Voltage Drop Calculator

[ramsy]

Max distance is 167'. This number was derived using ETAP and the parameters above.

ETAP is a black box, which won't estimate conductor temperature, and my NEC tables adjustments would need a 50% Power Factor to match @ 167 feet.

Please disregard Power factor comment, forgot to set Voltage to 208.

Now got 167 feet at 0.85 power factor, same as ETAP

 

[MyCleveland]

because it will only take a minute

v / z total ckt = i should = 40, not 36
that is where the difference phasor vs nec eff comes from
humor me
v/i = z total = z load + z line

you have v/i = z load = z total + z line or z total = z load - z line

40amps is the IDEAL number with Zline = 0.
Converting 40 into Zload.
Vdrop of line in our exaggerated example show how the series Z will impact actual load Voltage...Vload=Vcct-Vline.
Taking this into account the 40amp is no longer accurate...the 35+/- is.

The Zeff vaule is ineffective at PF of 1....as PF drops the percent error drops between Zeff calc and Zcct calc.

 

[MyCleveland]

Please disregard Power factor comment, forgot to set Voltage to 208.

Now got 167 feet at 0.85 power factor, same as ETAP

Believe he was using PF of 1....so its not the same.

 

[ramsy]

Believe he was using PF of 1....so its not the same.

I believe 0.85 is standard.
Point-to-Point Short-Circuit Analysis must set PF=85%, where NEC Tbl.9 impedances are converted 1/(Z per foot) to "C" table results, published by IEEE Std. 241-1990, IEEE Recommended Practice for Commercial Building Power Systems.

 

[Ingenieur]

40amps is the IDEAL number with Zline = 0.
Converting 40 into Zload.
Vdrop of line in our exaggerated example show how the series Z will impact actual load Voltage...Vload=Vcct-Vline.
Taking this into account the 40amp is no longer accurate...the 35+/- is.

The Zeff vaule is ineffective at PF of 1....as PF drops the percent error drops between Zeff calc and Zcct calc.

your load i is 40 amps NOT 36

you need to adjust so ckt i is 40 (or whatever you plug in)
you want v drop at 40 not 36
if motor fla is 40 and pf 0.88 your drop is calc'ed on 36
in reality for a motor if v drops i increases making matters worse
changes will take a minute

v/i = total ckt z = load z + line z
you know total z since you spec v and i
you know line z from tables
calc load z to achieve 40 A

you want v drop not i drop
humor me

z eff is easily adjust for pf

 

[MyCleveland]

Ingenieur & ramsy

If I have a 10A-1200w-120v heater at the end of a 300' #10awg...will the cct amps = 10amp ?
We cannot force the load Z in order to model the cct at the loads RATED ampacity.

If the 0.85 is an industry standard, then the entire discussion is moot.

 

[Ingenieur]

Ingenieur & ramsy

If I have a 10A-1200w-120v heater at the end of a 300' #10awg...will the cct amps = 10amp ?
We cannot force the load Z in order to model the cct at the loads RATED ampacity.

If the 0.85 is an industry standard, then the entire discussion is moot.

you size for rated nameplate load, in amps/volts, pf, kw, kva
not actual operating
not sure why you are so reluctant
change 1 sign in 2 cells

 

[ramsy]

Ingenieur & ramsy

If I have a 10A-1200w-120v heater at the end of a 300' #10awg...will the cct amps = 10amp ?
We cannot force the load Z in order to model the cct at the loads RATED ampacity.

Got 6.24 Volts Dropped @ Pf=1, no information for cct amps?

If the 0.85 is an industry standard, then the entire discussion is moot.

The NEC does offers an alternative to Table-9 values for Z, and its 0.85 limit below:

310.15(C) Engineering Supervision. Under engineering supervision, conductor ampacities shall be permitted to be calculated by means of the following general equation:

I = √[(Tc-Ta)/(Rdc(1+Yc)Rca]x1000 Amperes

where:
Tc = conductor temperature in degrees Celsius (°C)
Ta = ambient temperature in degrees Celsius (°C)
Rdc = dc resistance of 305 mm (1 ft) of conductor in micro‐ohms at temperature, Tc
Yc = component ac resistance resulting from skin effect and proximity effect
Rca = effective thermal resistance between conductor and surrounding ambient

 

[ramsy]

NEC 310.60, for Conductors Rated 2001 to 35,000 Volts, a different formula is required under engineering supervision, which uses ΔTd = dielectric loss temperature rise, which Mivey used in his equations.

 

[mbrooke]

Rather than working with Black Boxes, a community spreadsheet project may help settle this.

Lesser efforts get certified under open source licenses, are more accessible than ETAP, and more usable than NEC idiot tables, where bending over backwards --for cross referenced adjustment factors-- makes casualties of Motor loads, Termination Temperatures, and Voltage Drop spec. limits.

But there may be a reason for that as often its better to assume worse case. For example, the voltage drop itself changes the theoretical current draw of the load. A restive load will draw less current with VD, however other loads may pull more creating greater actual drop than a 60*C conductor assumption does.

 

[Ingenieur]

depends on how you do it

assuming total L = 600'
Z line = 0.72 + 0.0378 j ohm = 0.721/3.00 deg
Z load = 120/10 = 12 ohm
Z total = 12.72 + 0.0378 j = 12.72/0.17 deg or pf 0.999996

i = 120/0 / 12.72/-0.17 = 9.434/0.17
v drop = 9.434/0.17 x 0.721/3 = 6.8 v or 5.67%

or
Z eff corrected to pf of 0.999... = 1.2 cos 0.17 + 0.63 sin 0.17 = 1.2002
600/1000 x 1.2002 x 10 = 7.2 or 6.00%

or using only ac R since load pf is 1
600/1000 x 1.2 x 10 = 7.2 or 6%

lol

 

[mbrooke]

^^^ Haven't though about it like that in terms of including the load Z in the equation itself. This yields a closer value to reality in that it basically put the power factor in correct proportion?

 

[MyCleveland]

Ingenieur

Of course your prior recommendations are correct....so much for my logic.

Working on IEEE 141 calculation method.

Will come back if I get it operational.

 

[Ingenieur]

Ingenieur

Of course your prior recommendations are correct....so much for my logic.

Working on IEEE 141 calculation method.

Will come back if I get it operational.

it is the only way to learn
by working thru things you gain understanding

I've learned more by being 'wrong' (perhaps not the right word, there is no wrong when trying, NOT trying is wrong) than when I was 'right' (as you see there is no black and white answer, that is where your professional judgement comes into play)

your spreadsheet is a good effort...keep fine tuning
for <1000 vac bldg wire it will be great

 

[mbrooke]

Ingenieur

Of course your prior recommendations are correct....so much for my logic.

Working on IEEE 141 calculation method.

Will come back if I get it operational.

Cool You will be a hero to many.

Once finalized I think we should make a sticky with that and Smart's spreadsheets.

 

[ramsy]

..we should make a sticky with that and Smart's spreadsheets.

Anyone have a lnk to Smart’s work?

 

[mbrooke]

Anyone have a lnk to Smart’s work?

I can't find the exact threads at the moment however I will continue hunting.


Here is one of them in XLS format (hoping it will work)

Pass word for editing is (should be) abracadabra.

 

[mbrooke]

Here is another one:


http://forums.mikeholt.com/showthread.php?t=161205&page=3&p=1562971#post1562971

 

[ramsy]

Here is another one:..

Thank You

 

[mbrooke]

Thank You

No problem