you compensate for voltage drop, and compare it to the size of the wire you could have used, if VD were not an issue. This is the context in which the phase wires will have been "increased in size." This is what you use to determine the EGC size.

Perhaps I don't understand, but are we saying that if you increse the phase condcutor size to compensate for conduit fill (derating), then it is also necessary to increase the EGC size ?
 

charlie b

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augie47 said:
Perhaps I don't understand, but are we saying that if you increse the phase condcutor size to compensate for conduit fill (derating), then it is also necessary to increase the EGC size ?
That is not what I had intended to say, and I hope I didn't give that impression. You "size" the conductor that way. When you deal with temperature or multiple current-carrying conductors in a raceway, you are establishing the "correct size" of the conductors. At this point, the EGC is still sized on the basis of the OCPD setting. That is the EGC size you start with, when you next "biggie size" the conductor because of VD concerns.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
Charlie,

I disagree with your interpretation of 'increased in size' 250.122(B).

The code section doesn't say 'increased in size after you make consideration for conduit fill or for ambient temperature', it simply says 'Where ungrounded conductors are increased in size....'.

I contend that increasing the size of the conductors for conduit fill or for ambient temperature constitutes an increase in size that triggers 250.122(B).

Consider a long 30A circuit. For reasons of voltage drop you have increased the ungrounded conductors to #6, necessitating the use of a #6 EGC. Now consider this exact same circuit, of the exact same length, but in a raceway with 10 other current carrying conductors. In this second case, 310.15(B)(2)(a) forces the use of #6 circuit conductors. I contend that you must still use a #6 EGC.

Where conductors are increased in size for any reason, you get a reduction in voltage drop. When this reduction in voltage drop is not needed by the design, then increasing the EGC is probably a waste of material. So I understand _why_ your interpretation of 250.122(B) makes good sense in the case of short distances and packed raceways. But that isn't what the code says.

-Jon
 

charlie b

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winnie said:
Charlie, I disagree with your interpretation of “increased in size” 250.122(B). The code section doesn't say “increased in size after you make consideration for conduit fill or for ambient temperature,” it simply says “Where ungrounded conductors are increased in size....”
This has been debated before, and I don’t wish to rehash old arguments. Let me simply summarize my perspective.

There is one important word missing from that code article. The word, “from.” It should say, “increased in size from . . . ,” and then convey the starting point. Lacking that guidance, we could start from the body of Table 310.16 (your point of view), or we could start from Table 310.16 as modified by the temperature correction at its bottom and as modified by Table 310.15(B)(2)(a) (my point of view).

Here’s an example:

Suppose a pair of single phase circuits (not a MWBC) each has a calculated load of 50 amps. Suppose they share a conduit (i.e., two phase conductors, two neutrals, one EGC, all in the same conduit). Table 310.16 says a #8 is good for 50 amps, but only in the “conditions of use” shown at the top of the table. We don’t meet those conditions (i.e., the condition of “not more than 3 CCCs in a raceway”). So a #8 is not, in fact, good for 50 amps under our conditions. Even if you took the 90C rating of 55 amps, and multiplied it by the 80% factor of Table 310.15(B)(2)(a), a #8 is still not good for 50 amps. So what conductor size is good for 50 amps? Do the math, and you will see that a #6 has enough ampacity for our conditions of use. Therefore, we select a #6 for these circuits. I then pick a #10 EGC, based on a 50 amp breaker. That is the design process. I haven’t “increased in size” any conductor. I didn’t increase from #8 to #6. Rather, I determined that a #8 was not acceptable, and that a #6 is acceptable. I don’t increase the EGC, since I didn’t increase the phase conductor.
 
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winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
charlie b said:
There is one important word missing from that code article. The word, ?from.? It should say, ?increased in size from . . . ,? and then convey the starting point.

I concur wholeheartedly!

Depending upon circumstances, one can easily come up with situations where your interpretation or my interpretation results in unnecessary waste of materials or undersized EGC.

I vote for ditching this code section entirely, and instead providing a clear and enforceable definition of 'effective ground fault current path' :) But that is a fight best left for the concerned commercial interests to hammer out :(

-Jon
 
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