Voltage Drop & Cable Sizing

Since your question indicates that you know very little about this type of work you should consider hiring an engineer who does. It is unethical and costly to your client to get paid for work you are unqualified to do.

This is not for doing a job. I just want to know the most accurate method. The only derating I consider is of temperature. How much of an impact can the other factors make? Like soil resistivity, being laid in air or ground, etc make?

Sounds like homework, or a job interview. You just asked another elementary question here.

Guide to Construction of ArTu switchgear - ABB
So, if you are interviewing for a job, don't bother. If you have a job, quit it and get a job you are qualified to do.
If it's homework, read the book.

If all that fails, use a search engine, like everyone here does to get information.

The search engines do not provide the answers I expect.

So, you expect someone else to do the work for you?

You obviously don't know How to Use a Search Engine to Search the Internet

The answers to your question are easily found. Just as your other question demonstrates a lack of knowledge of the subject, learning how to search will solve these elementary problems.

I was hoping to get a reliable answer from people with good enough knowledge on the subject.

As I said, there are numerous tables, documents, specifications and manufacturer's data that freely and concisely provides the information you are incapable of finding.

I'm not going to waste any more time on you, or this thread!

<unsubscribe>

If you refuse to do the homework, you won't understand the answer.

If you understood high school level electricity, you'd know that the attributes of the conductor and load will determine the voltage drop.

Now do your homework.

My oh my! Somebody here is having an OT temper tantrum. The irony of this entire thread is that the OP posted a table that includes the voltage drop. If you blow up the image and read the blurred title text for the second to last column it states "Voltage Drop mV/A km". [At least I think the length is in kilometers.]

As another CR4 member said earlier, "You can take a horse to water. But you shouldn't be expected to suck on the horse's anus to make them thirsty."

ROFLMAO!!!

OP has a partner in whine.

Considering the blurred table, the question, the apparent unwillingness of the OP to take the answers (many good ones) and apply them, the attitude of the OP and a couple of other things I won't mention here the answers are fitting for the question and the OP's comments.

As you have been told several times, search for an answer on how to do it. If one engine doesn't help go to another one. If that doesn't help go to another one. There is more than Google out there! We want to help you but a bad question and a bad attitude are not conducive to having others do your homework or the work problems.

Good questions begat good answers, bad questions begat bad answers.

Oh, by the way, I know how to get an answer for you but this is probably going to help you more. Although if you don't read it and utilize it your not going to get any other answer than "6.28"

Good Luck, Old Salt

One thing everybody assumes is that I do not understand basic electrical engineering. The only thing I wanted to know is that how much of an impact can factors like soil resistivity, conductor spacing etc. make in real life. The more feedback I get from people who are experience in field as well as in theoretical things, the better it is.

I wasn't just posting a picture and saying, Hey people! tell me how to calculate voltage drop.

If that is what you wanted then you should have asked it that way. No one can guess what you want, you have to express it clearly. Good questions get good answers, bad questions get bad answers.

Something to the sort of "What affect do _____, _______, _______, and _______ have on the _____, _______ and ______ in terms of the XXXXXX?

Most of us are not mind readers and can not read what is not there in the question. Me, I can't even read my own mind let alone someone elses!

Good Luck, Old Salt

Every cable book written and every cable manufacturer offers documents on spacing, depths, soil resistivity(wet and dry), soil thermal ratings, (geotechs), proximity to other buried services. You current best bet to save yourself much taunting is to do your research with some documents from libraries or cable supplier.

Nexans, Brugg, Prysmian, Pirelli, NKT (Ericsson), Draka, Cable Corp of India, Tihaan, Parker Scanrope, they all have the ready made data you seek.

"We shall find peace. We shall hear angels. We shall see the sky sparkling with diamonds. And we shall know cables are rated and installed properly" -- Anton Chekhov & IQ

Ok you deserve an intelligent helpful reply.

The other variables you mentioned have a lot to do with the temperature, ergo derating.

Read this, play with this and consider buying this.

Sorry to say,

OP has already gotten intelligent, helpful replies to their question, "I need to calculate the voltage drop".

If they don't know how to use them, is that our fault?

Your links give cable sizing tables, the Wiki link directly answered OP's request.

This is just another poster who doesn't want to do their own work.

Then read and apply BS7671. Many people have applied good knowledge in order to compile and publish it.

That'd be a big problem!

Live not always holds what we expect!

Websites telling stuff that we do not expect!

What you need to do (since you cannot change the web sites) you need to change your expectations! Further I suggest to free up your mind and dont expect anything but rather read and digest! Learn and do not form your knowledge about your expectations.

You have it the wrong way around.

It's all in BS7671.

Cable sizing is one of CR4's most frequent topics, so reporting this thread as a duplicate is very tempting.

The chart is dimmed out and unreadable (a common graphics problem here).

Wire resistance increases slightly with its temperature, but I haven't seen this factored into normal voltage drop calculations, which depend on load, wire resistance, and number of phases. Basically just Ohm's Law.

As you know, the temperature will far more effect the breakdown voltage of the insulation than the voltage drop.

But a 20 degree change in temperature seems to have a derating effect of upto 18%

On ampacity, yes; but not significantly on voltage drop.

Consider this: At 30 degrees, 380v, 0.8 pf, 4% max. allowed volatge drop, 210 kw over a 200m length can have a voltage drop of max. 0.191 mv/A/m.

What if the temperature is 50 degrees? How will the derating factor come into play?

It doesn't*; those are separate topics, and you are trying to conflate the two.

*Except for the very minor resistance increase in the wire.

So, the cable length and load current are the only factors that needs to be taken into account?

After resistance, yes.

What about ambient temperature and method of installation? It's all in BS7671!

Hi!

1) Nice data

2) Why would I solve the whole thing for you? I mean what is it for me?

3) Lyn has already explained it to you.

4) Your uploaded chart isn't even readable...

5) People get to pay thousands of dirhams attending the ADDC exam and pass it, then why to give out paid info to you easily?

6) It is just as easy as anything to solve it

7) It might already be or similar examples be done on this forum, just need to spend sometime...

8) Etc.

:)

Read the rest of the posts... You have misunderstood what I want.

You have no clue.

You have no clue how to search for an answer.

You have no clue how to ask for help.

You have no clue what you are doing.

You say:

"I need to calculate the voltage drop"

"The only derating I consider is of temperature"

"soil resistivity, being laid in air or ground"

"The search engines do not provide the answers I expect"

"I am looking at making an excel sheet taking into account all parameters"

" the cable length and load current are the only factors that needs to be taken into account?"

"Read the rest of the posts... You have misunderstood what I want"

Nobody can understand what you want.

<unsubscribes shaking head wearily>

>>"Read the rest of the posts... You have misunderstood what I want"

>Nobody can understand what you want.

><unsubscribes shaking head wearily>

Lyn, thank you! You have very good insight to it. I would like to put the two important factors need to be considered in such calculations for your kind attention only. These factors are: 1) Temperature factor 2) Grouping factor. Temp. correction factor for cables is self explanatory and the grouping factor means the number of cables are installed close together in a run where mutual heating effects may be taking place, and the "method of installation" like for e.g enclosed in a conduit (surface or buried), on cable tray touching, etc.

Thanks in advance,

From your original post, "...I need to calculate the voltage drop..." Since you are acting as a "handbook engineer" here's how it's done:

1. Choose a wire size,

2. Choose a location (air or buried),

3. Choose a current,

4. See the voltage drop.

5. If you don't like the voltage drop, choose a different wire size and repeat.

6. Sooner or later you will come up with a combination that suits your (unstated) needs.

If this doesn't meet your criteria why don't you restate exactly what you want.

Could you put that on an excel sheet for OP?

I know, I know, I said I was leaving.

Let's face it, OP doesn't quite get the fact that the data in front of him comes from a spreadsheet, duh!

Take a look at the two posts together, posted at the same time, and Blackpaper is an Arc Flash disaster waiting to happen.

What I wanted to know was whether factors like ambient temperature and method of installation can have any impact on voltage drop.

Yes, weather and temperature can make an impact on the voltage drop across a long distance. Particularly the effect of corrosion from a poor installation connection but this should be considered a faulty configuration. There are additional non-linear effects (corona discharge) that weather, temperature and even barometric pressure can change with the higher voltages used for long distance power distribution. Unfortunately, you have not demonstrated here any grasp of the fundamentals of local power distribution losses at lower voltages.

You also do not show respect for our knowledge and experiences. You seem to believe this should be trivial information available for free. It is not free for a very good reason. A misunderstanding or misapplication of power distribution systems can easy destroy property and kill innocent people. This is why electricians and electrical engineers require a bonded license to do this kind of work.

Then you should have asked that question upfront. Unfortunately you are trying to blend two separate but related concepts.

The unqualified answer is no they don't; a single piece of 12ga. wire 100ft long, subjected to 10A of normal household AC voltage will have the same voltage drop whether it is hung in air, submerged in water, buried in dirt, etc., the only effect will be from the small change in resistance as the temperature changes (usually ignored in calculations that engineers do).

However the Ampacity does change significantly; if 50ft of the same wire is in still air and 50ft is buried, the voltage drop is the same, but the Ampacity is limited to that of the lower of the two conditions.

Ampacity is more of a mechanical engineering concern, how much does the current flow heat up the wire vs. how fast can the heat be removed from the conductor; whereas voltage drop is more of an electrical engineering concern, how much does the current flow through a resistance change the end to end voltage.

n.b.- I'm ignoring all the "stuff" about insulation types, nearby conductors, conduits, cable trays, mutual coupling, conductor geometry, etc., which is clearly beyond the scope of a handbook engineer's understanding anyway.

Why would ampacity be a mechanical engineering concern? Wouldn't a larger size cable be required if the derated ampacity is lower than the load current?

It's a mechanical engineering concern in its calculation since it's primarily a heat transfer problem. The ME sees a mass with certain properties at a constant temperature, surrounded by insulation with different properties, immersed in a conductive medium with yet another set of different properties and temperatures, and starts calculating the rate at which heat can be conducted away and the resulting temperature profile.

He's not concerned about volts, amps, resistance, etc. or anything electrical; for all he cares it's hot freon is passing through a copper tube buried in a trench giving off its heat from the high pressure side of a mechanical compressor driven by a steam engine, the calculations are virtually the same. And yes, if the end result is that the temperature is still too high he'd have to decrease the energy input. You have to admire how dualistic Mother Nature is.

The problem may be mechanical in nature, but the electrical engineer will have to resize his cable if the heat dissipation is not fast enough...

Will you need help with that, too?

Lyn writes:
>Will you need help with that, too?
I have already written in response # 53. If you think he needs help in calculating, then I would help him in calculation. He must put up the method of installation he would be applying and the number cables grouped along with this one...He would know the number of cables run there already for other services than the cable would be run for motor etc. :)

One last thing. Why is there multiple values for the current ratings and voltgae drop for the same size cable. The chart is based on a constant temperature and soil thermal resisitivity.

Because they are not the same cables. Cables do not fail because of just the conductor cross sectional area. They fail because the cable insulation has broken down. Insulation will fail from heat, abrasion, mold, vermin, oxidation and a host of more complicated reasons.

This is why the wiring code is not based on a theoretical normal operations conditions but failure analysis of real world conditions.

I did not say this before, I should have said this. I will now.

Stop trying to figure out a spreadsheet algorithm for any wiring code! By your own admission, you have no comprehension of the circumstances that generated these codes or what the codes themselves mean. The codes are complicated and convoluted because the world is complicated and convoluted.

Most importantly you and your spreadsheet do not have any authority to define the wiring needed in a location. Anyone following your spreadsheet may choose the wrong wires and kill innocent people.

STOP!!!!!

300 mm2 cable has values ranging from 0.185 to 0.174

You do not even know how to ask a concise question.

Stop

what is it that you dont understand?

Why the questions are being asked when a full protocol exists in standards such as BS7671.

If you are going to quote from tables then either post them or provide a link to them...aka citation.

Then "we" might, maybe, possibly... be able to help you bridge your comprehension gap.

I am not going to google your citations for you....that i$ WORK.

http://wikisend.com/download/689560/XLPE_Insulated_Low_Voltage_Cables_2012.30.pdf

Well before this link disappears into a bit bucket I'll attempt to answer the question you probably meant to ask as complete as I possibly can.

The ±6% deviation in voltage drop per ampere length values correlate with a broad ±47% change in the current flowing through the cables. This is probably most likely due to a self heating effect that changes the conductivity of the metal. For an understanding of how the temperature of solid state material can change the conductivity I recommend that you study this tome.

You should notice that different nominal currents through the wires produce the same voltage drop relationship depending on the cable being buried or not. This is because heat transfer of a buried and suspended cable is different. The earlier tome does explain very well the phonon effect in solids. Fortunately for you there are an easier set of equations for conduction through a solid. You should remember that these equations apply to both the self heating conductors and the just heat transferring insulators. The same heat transfer to a solid set of equations can also be applied to the earth around the buried cable but cannot be applied to the suspended cable. Natural convection comes into play here because the very low mass of air rises to take this heat away. The physics theory of fluid mechanics and heat transfer to a compressible fluid is even more complicated than the phonon theory presented earlier.

Now that I have "cast these pearls before...." STOP trying to derive a cable selection spreadsheet. Copy with citation the tables of a reputable code into your spreadsheet. Remind the users of your spreadsheet that you provide only a guide. They are responsible for meeting their local code standards.

I am paranoid. I am lonely. Am I in a Sometime now?

I need to point this out as it is important for all the electrical folks to know. I am in the cable business, MV, HV EHV. You may not have noticed but insulation thickness's are reducing in wires and cables, due to 'improvement' in polymers. (of course it has nothing to do with costs, dont be crazy). Where a 1200mm2 cable at 132kV had 22,5 mm of insulation thickness, you may note that it is down to 17.6mm and possibly 13mm thickness over the core. So this will mean that the existing tables will change and each country may have differing values, depending on their installation codes which vary from country to country. feel the house wiring and you will notice a doifference in the feel of the insulation and if you look a bit closer, you may even notice it is thinner.

An IEEE standard is fine in the Americas, of no value in Africa or Europe. The IEC standard is of value in Europe and the same IEC standard has different values in Africa. So Blackpapers spread sheet is only of value to him in his own country. I advise using the values as guides only as thermal resistivity of soil, both wet and dry affect his cables, the thermal rating of the air, the buried depth, the type of duct used, the proximity to other circuits, the type of laid formation, (flat, trifoil), buried, suspended in air etc. etc.

My advice, use the cable manufacturers tables, the latest tables, as they have done all the hard work for you and offered a pretty good guide based on their cable designs and materials used for the insulation, sheath, armouring and conductors. I think you might find many agree and will advise you the same.

My work here is done! I am off to Sometime, Somewhere, Somehow and I might go with Someone.

Still no answer for the different values.

This seems to be pretty clear.

But what could be the reason for multiple values?

Read man!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

and then,

read some more!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

Eric Idle; Galaxy Song. Last words before he returns to the fridge.

https://www.youtube.com/watch?v=44DlSj6bnn4

There are none so blind as those who will not see....that's why....

This was a lot more than what I asked for... But thank you so much anyway. Things are pretty much clear now.

It's all in BS7671.

BS7671 explains it.

I give up.
Voltage drop - Wikipedia, the free encyclopedia

lyn Do you sometimes get the feeling that you're flogging a dead horse?

Refer BS7671

BS7671. The IET's licence to print money. They must be running out of funds, the 2^nd revision is due June 2014 and come in to force January 2015.

Sorry gentlemen, even though I'm a member of the IET (Institute of Engineering and Technology) I'm disgusted at the frequency new revisions and guidance notes appear.
The average domestic electrician must have access to the current regulations. From reading the latest revisions nothing really affects them, but they still have to pay near £200 for the set. The IET has a bad habit of changing regulation numbers so addendums are impractical.

Sorry for the rant.

Why BS7671 is limited to buildings or homes or offices,do they have separate regulations similar to BS7671 for industries/factories?.

BS7671 is not limited. It is applicable across the universe.

dear

are many ready made software available to solve this issue.

mainly you need to calculate the cable size based on current.

insulation based on voltage .

voltage drop based on length

current rating based on temp. ( ie open or closed atmosphere )

type of insulation based on environment like XLPE or PVC etc.

normally all this read made software have ready to use program. just incent the cable sixe and length and voltage it will give rest of all

one of the software I use is bus bar calculation

I am looking at making an excel sheet taking into account all parameters,

I feel a headache coming on

What happens when trying to educate the inane. Some refuse all genuine efforts.

Which tool is best for this job: Maxwell's silver hammer, or Gallagher's watermelon mallet?

Happy to swap some Unicorn poop for a watermelon mallet. Deal. No Deal!

"I am looking at making an excel sheet taking into account all parameters,"

For whom is this excel sheet intended?

It's possible, but I don't know why you would want to.

First you need to purchase the relevant cable and equipment standards as the bulk of the derating factors for different situations can be found in there. Additional information can be found from major cable manufacturer's websites (which again reference those standards).

Go for it.

There are many factors to consider.

Read the Standards, IEC, IEEE, GIGRE, ASTM, BS, etc, and the cable manufacturers documentation. It is all clear and understandable and if you have the page showing this Table 31, you should have the entire document,.....to read.

Dear Mr. Blackpaper

The table given by you is not readable. Pl. provide the link so that we can open or provide in word doc.

Pl. open the following links and read. I am sure you will be benifitted and your doubts are cleared.

1. http://electrical-engineering-portal.com/an-example-how-to-calculate-voltage-drop-and-size-of-electrical-cable

2. www.electrical-engineering-portal.com/cable-sizing-of-sub-main-circuits-working-examples

3. http://electrical-engineering-portal.com/cable-sizing-of-sub-main-circuits-working-examples

4. http://cr4.globalspec.com/thread/99414/LV-Cable-Sizing

5. Cable Sizing Calculation - Open Electrical

• An example how to calculate voltage drop and size of ...
• Cable Size Calculations | EEP
• Worked example of cable calculation - Electrical Installation ...
• Cable Sizing Calculations to IEC and NEC - Cablesizer ...

6. http://www.electrical-installation.org/enwiki/Worked_example_of_cable_calculation

DHAYANANDHAN.S

If in doubt, consult a qualified local Electrician.