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#95588 09/23/05 03:21 PM
Joined: Jan 2003
Posts: 4,391
Also and very important, voltage drop can very much be a safety issue!!!

Please explain how voltage drop can be a safety issue if the rest of the installation follows the NEC. [Linked Image]


Bob Badger
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#95589 09/23/05 03:24 PM
Joined: May 2002
Posts: 1,716
Okay, I have to ask, what POCO provides a stable enough system that a "precise" VD can be calculated?


#95590 09/23/05 03:55 PM
Joined: Jan 2004
Posts: 1,507
Your right Roger, and that's why we can use the simple approach we do for voltage drop and get away with it. NOW Article 647 on the other hand may be somewhat more stable. As Bob pointed out it's not a popular Article. I only know of one contractor who gets involved with "Technical Power" as it is called and it gets kinda accurate. They use Technical Power also in some audio systems.

George Little
#95591 09/23/05 04:50 PM
Joined: Sep 2005
Posts: 99
Tripp Offline OP
Holy Toledo, Diver! I'm almost sorry i asked what formula you use! [Linked Image] Except that it's good to know it exists and who to go to if i really want to know! Still, i think i'll stick with my Uglies book for now.

Nevertheless, since i sent you my distances in my last post, did you calculate my voltage drop? I'm curious to see if we match up.. Also, still not sure if you think i should calculate my cmils for each conductor (aluminum and copper) using the total distance involved or whether i should calculate the size of each based only on the distance that conductor runs.

#95592 09/23/05 05:28 PM
Joined: Mar 2005
Posts: 38
Hi All,

Roger, you're right!!! The best VD computaton is only an approximation. POCO, conductor age, connections, circuit breaker age, etc. are all non-calculated contributors that cause computation vs actual VD variations.

iwire, A simple case is when you run conductors over a roof or attic where the ambient temperature can reach high levels. Unless the conductors are sized with regards to VD, this will cause a large VD that will start the insulation degradation process that will lead to a fire.

Tripp, I forgot about your VD, sorry.
I can see that the runs are 45' for aluminum and 87' for copper. Please post the conductor size, ambient temperature, and the raceway type and I'll knock it up right now.

Dolphins Software
#95593 09/23/05 05:49 PM
Joined: Mar 2005
Posts: 38
I just check back to get Tripps info and though I add a few more notes.

As I stated prior, regardless of the VD formulae you're using, the fact that you all are using VD in conductor sizing gains my respect and attention. I would like to note that I forgot to mention probably the largest contributor to VD variation is ambient temperature. The only problem I have with 2KID as a bases of VD is that it doesn't take temperature into account. Yesterday I had a conversation with a contractor who went stricky by 310.16 and wondered why Volts upsized a conductor. I point out to him the roll that temperature played in VD and that the NEC does account for it with the temperature Correction Factors located just below the 310.16 table, something that he had never noticed or done prior.

Dolphins Software
#95594 09/23/05 07:56 PM
Joined: Apr 2002
Posts: 7,333
Likes: 7
I have to copy Divers formula and in about 7 weeks, when I give the entry level guys (In Basic Electricity) the KIL way, and that spins there heads; wait for Diver's, that may be incentive to learn KIL.



#95595 09/23/05 11:21 PM
Joined: Sep 2005
Posts: 99
Tripp Offline OP
Hi Diver - here are my numbers:

copper: 82' (not 87')

wire size: inverting KIL to find cmils, and using a desired Vd of 3%, i got #4 for the SE aluminum cable and #6 for the copper.

Ambient temp: good question. we're in Montana; the wire will be run through a basement crawlspace (alum.) in no conduit; and then outside along a cement pad in PVC sch.80 (copper).

Take it from there.

#95596 09/23/05 11:36 PM
Joined: Oct 2000
Posts: 2,749
Don't forget the following rule:

250.122(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.

Joe Tedesco, NEC Consultant
#95597 09/23/05 11:40 PM
Joined: Sep 2005
Posts: 99
Tripp Offline OP
Thanks Joe - i have every intention of increasing EGC accordingly. Thanks for being on top of it.

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