Is Bus-Bar Size Related to Connecting Cable Size?

Perhaps the instructor wants you to make an educated guess.

Maybe you should ask him if the bus bar is properly sized.

That's what happens when you miss lectures.

Go to class!!!

the question was not asked by any instructor and too bad i can not go back to school at the moment.

Actually i do not have experience in conductor sizing for such huge currents.

What i would like to know dear Iyn is, is there a specific calculation method or industry standard that can be applied to the case or will this be just guess work. And one more thing while sizing cables for much smaller loads we consider a few factors like conductor, Voltage drop, ambient temperature, protection, laying style etc etc, will all these factors remain same for such high current values and even relatively HIGH voltages.] or some new ones will come into play as well.

Of course i don't expect anyone to take me through the entire process step by step on this forum but some guidelines can be provided.

If you do not have experience, how are you possibly involved in cable sizing?

Your lack of experience and education indicates that you have no business sizing anything.

The entire set-up very likely will explode in a ball of flames.

Where is the project, so that we may all avoid the location?

There are many industry standards, depending on where you are, that cover this.

YOU SHOULD KNOW THEM!!!!!!!!!!!!!

<un-subscribes in disgust>

Some guys here know how to do the elementary stuff without asking how.

I doubt asking being a serious offense.

No, but when people, like you, waste time asking questions whose answers they should already know, it does detract from the technical interchange of those with real problems.

You just want someone else to do the work for you.

What do you mean 'should already know'?

You demonstrated Lyn's point. You do not understand a simple sentence.

Why don't you do something useful by answering the question rather than arguing with me?

Useful to whom?

the asker

This is not a free tutoring service, as you seem to believe, based on your previous questions and your interference here.

It is the "ENGINEER'S place for news and discussion".

It is not a place where arrogant upstarts are well received.

So, do something useful, for once, and answer the "asker's" question if you can.

Why bother to respond to the arrogant upstarts' questions?

That's not a very precise answer. Who do you consider to be the "asker"?

You have any plans on affecting my beliefs?

I'm not even sure if you have beliefs. Frankly, I do not care if you do or do not have beliefs.

I guess that you only demand answers but will not provide any when asked. That's useful to know.

I couldn't guess that was a serious question :O

"Do not argue with an idiot. He will drag you down to his level and beat you with experience"

Never argue with a fool, onlookers may not be able to tell the difference."

http://cr4.globalspec.com/member/61832/SArsalan

is there a specific calculation method or industry standard that can be applied to the case or will this be just guess work

GUESS WORK?????!!!!!!

In small electrical work a trained electrician take clue from cable manufacturer's catalogues and technical papers to decide the size of cable.

The selection of cable for any large electrical project is not a simple decision to be taken by an armature but is a highly technical matter as many parameters have to be considered. It is so elaborate subjects that we spend two full days of discussion and presentation of papers by eminent personalities in cable manufacturing and top brasses of electrical industry in India. So you can understand the importance of the subject matter

Cables to be used for a particular job is recommended based on various aspects

1. Load in Ampere. The current rating of each cable is recommended by the manufacturer depending on

(a) The type of cable like PVC, XLPE, etc., single core, multi core, armored, unarmored

(b) Conductor material e.g. copper or aluminum Insulation or cable type - e.g. PVC, XLPE, EPR

(b) Voltage rating of the job LV, HV, EHV

(c) Laying conditions like Underground in direct earth, cable ducts, and cable trays exposed to atmosphere etc.

2. De-rating factor depending on ambient temperature, Cable laying conditions etc.

3. Voltage drop depend on length of cable

4. Power loss depending on conditions of heating of conductor, sheath, armor and environmental conditions.

Load Data calculations by studying the characteristics of the load that the cable will supply, which includes number of phases, e.g. three phase or single phase, system / source voltage ,full load current, rated kW, diversity factor ,full load power factor. Length of cable run from source to load.

Method of cable installation e.g. cable tray / ladder, in conduit / raceways, against a wall, in air, directly buried, etc. Ambient or soil temperature at the installation site, cable grouping, i.e. the number of other cables that are bunched together or installed in the same area. Cable spacing, i.e. whether cables are installed touching or spaced, Soil thermal resistivity (for underground cables)

There are specific calculations to know the effect on cable size calculation.

As stated, there is not enough information even for an educated guess.

Your impression is correct, the assertion is absurd, the bus bar could be oversized or undersized. The answer is "no", unless you want to qualify it by responding "...not without much more information..."

Yes, they are related. They both conduct. Like all families, there are good and not so good relatives.

You can specify the cross sectional area of the cable just using the 2500amp information that you already have. Include in your answer worst case assumptions about how the cable will be installed, (ratings are different based on the amount of heat that the cable can dissipate) and state which assumptions you have made. Without more information about the voltage and application you cannot specify the type of cable.

Ignore any derogatory comments about your lack of experience. Some of us are too old to remember falling asleep in boring lectures.

Look my friend, please post the right and detailed info.

1) What SIZE would your Genset be?

2) What would the purpose be for the Genset to provide to i.e for emergency load etc.

3) What kind (ops brand) of Genset? I would prefer Perkins :)

4) What would be your total load and emergency load?

5) Would you want motor and elevators (In British English Lift) tied to emergency load or NOT?

6) My favorite part but next time about whether or not you would want an ATS (Automatic Transfer Switch) in case of utility main power shut down? A lifelong tale...W/ style from an Eng.

Use the cable manufacturers cable information tables. You have the amps, just go across the table to the related cable size. It is that easy. and you will find it may be rated at 2580A and the cable size is 1250MCM. Same rating as your busbar.

My my, these cable manufacturers make our lives so easy. Thank you.

Best answer I've seen, after all, (as I was taught) you don't have to remember everything, just have to know where to look it up, and with the internet these days, it should be easy to find, not like the way it was "back then" when every designer had an extensive "library" to find things.

Sure, and I can look up a myriad of texts on open heart surgery, but that doesn't make me a cardiac surgeon! Just suppose the busbar was oversized in the past and other handbook engineers have come along and said "...Wow, I have a 2,500 Amp bus bar, I guess I can throw another generator on as long as I size the cable to handle 2,500 Amps..."

The point is that you know nothing about why that busbar was sized the way it was, maybe it was oversized for future expansion, and that has taken place, and it's now undersized for the loading, and you're going to add more to it???

That's no different than looking at a 42 position breaker panel with 32 open spaces and saying "...Wow, I can fill this sucker up with another 32 20 Amp breakers and pull 640 Amps more out of it...", while ignoring the size of the main breaker and the feeder and transformer behind it. It's a system and it must be taken as a whole, not piece by piece.

Anyway OP's question was probably an interview question whose answer indicated his critical thinking skills, it has a different answers depending upon the experience level of the interviewee.

After a certain distance and specific conditions, you must consider that in case of a short circuit at the cables, the busbars and the generator are the sources of current. Then, the generator`s circuit breaker is not enough and you need to install a cable protector at the busbars end of the cable.

Check with your electrical code for the details.

Who told you that? And did the person tell you that the bus bar was of the proper size?

Ok guys the purpose of asking was not that i was going to select a cable based on your suggestions, the question was only put here because i was curious if there are other methods of sizing cables apart from the one i know about (calculations based on different factors). Like some sort of guideline where the minimum size of cable is suggested on the basis of a correctly sized bus bar. From all your suggestions i figure out that it is not possible to do so.

Sorry to all those who thought my curiosity to know something was a bad thing, and thanks to others for the info provided.

It is possible if the bus bar is correctly sized, and if other factors like distance and temperature do not come into play.

If you had only used the effort spent here looking on the web:

Cable Sizing Calculator - myElectrical.com you would have found:

Cable Sizing Calculation - Open Electrical
and many more:

Cable Sizing Software | Voltage Drop Software | Motor ...
Same for the mouth, Blackpaper.

As I said simple stuff is easily found with just a little effort.

It is called the "ENGINEER'S place for news and discussion"

Has the definition of 'Engineer' changed to 'a person who cannot give a simple straight forward answer'?

"Has the definition of 'Engineer' changed to 'a person who cannot give a simple straight forward answer'?"

Yes and no.

In jest, an Engineer is 'a person who, when asked for the time of day, will tell you how to build a watch.'

In all seriousness, fewer questions than you might think have simple, straightforward answers - especially when the question, such as the OP's original question, is incompletely specified. If the OP had asked the *actual* question in all its glory, and not the one the OP asked, then we could have cut to the chase right off the bat and answered *that* question instead. It might not have a simple, straightforward answer, but if the question is well-framed and complete, so will the answer be, straightforward or not.

By the way, the answer "We don't know, but we'll find out," is a *perfectly legitimate answer* when that answer is closest to the truth.

I worked with an engineer who was pathologically incapable of saying "I don't know" and would instead answer some other question that wasn't asked and whose answer was completely irrelevant. He was huge time-waster and others who needed answers bypassed him completely because they didn't have 45 minutes to listen to his bullshit. It is this type of engineer who will tell you how to build a watch. When asked the time, this guy would tell you how to ride a motorcycle.

The best questions are those which suggest their own answers. Asking a good question is much harder than answering the wrong one and saves everyone a lot of time.

CR4 *is* a good place for Engineering News and Discussion. In my opinion it is unmatched in terms of the competency and willingness-to-help of its members.

On the flip side, this forum insists upon well-framed and succinctly-phrased questions. Woe be unto those who cannot be bothered!

If one wants a good, quite often excellent, quite often by necessity not-very-straightforward yet complete, accurate and timely answer, one had better put a little effort into the question before posting it.

It's only fair. To everyone.

My apology in advance! Lyn would be a good person (woman or man?) to reply to such queries? Again, CR4 is very good and informative forum. I have not spent much time in here but whenever done so, learnt and contributed to it as well. HOWEVER, in my PERSONAL opinion it ISN'T a "technical solution forum" for all there are questions and answers? That would be like a paid department in Intel or Microsoft corporations where they would get to sell products and whenever there would be a problem with the product, the customers could call a technical division center to help WALK THEM through STEPS...One could request as things don't appear to be clear or so? One could not force anyone for a final and accurate answer. It is just a discussion forum where some knowledge is a prerequisite in order to know in depth or else on any subject matter. The old archives would the best place to begin with? Again, my apology as everyone is entitled to his/her rightful opinion as a part of free speech?

Sure it's a 'technical solution forum' for questions and answers, but as folks here put time and effort into their answers, sometimes a bit of research, too, for the questioner to do the same at the outset is just common courtesy.

I agree to what you write!

Yes, there are other methods to select a cable. Regardless of the selection method used, you should be aware of the local wiring codes for where the cable will be used. You should know if your selection is in compliance with the code or has a good reason for not complying with the code. If the latter, be prepared to document your reasons for varying from the code.

With few exceptions, bus bar ampacity > cable ampacity.

That is all ye know, and all ye need to know.

--John Keats

ξ

It's bolx; the cable size is only related to the genset rating, the method of installation and the length of the cable. British Standard 7671 again.

Are all three conductors in "free" air or contained inside one (1) conduit?

What is the ambient temperature derating factor?

If in free air and at 30C/86F temperature then 2 each 1750kcmil conductors will suffice.

If all three are inside conduit and at 30C/86F temperature then five (5) each 1500 kcmil conductors will suffice.

First identify the environmental variables, then look at the "allowable ampacities of conductors" charts to determine the size of conductors required.