best approach for calculating voltage drop in parallel service conductors

Status
Not open for further replies.
Three questions in here:

I recently had a design where the civil engineer chose to place the utility transformer over 200' from the MDP. I was running over 1000A through 4 sets of parallel conductors and I wanted to verify that the long run and high amperage weren't going to create an unacceptable voltage drop.

1) Would using the calculated demand load be acceptable (it's a restaurant so the demand load is significantly lower than the connected) or should the connected load be used?

I was doubtful that the relationship between the calculated voltage drop and the number of parallel conductors was linear; that is, 4 times the cross section = 1/4 the voltage drop. So I used Chapter 9, Table 8 to get the total cross-section and then used the largest wire size that was less than that and did the calculation by hand. So my questions are:

2) What's the most accurate calculation, and;

3) What's the most practical, that is, what's the easiest calculation that doesn't introduce excessive error?

I found what I thought was a pretty decent calculator online that provided parallel solutions, but as I used it I discovered it had prohibitive limitations - it chose the number of conductors and size, and you couldn't play around with configurations. So it ended up being pretty worthless, and I couldn't find any decent calculator that let you input the desired number of conductors. If anyone knows of a really good calculator, I'd be really interested in that - especially if it was available as an iPhone app.

Thanks...
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Three questions in here:

I recently had a design where the civil engineer chose to place the utility transformer over 200' from the MDP. I was running over 1000A through 4 sets of parallel conductors and I wanted to verify that the long run and high amperage weren't going to create an unacceptable voltage drop.

1) Would using the calculated demand load be acceptable (it's a restaurant so the demand load is significantly lower than the connected) or should the connected load be used?

I was doubtful that the relationship between the calculated voltage drop and the number of parallel conductors was linear; that is, 4 times the cross section = 1/4 the voltage drop. So I used Chapter 9, Table 8 to get the total cross-section and then used the largest wire size that was less than that and did the calculation by hand. So my questions are:

2) What's the most accurate calculation, and;

3) What's the most practical, that is, what's the easiest calculation that doesn't introduce excessive error?

I found what I thought was a pretty decent calculator online that provided parallel solutions, but as I used it I discovered it had prohibitive limitations - it chose the number of conductors and size, and you couldn't play around with configurations. So it ended up being pretty worthless, and I couldn't find any decent calculator that let you input the desired number of conductors. If anyone knows of a really good calculator, I'd be really interested in that - especially if it was available as an iPhone app.

Thanks...

What I do is divide the current by the number of parallel conductors and calculate Vd for a single conductor.
 

Smart $

Esteemed Member
Location
Ohio
Total current is divisible by the number of conductors per phase (1/4 total current per conductor in your case).

1) Demand load. All your equipment minimum rating are based on this value, so actual current should not go higher ever. But if there are known future additions planned, these should be considered in with the demand load calculation.

2)...
download.aspx?folder=2d3c568d-f846-4534-885b-54a329e97b4c&file=IEEE_Std_141-1993_3.11.JPG

3) VD = I × Z
...where Z is equal to conductor length in feet divided by 1000 times the impedance to neutral per 1000 ft from Chapter 9, Table 9 for wiring method. For the typical 2-wire circuit, multiply result times two. For the typical 3Ø 3-wire circuit, multiply the result by the square root of 3. This is essentially saying the same thing as the approximate formula in #2 above.
 

mpoulton

Senior Member
Location
Phoenix, AZ, USA
Voltage drop scales linearly with conductor resistance, which goes with the inverse of cross sectional area. It's actually really easy to calculate for parallel conductors: just divide the current by the number of conductors, then run the calculation for a single conductor. You'll want to consider the peak load current that is likely to occur in reality, which may not be exactly the same as the demand or connected load calculations. You may need to consider motor starting transients if there are any large single motor loads (big HVAC unit?).
 

Ingenieur

Senior Member
Location
Earth
Doesn't really matter, all shakes out the same
divide current / # cond
divide pu length Z by # cond
calc drop and divide by # of cond
mult area by # of cond

as to load, engineering judgement
for me it depends on demand/connected ratio
if over 60% and drop < 2% perhaps ok
if <40% and drop is 3% perhaps not
or do connected and lose no sleep unless spending a bit more of the clients money bothers you
 
Status
Not open for further replies.
Top