Voltage Drop Video

[Carl Trost]

Mr. Holt,

I watched your Voltage Drop Program today. I was hoping you could explain something to me. One of the problems you went over in the program involved a 240V, 5HP, single phase motor with a name plate rating of 26 Amps. You were sizing the conductors for the given distance. I stoped the video once the question came up and worked it through. Upon restarting the tape I was a bit baffeled at why you worked it the way you did. Why did you not go to Table 430.258 to get the FLC of the motor and use that time 125% for the current? Why use name plate? Won't the resistance of the wire affect it's ability to carry the required current for the load? Are we increasing the ampacity of a motor's conductor to compensate for it being a continous load (That would be an assumption on the part of the code) or to handle the high in-rush of current as the motor starts up? Either way the conductor has to handle any current that may be applied to it. The increase of resistance due to the length of the run is the same as adding an additional load to the circuit. So would you not use the FLC x 125% in the voltage drop formula to size the conductors? Would not the conductors exceede their operating temperatures during the start up of the motor. Over time will this not damage the insulation of the conductors? I would think this would be even worse at points of termination where the resistance is highest. Working it the way I did I came up one wire size higher than you did in the problem. (a #8 and you got a #10) Both sizes were among the possible answers. Thats a lot to know to still get the answer wrong. Any insight on your methods with this would be very much appreciated.

Carl Trost
Canon City, CO

PS - Any hints on the Colorado Master exam administered by "Promissor"? If you know how does it compare to the Master Exam in the state of MI?

 

[Snorks]

Carl,

You are correct in your thoughts about conductor sizing. There are additional "derating" factors that also apply such as ambient temperature, power factor, raceway type, terminal temperature, insulation temperature, etc. But, all that said, your thought processes are 100% correct.

 

[don_resqcapt19]

When sizing conductors for voltage drop, you use the actual load, that is the nameplate, current.

Won't the resistance of the wire affect it's ability to carry the required current for the load?

Yes, but the load is the nameplate current, not the table current multiplied by 1.25.

Would not the conductors exceede their operating temperatures during the start up of the motor.

No.

I would think this would be even worse at points of termination where the resistance is highest.

The resistance should not be higher at the terminations.

Working it the way I did I came up one wire size higher than you did in the problem. (a #8 and you got a #10) Both sizes were among the possible answers.

When sizing the conductors for voltage drop, you must also make sure that they meet the code minimum. In this case that would be 125% of the table current of 28 amps. That value is 35 amps and the ampacity of #10 is 35 amps at 75?C so you can use #10 as long as the terminations are suitable.
Don

 

[charlie b]

Mr. Holt. . . .

You might as well address your questions to the membership at large. Mike seldom visits the forum, and even less often does he post his own comments. He has been a registered member since 2003, and has a total post count of 22.

Mike set up this forum, and pays for its upkeep out of his own pocket. But he lets the membership in general and the moderators in particular run the question and answer stuff on our own.

By the way, welcome to the forum.

 

[Snorks]

Don,

I disagree with a few of you comments.

"When sizing conductors for voltage drop, you use the actual load, that is the nameplate, current. "

Not necessarly. The NEC 430 tables present a "normal worst case" senerio. Therefore, sizing the conductor based on the 430 tables will establish a conserative conductor size that will be appropriate if in the future the existing motor is replaced with a different manufactures' motor with slightly nameplate values.

"When sizing conductors for voltage drop, you use the actual load, that is the nameplate, current."

Once again, are you estblishing a conductor size based on the actual motor values or a consetative NEC 430 table value.

"When sizing the conductors for voltage drop, you must also make sure that they meet the code minimum. In this case that would be 125% of the table current of 28 amps. That value is 35 amps and the ampacity of #10 is 35 amps at 75?C so you can use #10 as long as the terminations are suitable. "

There are so many factors to consider to correctly (per IEEE Std 141 Exact Formula) compute VD that this vague statement should be completely omited.

As a note:
Neher and McGrath established the current methods and supporting formulae for voltage drop computation. These formulae are the bases of IEEE Voltage Drop formulae Standards and the NEC ampacity tables.

 

[don_resqcapt19]

Snorks,

Don,
I disagree with a few of you comments.

"When sizing conductors for voltage drop, you use the actual load, that is the nameplate, current. "

Not necessarly. The NEC 430 tables present a "normal worst case" senerio. Therefore, sizing the conductor based on the 430 tables will establish a conserative conductor size that will be appropriate if in the future the existing motor is replaced with a different manufactures' motor with slightly nameplate values.

The orignal question and my comments are based on the minimum requirements of the NEC. There voltage drop conductor sizing rules in the NEC. You can choose what ever method you want to make that calculation. In most case the nameplate FLA is use for this purpose if it is known.

"When sizing the conductors for voltage drop, you must also make sure that they meet the code minimum. In this case that would be 125% of the table current of 28 amps. That value is 35 amps and the ampacity of #10 is 35 amps at 75?C so you can use #10 as long as the terminations are suitable. "

There are so many factors to consider to correctly (per IEEE Std 141 Exact Formula) compute VD that this vague statement should be completely omited.

My comment is correct...the minimum size conductor for this load is a #10 assuming that the conductor and the terminations have a 75?C or higher rating. This comment is also saying that in the case where the voltage drop calculation would permit the use of a conductor with an ampacity that is less than 125% of the table FLA, it would be a code violation to use the smaller conductor. In this example, as is often the case, the code minimum conductor will also provide an acceptable voltage drop.
Don

 

[iwire]

snorks.

Why 125%?

What significance does that figure have in figuring voltage drop for a motor?

A typical motor will have a starting current of about 6 to 8 times the FLA if the circuit conductors can deliver it.

So if you want to figure voltage drop for start up you may have to use 700% of the FLA.

It's all a design consideration.

 

[Snorks]

Hey Carl,

I'll run this scenario through Volts Electrical Design Software and see how it compares to your computations.

I am assuming:
125% of load
100% continuous load
Terminal temperature at 75?C
Induction type motor

I need the following:
Conductor material - Cu or Al
Insulation Type
Ambient temperature
Raceway type
Voltage
Phase
Ampacity value (either from the nameplate or the 430 tables)
Distance
Voltage Drop limit (5%, 3%...)
Any other conductors that will be in the same raceway

Let me know and I'll have your conductor sized per voltage drop limits in a couple of seconds.

 

[roger]

Snorks, forget Volts, this is an exercise for exam preparation, Volts (or any other software for that matter) will not be allowed in any exams I know of.

Neher-McGrath formulas are not needed for this exercise either, although included in 310.15(C), if you read the heading of this subsection you will see this particular equation (or any of these calculations or equations) does not apply to us electricians nor are they needed.

Roger

 

[Carl Trost]

I will have to go back out to the fortress of solitude and check my notes to get all the numbers given in the problem. I will try to remember to bring it back in with me after my next study session out in the FoS.
I am busting my brain getting ready for the CO Master Exam. I have a Master license in MI but was not able to get a license by endorsement in CO because I tested under the 1999 code. If I had tested under the 2002 or 2005 code I would have been golden. I meet every other requirement in spades.
Being new to Colorado I don't know very many people. I have yet to meet anyone who has taken the CO Master exam. I have no idea what to expect. I took a local test prep course, twice in fact, but it was very loosely run. 10minute breaks turned into 20+ minute breaks. A bit to much time was spent on tangent conversations. At the end he had to just brush over an area or two that I am not fully versed in. I went the second time hoping the first was just a fluke. No ah ah. I took an excellent test prep class in MI, so I know how one should be done.
Nobody seems to know how the CO exam is graded or even what the passing grade may be. There are very well sourced rumors that the test is broken up into "areas" or "disciplines" and that you must get a passing percentage in each "area" to pass. So if they have only 3 questions on motors and you miss one........ to bad, see you next time you have another $73 to take our test. This can not be confirmed. At least those in the know are being very tight lipped about it.
SO I am really sweating the details in my preperation for this one. I knew lots of people who took the test in MI, so I knew exactly what to expect and I studied as to those expectations. But here I just don't have that confidence that I know what I am up against.
The test is administered by "promissor". Anyone out ther who could point me in a direction or help restore my confidence with a little first hand knowledge?
This is turning into an interesting thread. I hope I did not take it to far of topic.

Carl

 

[resistance]

Ok, what happen to the OP

 

[George Stolz]

Carl, welcome to the forum, and to Colorado.

Being new to Colorado I don't know very many people. I have yet to meet anyone who has taken the CO Master exam. I have no idea what to expect.

I have not taken the Master's Exam, but I have taken the CO Residential Wireman, and the Journeyman exams.

They are all basically the same test, only a bit more difficult as they go. When you register with the state to take the test, you will receive a brochure in the mail giving you percentages of what types of questions will be on the test.

If I remember correctly, the RW leans heavily on 210, Chapter 3 wiring methods. The JW leans heavily on 430, 250, 450, with enough Chapter 8 questions in there to throw you off.

The master has a reputation for voltage drop questions, load calculations and grounding. The percentages given in the brochure would be helpful, but I don't have mine anymore.

There are very well sourced rumors that the test is broken up into "areas" or "disciplines" and that you must get a passing percentage in each "area" to pass. So if they have only 3 questions on motors and you miss one........ to bad, see you next time you have another $73 to take our test. This can not be confirmed. At least those in the know are being very tight lipped about it.

That's not true to my knowledge. If anyone fails the test, they are given bar graphs with no numbers to show where they were weakest. These are broken down by topic (motors, grounding, calculations, etc) so that is probably the source of the rumor.

I think it's 110 or 100 questions, and you have around 4 or 4.5 hours to correctly answer 70% of them, and they throw ten questions out the window without grading them from whimsy. The brochure lays this out exactly.

You will not know your percentage whether you pass or fail.

You bring in nothing except a calculator, they provide pencils, the NEC, scratch paper, and that quietly stressful testing environment.

I'd study, study, study, and go take the thing. If you're as studied up as you seem to be, you shouldn't worry. (Between you and me, I didn't get time to study (aside from forum time) before taking the JW, and still passed. Take Mike Holt's free exam before taking the real thing, it'll have you in the right mindset. )