Engineer Boards

# Voltage Drop

## Recommended Posts

I have some extra time today and decided to compare all the various voltage drop methods (there are a few)...both single & 3 phase problems.

Methods are Ugly's handbook, Bussmann chart, and of course the PE NCEES method using Chapter 9, Table 9. I wanted to see if there were differences...and the results were interesting.

Here's what I came up with:

Problem #1

Single Phase, 240 Volt, #8 THWN, 30 amperes, 150 feet

Ugly's 2008 handbook:

Vd = (2 x K [copper constant] x L (distance) x I ) / circular mills

= (2 x 12.9 x 50 x 3)/16,510

= 7.03 volts

Bussmann: 150 feet x 30 amperes = 4500

4500 x 1520 [100% pf, single phase for #8 wire] = 6,840,000

Divide by 10^6: 6.84 volts

PE method: Z = (.78 + j.065) * ((150 * 2)/1000)

= .234 + j.0195

I * Z = 30 (.234 + j.0195)

= 7.04 volts

So far so good...all 3 yielding similar results.

3 phase is next and this is where the fun begins.

##### Share on other sites

Now I'm taking problem #129 from the Power NCEES problem set:

Three phase, 480 Volt, 500 kcmil, 400 amps, 250 feet, .8 lagging pf

Ugly's:

Vd = (1.73x 12.9 x 250 x 400)/500,000

= 4.47 volts

Bussmann:

250' x 400A = 100,000

100,000 x 85 [bussmann table, 500kcmil @ 80% pf, 3 ph] = 8,500,000

Divide by 10^6:

8.5 volts

PE method gives 9.72 volts....I can type it out if people want.

A bit of variance here....makes you think a little.

What's annoying is the company excel spreadsheet seems spot on with Ugly's...which comes out low. Ugh!

Maybe I made a mistake (doubt it since I checked it over a few times)....but something is up.

Any ideas?

##### Share on other sites
Any ideas?

The second line on the page: "Inductance Negligible"

##### Share on other sites
Any ideas?

The second line on the page: "Inductance Negligible"

Sorry, I don't understand.

##### Share on other sites

PE method, Ohms law, all the way, all the time. Ohms law is all that is required.

##### Share on other sites
PE method, Ohms law, all the way, all the time. Ohms law is all that is required.

Makes me appreciate my degree that much more.

BTW, power factor seems to be irrelevant to the actual value of voltage drop (who cares about angle?) but the Bussman tables seem to think they matter.

Interesting that they could make such a mistake.

##### Share on other sites

^The thing to remember for things like the Bussman tables is that they are providing direction to select a protective device. The numbers they generate for the tables will be generated ignoring details that, while affecting the actual numeric answer, will not affect which device you select.

##### Share on other sites

In smaller gage cables and shorter runs resistance is what will cause voltage drop so the higher the power factor the more voltage drop you can expect. Take for instance the IEEE Red Book's approximate formula V=IRcos(angle) + IXsin(angle) the higher the angle the smaller the pf the less percentage of IR contribution and the more percentage of IX, but since X is low for small gage short runs the voltage drop will be small. At 500kcmil the reactance is higher than the resistance so smaller pf's will have a higher voltage drop. It's not until 300kcmil where the reactance starts passing the resistance.

The Ugly's book is intended for residential and small commercial building applications where resistance dominates and power factors are typically high. Certain references in the Ugly's book is intended for on the fly calculations which is not acceptable for Engineering applications.