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| Subject - Equipment Grounding Conductor Size | |
| iwire |
There was a post about wire sizes for a long run, I asked about the Equipment Grounding Conductor (EGC) size. I think a lot of people are unaware of the requirements of 250.122(B) so I thought I would ask some questions and hope to start some discussion on the subject. First question: You have to feed some post lights at the end of a very long driveway, 12 amps of 120 volt lighting 500' from the house from a 20 amp breaker. If you decide to keep voltage drop to 5% you will need to run 6 AWG for the ungrounded and grounded conductors. Would you be able to use 6/2 UF cable for this branch circuit? Question 2. Commercial outside lighting circuit, 208 volt with a 50 amp breaker 75 C wire. Due to the long length of the circuits you use 1 AWG for the ungrounded conductors. What size Equipment Grounding Conductor will be required for this circuit? Question 3 Same circuit as question 2 but now with a 20 amp breaker. What size Equipment Grounding Conductor will be required for this circuit? The answers to all of these questions are surprising, and if ignored could be costly if the inspector keeps an eye open for 250.122(B) violations. |
| David Hyatt | question 1) Yes 6/2UF with ground would work. At 5% instead of 3%. That I would use for branch circuit. Your ground would be increased proportionally to ungrounded conductors. 250.122(B) question 2)I would say by the chart 250.122 #10 question 3) 12 per tbl 250.122 But I know these are not the answers your fishing for. I would say seeing that you increased the size for VD and the ground is not a current carrying conductor 250.122(b) would not apply. Thats just my thoughts, let us know what you would say. Anyone. |
| iwire | Your right those are not the answers I am fishing for. quote: The 6/2 UF will not provide the correct (large enough) equipment grounding conductor. |
| Ryan_J | Come on Bob. Question one: 6/2 UF won't comply Question two: Not enough information to answer the question. What is the rating of the breaker? 110.14(C) tells me that the breaker is 60 degrees, so If I were taking a test, my answer would be: 3 AWG for the equipment ground. If the breaker is labeled at 75 degrees, you would need a 4 AWG equipment ground. Question three: 1 AWG. Perhaps I am that inspector you are referring to? |
| karlwayne | hey iwire I'm not the brightest apple in the bushel, but I did find two cents in my pocket. #1 - increase to #6 #2 - increase to #2 #3 - increase to #4 slay me |
| iwire | No Ryan, you would be the inspector to catch the line side bonding issues. 1)I agree  2)75 C wire and breaker. To make it easer assume I use 8 AWG for a 50 amp circuit. 3)I agree Bob |
| iwire | OK so far we have correct answers for 1 and 3. 1)Needs a 6 AWG grounding conductor 3)Needs a 1 AWG grounding conductor 2)Is a tough one and you can come out with different answers. But if we say the 'normal' size 50 amp conductor is a 8 AWG and increase that to 1 AWG you will need a 6 AWG grounding conductor. It is strange but true that the 20 amp circuit may end up requiring a larger grounding conductor than the 50 amp circuit. |
| Ryan_J | An increase from 8 AWG to 1 AWG is an increase of 500%, circular mil. An increase of 500% from 10 AWG would be 3 AWG, right? The numbers I used (circular mils) from table 8, chapter 9: 10 AWG: 13,380 1 AWG: 83,690 8 AWG: 16,510 The math I did: 83,690/16510=5.07 5.07*13,380=67,823 circluar mils required. |
| iwire | quote: 10 AWG: 10,380. |
| Ryan_J | DOH!!!! Right math, wrong number!
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| iwire | No big deal, you did a better job than I would have at showing the math.
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| iwire | I have to apologize I misread Table 8. The answer to question 2 is 3 AWG as Ryan had said not 6 AWG as I first stated. 
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| David Hyatt | I see that ya'll are raising the wire size to the ratio of CM, but like I stated in my post would this apply to up sizing for VD? With all of our minds together we can come up with the right answer. Lets be nice.
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| Ryan_J | Hi David. Bob (Iwire) and I know each other from a few different sites, like about 5. Beleive me, wer'e not mean to each...quite to the contrary actually. Bob and I are very good friends. As far as the application of 250.122(B) goes, I think voltage drop concerns are exactly what is trying to be addressed in this section. Think about it: In Bob's first example with the UF. you have a 20 amp breaker, therefore you want to create about 80-100 amps of fault current to trip the breaker. 120 Volts, 6 AWG ungrounded 12 AWG EGC. 500 of 12 is 0.99 Ohms. 500 of 6 AWG is 0.49 Ohms. Total impedance of circuit (not including reactance) is 1.48 Ohms. 120E/1.48Z=81.08I. Now, same circuit, with 6 AWG as the EGC. Total impedance is 0.491 Ohms. 120E/.491Z=244I I think the intent of the section is to ultimatly comply with 250.4(A)(5). 244 Amps fault current will certainly trip the 20 a OCPD faster than 81 Amps of fault current. Does that make sense? |
| iwire | quote: I think I am nice. Here is the rule in the 99 code. quote: Here you see they specify conductors increased in size for voltage drop. Now the 2002 version. quote: I have been told by people that follow the code making process the change was made as people where saying the wire size had been increased for other reasons than voltage drop just so the could get around up sizing the EGC. The whole point here is to make sure the resistance in the grounding conductor does not become so high that it will not trip the breaker. |
| David Hyatt | I agree totally, now that you showed me that from 99 code. I did look at 250.4(A)5 earlier and started to post it but didn't. Thanks for clearing that up. Makes perfect sense to me now. |
| Scott Vickrey | I also believe this to be an overlooked section of the code. This is a very informative topic and will benefit all who read it. Thanks for writing it.
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| frenchelectrician | hi guys,, i am sure some of you allready know me now ,,ok i will cut to the chase but this part really open my eyes about the grounded conductor , yes i was well aware about it but i never thought about the 6/2 ug but one thing it still nagging me,, but let me speak up for a se if the 6/2 ug conducter have grounded conductor let me say size 8 which i know some grounded conductor have " undersized " conductor and for that long run i know it dont really meet the code at all but i am glad i learn something new here now .. merci, marc |
| tony thrower | NEC 250-122(B) Where ungrounded conductors are increased in size, equipment grouding conductors, where installed, shall be increased in size proportionately according to the circular mil area of the ungrounded conductor. Question #2: The minimum size copper ungrounded conductor under any condition is #8 75c. The cir. mil area is 16,510. NEC table 250.122: The minimum size copper EGC for a 50 amp branch circuit is #10. The cir. mil area is 10,380. In order to prevent an undesirable voltage drop on the branch circuit conductors the #8 conductor size is increased to #1 copper. The circular mil area is 83,690. 83,690 cir. mils / 16,510 cir. mils = 5.069. This is an increase in size of 506.9% The minimum EGC size #10 must be increased by 506.9%. 10,380 cir. mils x 5.069 = 52,616 cir. mils. NEC table 8 chapter 9: #3 is the correct size EGC having a cir. mil area of 52,620. |
| Ryan_J | Tony: It would depend on the temperature rating of the breaker as to whether you start your calculation with 8 AWG or 6 AWG. |
| Dave Nix | Hi Guys, I believe Tony is correct in his calcs. I also believe that the termination's rating only comes into play when we are considering ampacity. Volts is volts! |
| Dave Nix | Sorry, what I meant to say is that when considering voltage drop, the size of the actual copper conductor is what is considered in relation to it's resistance rather than it's current carrying ability. |
| tony thrower | I will be using an O.C.P.D. that is rated 75 c. |
| Ryan_J | Hey guys. I agree with your statements regarding voltage drop, but for the calculation that Bob posted we would need to know the temp. of the breaker. |
| tony thrower | My OCPD has a temperature rating of 75 c. |
| DANO | Very interesting. Thanks for the information. Be safe. |