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#95608 09/24/05 02:02 PM
Joined: Jan 2004
Posts: 1,507
G
Member
Okay- I'll ask it one more time- What is "I" and what is "E". Unless I have these values I can not calculate voltage drop. Maybe I'm green I've only been an electrician for 40 years.


George Little
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#95609 09/24/05 02:22 PM
Joined: May 2002
Posts: 1,716
R
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Tripp, George has asked for the I & E values, please let us know what they are so that those who may want to can perform the calculations for comparisons.

With out knowing these numbers even Dan had to assume, and that pretty much reduces his high dollar precise software to a guessing program.

Roger

#95610 09/24/05 02:55 PM
Joined: Mar 2005
Posts: 38
D
Member
Ok, after another soda I have found a large error in my assumptions.

Since this is SE cable, I am assuming that the conductor connects to a CT then a circuit breaker. Both these devices have a 75ºC terminal temperature rating. With that the Al and Cu conductors carry a max of 65A(I) for #4 & #6 respectively.

I am also assuming V(E) at 208, ambient for the ground at or less than 36" depth at 60ºF and above ground at 105ºF, 3 phase power. Soil thermal conditivity(RHO) is also assumed at 90.

The conductor impedance(Z) values are .02571 for Al and .05566 for Cu at 45' and 82' respectivly.

Ztotal = 0.08137 ohms
Vd = I * Z = 65 * 0.08137 = 5.28905
%Vd = Vd / V(E) * 100 = 5.28905 / 208 * 100 = 2.54% Vd with the above assumptions. If V(E) = 480 or the SE cable is connected differently or any of my other assumptions are different, then these values are wrong.

And I agree with George Little, there are too many assumptions.
Opps, I guess I was writting this when Roger posted, I also agree with Roger.

[This message has been edited by DiverDan (edited 09-24-2005).]


Dolphins Software
#95611 09/24/05 07:11 PM
Joined: Sep 2005
Posts: 99
T
Tripp Offline OP
Member
Well, i did give you guys my "I" in the last post, but i'm sorry i forgot to give "E". As i mentioned earlier, amperage is 50amps. Voltage is 240. This is a residental addition - power to a detached shop.

#95612 09/24/05 09:59 PM
Joined: Jan 2004
Posts: 1,507
G
Member
Well I used the NEC Table 8 and given the length of 45' #4 Aluminum and 82' of #6 copper, 240v. and 50a. and came up with voltage drop of 6.3v.

If I'm right this is less than the recommended 3% for feeders.

Someone either agree or disagree with me [Linked Image]


George Little
#95613 09/24/05 10:06 PM
Joined: Aug 2005
Posts: 44
Member
Quote:
___________________________________________
"However, since ambient temperature is a major contributor to VD, the NEC included the temperature Correction Factors to adjust the conductor size accordingly."
___________________________________________

Ambient Temperature Has Nothing to do with Voltage Drop of the wire. Voltage Drop Increases Wire temp and then you have to take that into account.
% Volts Dropped is % Power Wasted. The Customer IS Paying for the power wasted in the wires. As far as I'm concerned all we as electricians have to worry about is Volts Dropped After the Meter. Who Cares about what the POCO supplies to us at the meter. We need to keep Voltage drop to a minimum after the meter. During my apprentiship days we did alot of calculations along these lines. Upsizing wires WILL pay for itself within a couple of years, Especially with continuous loads.
Pretty much Any wire run over 100' needs to be upsized! I can't stress this any more The Customer IS paying for the power wasted in the wire due to Voltage drop. They Pay for it but don't get to use it! Think about that when you use 5% Volts Dropped. That is basically 5% tacked onto their electricity bill. Code is a minimum and in this case just reccomended, It's hard to bid jobs this way. Ain't it great being an Electrician Competing to do cheaper work!

Justin


It's Not The Fall That Kills You...
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#95614 09/25/05 06:43 AM
Joined: Jan 2003
Posts: 4,391
I
Moderator
Quote
Think about that when you use 5% Volts Dropped. That is basically 5% tacked onto their electricity bill.

That is a not really the case, a 5% VD will not directly mean a 5% increase in the electrical usage. It will not even be close, the wasted power due to VD will be an very small percentage of the total bill.

That aside I do agree the customer ends up paying for electricity not directly use by them and if they are willing to pay up front for larger conductors it is a better job overall. [Linked Image]

Bob


Bob Badger
Construction & Maintenance Electrician
Massachusetts
#95615 09/25/05 07:30 AM
Joined: Jan 2003
Posts: 4,391
I
Moderator
DiverDan

Quote
However, since ambient temperature is a major contributor to VD, the NEC included the temperature Correction Factors to adjust the conductor size accordingly

Here you are mistaken, the temperature correction factors are not in the NEC for voltage drop reasons.

The correction factors modify the rated ampacity of a conductor in order to keep the conductors insulation operating within it's design when the ambient temp is above or below 78–86 F.

[Linked Image]

Bob


Bob Badger
Construction & Maintenance Electrician
Massachusetts
#95616 09/25/05 01:16 PM
Joined: Mar 2005
Posts: 38
D
Member
I'm explaining how the wheel works here and that is not what I want to do.

I don't know who to use the quote tags so please bear with me.

From Justin:
Ambient Temperature Has Nothing to do with Voltage Drop of the wire. Voltage Drop Increases Wire temp and then you have to take that into account.

This is just plain wrong. Do some research.


From Bob,
Here you are mistaken, the temperature correction factors are not in the NEC for voltage drop reasons.

You have mistaken what I said. Voltage drop increases as ambient temperature increases. If you had ever used the above temperature formulae you would see how this happens. Conductors must be upsized to compensate for high VD. So, knowing the the NEC does not stress VD, the temperature correction factors signaled a need to develop the square of the temperature differences formula which is how the temperature correction factors are derived. All three, the VD temperature formulae, the square of the temperature differences, and the NEC temperature correction factors accomplish the same thing!

As a commentary:
Your "beliefs" do not change physics. It would only benefit your expertise to do a little more research into these subjects.

And while you're researching, lookup Neher-McGrath as their formulae was used to establish the NEC ampacity tables in the same way that the above temperature adjustment VD formulae was initially used to establish the NEC temperature correction factors.

[This message has been edited by DiverDan (edited 09-25-2005).]


Dolphins Software
#95617 09/25/05 02:18 PM
Joined: Sep 2005
Posts: 99
T
Tripp Offline OP
Member
Well, i am very satisfied with all the feedback i received and i thank everyone for their attention and furthermore i am continuing to enjoy the ongoing discussion.

the more i know, the more i see there is to know; or...so much to learn, so little time to spend on the computer. [Linked Image]

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