voltage drop 425 feet help

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tom baker

tom baker

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Deanos said:
We're still on the 05 and I disagree on the ground issue. I think I can run two 3/0 and drive two rods at the barn and call it a day. No?
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You can not. The 05 prohibits regrounding the neutral if there is a parallel path to ground IE water pipe, coax cable. A grounding electrode system is always required. 08 code says you can never reground the neutral.

Have you considered install a step up transformer (240x 480) and then back to 240 at the building? Use a Square D mini power zone that has the primary OCP, transformer, secondary panel in one unit.
 
cadpoint

cadpoint

Senior Member
Location
Durham, NC
Where you come up with your figures?
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Blame it on operator error with in an App! I knew I should have wrote down my calculations.
...
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That was also suggested in the other forum by a really smart and handsome guy. :cool:
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That was blood red not flush red... right :)

So they already embrassed themselves thus the bridge statement! Adding to the electrician
legend! Carry on!
 
J

JDBrown

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California
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Electrical Engineer
Deanos said:
Ok here is the thing, pipe is existing 1&1/2 pvc 425 feet long 60 a overcurrent at the main panel,, the load at the barn is about 48 amps as a guess. Whats the largest size copper conductors and or alr. conductors that can be pulled?? So far I came up with 3 3/0 alr. with no ground, drive a ground rod at the barn and call it a day, or three number 1/0 and one # 6 copper. What do you guys think?? The huge issue is the 1&1/2 pvc thats undersized.
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It looks like 2005 NEC 250.32(B)(2) allows you to run your feeder without an EGC, as long as there are no parallel paths for normal neutral current. So, that could be an option if there is no metal water pipe, etc. I don't particularly like going that route, but it appears it would be Code compliant, given that you're still on '05.

Now that we have that out of the way, you can't run 3#1/0 & 1#6 EGC (copper) for a 60A circuit. NEC 250.122(B) requires your EGC to be "increased in size proportionately according to the circular mil area of the ungrounded conductors." So let's do the math.

#1/0 circular mils / #6 circular mils = 105600 / 26240 = 4.024

The normal size for a copper EGC on a 6A circuit is #10 (see Table 250.122), so we need to increase it by multiplying its circular mils by 4.024:

10380 x 4.024 = 41773 circular mils

This is just barely larger than #4 AWG, so you need to go up to the next larger size -- hence, your EGC needs to be at least #3 AWG.

Assuming THWN conductors, 3#1/0 & 1#3 EGC will take up 0.6538 in2 (Chapter 9, Table 5). This will work in your 1-1/2" PVC (whether it's Schedule 40 or 80 -- see Chapter 9, Table 4).

You'll still need to check your pulling tensions in a run that long, though.

Oh, and by my calculations, for a 425' run of #1/0 copper conductors at 120/240V, single phase, drawing 48A, the voltage drop will be about 2.21%. At 60A it would increase to about 2.76%.

Hope all this helps.
 
augie47

augie47

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This may have coverd and I got lost in the forest.
Since you are on '05, as mentioned, the requirement for an equipment grounding conductor depends on the presence of any metallic path.
Assuming none, Table 9 allows (3) 2/0 XHHW in a 1-1/2 Sch 80 PVC. 2/0 would keep you below 3% drop.


If an equipment ground is required you can do the math for fill, but even if you dropped back to #1 (1-1/2 will handle 4-1s) at 48 amps your voltage drop si just a bit over 4%, normally a workable number.
 
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kwired

kwired

Electron manager
Location
NE Nebraska
Is this service supplied or feeder supplied? That can change the need for equipment grounding conductor even in the 2011 NEC.
 
E

edlee

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Location
Western Massachusetts
I used the VD calculator on the Southwire website, and for 48a 240v at 425 feet , #1 copper gives a VD of 2.53%.
No problem in the 1-1/2" pipe, which can handle 5 #1thhn conductors.

If OP wants to rate if for the full 60a, then 1/0 thhn will work, 2.76% VD. 1-1/2" PVC sized for four #1/0 thhn conductors.
 
ActionDave

ActionDave

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Seems like a lot of hand wringing about a non issue.
Count me in as one who is not seeing any problem with VD or the pipe too small.

If there were no requirement for an EGC I would not pull one.

3/0...... 480V transformers.....for 60A? Really? 425 feet is not that far. Fear of VD is often greater than the reality of it.
 
kwired

kwired

Electron manager
Location
NE Nebraska
ActionDave said:
Seems like a lot of hand wringing about a non issue.
Count me in as one who is not seeing any problem with VD or the pipe too small.

If there were no requirement for an EGC I would not pull one.

3/0...... 480V transformers.....for 60A? Really? 425 feet is not that far. Fear of VD is often greater than the reality of it.
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And the load being fed in the OP is probably nothing that over 3% drop will matter much either.
 
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