Voltage Drop Calculator

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infinity

infinity

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I've been trying to calculate the maximum distance on some feeders. I've tired two different apps and an online calculator and cannot come up with two values that are close. Here's my sample problem:

3Ø
208V
1600 amps
Copper
Installed in EMT
Max VD=3%
Parallel sets=4
Conductors=600 Kcmil
Max one way distance?

When using this online calculator I get up to 250' before exceeding 3%.

http://www.calculator.net/voltage-d...=250&distanceunit=feet&amperes=1600&x=70&y=18

The Southwire app gives me 239' and the General Cable App gives me 205'.

Screenshot_20180324-070742.jpg Screenshot_20180324-070628.jpg
 
M

MyCleveland

Senior Member
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Cleveland, Ohio
infinity

Here are numbers from my excel sheet I posted a while back.
Per your spec's...

PF=1, Distance = 360', VD = 3%
PF=0.9, Distance = 208', VD = 3.01%

I know the Southwire is set up using PF = 0.9....or so they say.
Based on the results, the General Cable does also and matches my result at PF=0.9

Why are you calculating at FULL capacity of feeder ?
 
I

Ingenieur

Senior Member
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Earth
using NEC value 0.047 Ohm (steel, pf = 0.85)

vdrop = 0.03 x 208 = sqrt3 x L/1000 x i x Z/4

L = (0.03 x 208 x 1000 x 4) / (sqrt3 x 1600 x 0.047) = 191.6'

if we assume 80% loading 1.25 x 190.6 = 239.5'
 
infinity

infinity

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Staff member
Location
New Jersey
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Journeyman Electrician
MyCleveland said:
infinity

Here are numbers from my excel sheet I posted a while back.
Per your spec's...

PF=1, Distance = 360', VD = 3%
PF=0.9, Distance = 208', VD = 3.01%

I know the Southwire is set up using PF = 0.9....or so they say.
Based on the results, the General Cable does also and matches my result at PF=0.9

Why are you calculating at FULL capacity of feeder ?
Click to expand...

It's the worst case scenario even though the actual load is unknown and is lower. We just have a feeder schedule and the numbers are all over the place. We might have one 1000 amp, 120' feeder long with 7 sets of 750 kcmil Al conductors and another with 4 sets and the same length. The engineer is reluctant to say that some of his calc's are wrong but in the above example there is obviously a problem.

Can you link to that thread with the spreadsheet?
 
kwired

kwired

Electron manager
Location
NE Nebraska
differences between results from different calculators probably result from use of different temperature or power factor. If the calculator doesn't give you the option to enter either of those then it uses same factor for all calculations. For power factor not entering anything at all in to the formula is going to give you 1.0 power factor results. If no temperature is requested then one would kind of assume the results will be for 30 Deg C mostly because that is the base for the ampacities in 310.15(16)
 
mbrooke

mbrooke

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kwired said:
differences between results from different calculators probably result from use of different temperature or power factor. If the calculator doesn't give you the option to enter either of those then it uses same factor for all calculations. For power factor not entering anything at all in to the formula is going to give you 1.0 power factor results. If no temperature is requested then one would kind of assume the results will be for 30 Deg C mostly because that is the base for the ampacities in 310.15(16)
Click to expand...

I agree. And for 14-10 gauge wire 1.0 PF will give the worse case drop and often what I assume when not known for general use circuits. However for larger sizes PF needs to be taken into account as its not the same and can throw the numbers way off from the real world.
 
M

MyCleveland

Senior Member
Location
Cleveland, Ohio
Infinity

"Can you link to that thread with the spreadsheet?"

I don't know how to do that. If you find it, download the SECOND upload....error in the first post.
You can PM an email address and I will send direct. Only request is feedback on operation.
You can also select bus duct instead of wire
 
M

MyCleveland

Senior Member
Location
Cleveland, Ohio
kwired said:
differences between results from different calculators probably result from use of different temperature or power factor. If the calculator doesn't give you the option to enter either of those then it uses same factor for all calculations. For power factor not entering anything at all in to the formula is going to give you 1.0 power factor results. If no temperature is requested then one would kind of assume the results will be for 30 Deg C mostly because that is the base for the ampacities in 310.15(16)
Click to expand...

I have never seen a freeware program that offered PF or temp inputs.
Thought is they are trying to be conservative and keep PF at 0.9.
 
mbrooke

mbrooke

Batteries Included
Location
United States
Occupation
Technician
MyCleveland said:
No argument...burden is on the user of the program.
Click to expand...

That is true- which is why I never trust online VD programs unless its for very rough values. Unless it goes into detail on how the # are derived. I got bit once cheating with Google- turns out the site I was using wan't even factoring in reactance. Long story short I was way off :ashamed1:
 
kwired

kwired

Electron manager
Location
NE Nebraska
MyCleveland said:
I have never seen a freeware program that offered PF or temp inputs.
Thought is they are trying to be conservative and keep PF at 0.9.
Click to expand...
The fact they don't offer those inputs means they have to be assuming fixed constant on those values. They may do so in an attempt to cover most general applications, but there will still be times that real world leads to significantly different results.

Most time we don't need to be too precise on voltage drop anyway.
 
I

Ingenieur

Senior Member
Location
Earth
am I missing something?
the pf ie 0.9, 0.85, etc is for the load, not conductor

the pf of the conductor (cos(arctan x/r)) has little impact on system pf for the v and i ranges under discussion

assuming the op's conditions
208 vac, 1600 A, 4 sets 600, emt, use 300'
Z cond = sqrt3 x 300/1000 x 0.047/4 = 0.0061 Ohm

load Z ~ 208 / 1600 = 0.1300 Ohm
a factor of ~20 difference

cable parameters from NEC table
x = 0.039 ohm/1000' (L only)
r = 0.025 ohm/1000'
Z = 0.0463
pf = 0.5397
Z = 0.0032 + j 0.0051 (300' loop)

load assume 0.9 and 0.13 Ohm
Z = 0.117 + j 0.0567

new system Z = 0.1202 + j 0.0618 = 0.1352/27.21 deg
or new system pf = 0.8893
a change of 1.2%
moot
plus this ignores cable C which would raise the pf a bit, ~0.893 range, 0.8% delta
 
infinity

infinity

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Staff member
Location
New Jersey
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Journeyman Electrician
So from what I've read in this thread so far there is quite a bit of leeway in the actual calculation of the VD number.
 
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