Electrical Forum - Need help with Volt Drop

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Subject - Need help with Volt Drop
Electricman	A friend of mine called today and asked about a 100 amp sub in a barn, the barn is 360 feet away from the house and he wants to know what cable to run. I come up with 2\0 copper which gives me a volt drop of 6.96 volts . The only things he has to power in the barn is 1000watts of lights and a 240v 20 amp air compressor. I was just wondering what you guys can help me out with in your calculations. Many thanks, Craig P.s. show your work please
Electricman	Ok, here is the formula that I used tell me if I did it right 2 X K X L X I divided by cmil using table 8 in chapter 9 2\0 cable has a cmil of 133100 I multiplied this by ohms per foot .0000967 to get 12.870 so 2 X 12.87 X 360 feet X 100 amps = 926695.44 I then divided 926695.44 by cmil 133100 to get 6.9624 which is less than 3%. Sound right? let me know
Ryan_J	Single phase, right? Lets eliminate about half of the work and use Ohm's law instead. Also, we're talking about A/C, so I use table 9, and in this case assumed a PVC raceway. Here goes: I=E/R or, E=IR Load (I)=100 Impedance (Z)=.072 Ohms(2/0) 100.072= (7.2VD) I think your ok
Electricman	240 volt single phase AC yes, Thanks Ryan
Ryan_J	BTW: The real load can be used for your VD calc's, not the rating of the panel. Using a load of 24 amps, which is what you have, you have a VDof less than 1%!!!
Ryan_J	quote: Originally posted by Electricman 240 volt single phase AC yes, Thanks Ryan You're very welcome Ryan hates doing voltage drop calculations :P
Electricman	He talked about future stuff(Dont they always)That was my concern again thank you. Craig And I guess you can thank me for making you do a V\|D calculation LOL
cs409	Lets see,,,,,360 feeeeeeeet,that will put you well over a 1000bucks just for wire! not to mention the labor to put the pvc into the ground etc etc.. think i would check into having the power company bring service out to the barn! just put a new service etc on the barn.....
Ryan_J	The math: Lets say I've got to provide a 20 amp circuit for a light pole that is 500' feet away from the panel. I decide to use #8 AWG for the voltage drop (I'm guessing). If we go right to table 250.122, we would pull a #12 EGC for the circuit. Here's what happens when a ground fault occurs: 500 feet of #8 has a resistance of 0.389, and 500 feet of #12 has a resistance of .99, for a total circuit resistance of 1.379 Ohms. 120/1.379= 87 amps of fault current...probably enough to trip a 20 amp breaker in less than 4-5 seconds. Same circuit, only with a #8 EGC, as required: Total circuit resistance of 0.778 Ohms. 120/0.778=154 amps of fault current...enough to trip a 20 amp breaker in less than one cycle (1/60th of a second).
David Hyatt	Ryan, I guess since code does not require VD I would agree you could use the load instead of amp rating of panel. But what about the future. The barn may never see 100 amps but what if it does. I haven't done the calculation yet but would think 2/0 would still work.
Electricman	cs, yeah I suggested that right off the bat but he has his heart set on this sub panel, I dropped the bomb on him today about the $ and he was a little suprisedto say the least.
Ryan_J	Challenge: What size equipment ground is required if we use 2/0 for this 100A panel?
JimmyDee	As per 250-66, #4 because the amps have nothing to do with the size. It is the wire size that determines the ground size. Jim
Ryan_J	It happens to the best of us.
JimmyDee	quote: Originally posted by Ryan_J It happens to the best of us. I glad you included me in that group. Or did you?
cs409	Electricman,,,what sort of material/labor did you drop on him,,and was this a friend to friend cost or was profit in it? and did he ask you for the utility co. phone number LOL
Ryan_J	Wrong table Jimmy. Go to 250.122, and see (B).
JimmyDee	Jim
Ryan_J	But of course. So, any takers on the EGC size for this circuit?
cs409	what prize do we get for the correct answer
Ryan_J	quote: Originally posted by cs409 what prize do we get for the correct answer Why, knowledge, of course :)
JimmyDee	Looks like #8 cu. Jim
Electricman	I agree #8 copper although I was gonna use #4
cs409	#3 cu. equipment grounding conductor... providing that your increasing your hot conductors from #4 to #02....
DavidJI1956	Use #4, it maybe oversized, if he ever does up grade it is already there. And just to say from Washington, I'm new to this place.
iwire	Ryan is my hero. quote: Originally posted by Ryan_J Challenge: What size equipment ground is required if we use 2/0 for this 100A panel? 8 AWG = 16,510 circular mils 2 AWG = 66,360 circular mils 2/0 AWG = 133,100 circular mils OK 100 amp feeder using 2/0 for the ungrounded conductors. Table 250.122 would require 8 AWG for a 100 amp breaker. I agree with cs409 that 2 AWG is the normal size copper ungrounded conductor for a 100 amp feeder. Divide the circular mils of the 2/0 by the circular mils of the 2 AWG to find the ratio of increase. 133,100 / 66,360 = 2.0057 ratio. Multiply the circular mils of the 8 AWG by the ratio. 16,510 x 2.0057 = 33,114 So we need an EGC of at least 33,114 circular mils or a 4 AWG as cs409 has said.
iwire	quote: Originally posted by DavidJI1956 Use #4, it maybe oversized, if he ever does up grade it is already there. David if we run 2/0 copper for a 100 amp feeder we are required to run 4 AWG for the EGC per 250.122(B). It is interesting that the NEC does not require conductors up sized for voltage drop, but if you decide you want to do that you are required to increase the EGC size.
cs409	i was doing my cal. on the assumption that the upgrade to #02 was due to the distance/aka voltage drop, where #4 could have been used as a starting point for the 100amp panel,,until of course you run it 360ft...so with 250.122(b).... u have to adjust the #8 wire for EGC .... soooo.... you take the #4 at 41740 cmil, and divide it into #02 at cmil of 133100 which gives u a ratio of...3.18 then * 3.18 x cmil of #8 of 16510 and you get cmil 52502 which equals EGC wire size #3 so u end up with a EGC upgrade from #8 to #3......what a big change!
JimmyDee	quote: 250.122 Size of Equipment Grounding Conductors. (A) General. Copper, aluminum, or copper-clad aluminum equipment grounding conductors of the wire type shall not be smaller than shown in Table 250.122 but shall not be required to be larger than the circuit conductors supplying the equipment. Where a raceway or a cable armor or sheath is used as the equipment grounding conductor, as provided in 250.118 and 250.134(A), it shall comply with 250.4(A)(5) or 250.4(B)(4). (B) Increased in Size. Where ungrounded conductors are increased in size, equipment grounding conductors, where installed, shall be increased in size proportionately according to circular mil area of the ungrounded conductors. This is one of the things that just kills me about the NEC. Why didn't they just give us the same chart that they have for the grounding electrode wire and all the controversy would be gone. I'll give you a for instance. quote: where #4 could have been used as a starting point for the 100amp panel,, Why do we assume we would start at #4 wire here for sizing. Why not #2 TW? I think this is one area that the writers could have simplified the whole thing by sizing the ground wire by the size of the feed wire and be done with it. It will probably work out that way anyhow. Jim
cs409	Ryan started all this mess LOL....and where the heck did he run off to?
Ryan_J	LOL!!! I didn't run off...I was just lurking in the shadows. Many thanks to Bob Badger (Iwire) who taught me this section of the code. It really does make for some interesting conversation. Where you been anyway, Bob?
cs409	any word as to if they plan to make any changes in this area?,,,i read somewhere some real hard info on this subject, which dates back 30yrs, after this study etc. that is one of the reasons for the increase in an EGC size when you increase the size of your ungrounded condutors....any way, if i run across it, i will post,,,it helps to understand why u do it...
veganfan	quote: Originally posted by JimmyDee quote: 250.122 Size of Equipment Grounding Conductors. (A) General. Copper, aluminum, or copper-clad aluminum equipment grounding conductors of the wire type shall not be smaller than shown in Table 250.122 but shall not be required to be larger than the circuit conductors supplying the equipment. Where a raceway or a cable armor or sheath is used as the equipment grounding conductor, as provided in 250.118 and 250.134(A), it shall comply with 250.4(A)(5) or 250.4(B)(4). (B) Increased in Size. Where ungrounded conductors are increased in size, equipment grounding conductors, where installed, shall be increased in size proportionately according to circular mil area of the ungrounded conductors. This is one of the things that just kills me about the NEC. Why didn't they just give us the same chart that they have for the grounding electrode wire and all the controversy would be gone. I'll give you a for instance. quote: where #4 could have been used as a starting point for the 100amp panel,, Why do we assume we would start at #4 wire here for sizing. Why not #2 TW? I think this is one area that the writers could have simplified the whole thing by sizing the ground wire by the size of the feed wire and be done with it. It will probably work out that way anyhow. Jim How did you get that quote from NEC? It looks like a paste, or are you just that good at editting your post? Is there a place to get quotes from NEC on line? Just wondering.
Ryan_J	Jimmy is probably using the NEC on CD ROM, like I do. It makes it very nice to copy and paste with, and for an instructor, it is an indispensable tool to be able to throw the code up on the projector for all to see.
JimmyDee	quote: Originally posted by Ryan_J Jimmy is probably using the NEC on CD ROM, like I do. It makes it very nice to copy and paste with, and for an instructor, it is an indispensable tool to be able to throw the code up on the projector for all to see. Yep, Ryan is correct. Jim
veganfan	Great thats what I asked for for fathersday/birthday present. Hope i get it taking fundamental wiring class now for NEC provided by work, but next semester plan to take 16 week class strictly on NEC which I will pay for.