LV Cable Sizing

Only partly correct. The wire size must also satisfy a cross-sectional-for-ampacity requirement, with possible adjustments for ambient temperature, number and configuration of adjacent conductors, etc. See local regulations for details.

I'm not fond of your formula but I cannot say if it is correct or not from the sparse information. Your formula does not take into account the cable insulation, number of conductors, cable path (residential wall, direct burial, conduit, cable tray, etc.), or any plausible indication of where this "rule of thumb" formula should be applied.

Hi,

Regarding cable distance correction factor: - is it for depth of laying or length of cables upto load centre?

The following table is not clear - what is Nil, cable diameter, 0.125m, 0.25 m, and 0.5 m? It appears 0.125m, 0.25m and 0.5m are cable diameter. Shall I understand for the blue marked cable distance correction factor that it is for 4 no of ckt (ie number of run of cable=4), and it is for a cable of diameter of 0.125 m??

Regards,

Inspection of that table shows that no consistent formula is applied, and that the distances have very limited applicability. GIGO.

You have chosen to use someone else's proprietary method(?) to do your work for you. You should contact the blog poster and ask him directly exactly when, where, why, and how you should use his "thumb rules", especially since he provides no references, standards, codes, etc.

...and now I see why there are no references, it was plagiarized verbatim with no attribution to the original author, you get what you pay for, shows what you can do with your "thumb rules" and where they were before you got them.

This I was referring to the link given by lyn above.....!!

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

That's better but as you correctly surmised the website does not clearly identify how to use that table. In both example cases the website demonstrates there is only one circuit making this derating factor moot. I have my suspicions what the other attributes mean but since this website does not identify which actual code this table comes from I will not post my guess. Code books are convoluted and complex for a very good reason. They attempt to handle the wide variety of real world conditions with the least amount of words. Removing any of the words will mean some scenarios will not be properly explained.

If you wish to know how to use that table, ask the one who posted it originally, not Lyn. Lyn just used a search engine, as you should have, to answer your original question.

There is also a question of relevance here. This website claims to be from an Indian electrical engineer. Presumably he uses the Indian electrical codes. They may not be relevant for your application. All of the codes used around the world are similar but they are not identical. That difference has more than once been the difference between a safe and a dangerous installation.

Take this lesson from this thread. You usually get what you pay for. You did not pay money for a universal wiring standard. This is a good thing because none exists. Your mind did pay all of us by recognizing a flaw in this website's presentation. Keep thinking clearly and you will go far.

Lastly, please come out of the shadows by using your CR4 handle instead of the anonymous posting option. You are polite and well reasoned. You will be granted more respect here by using your handle.

It's interesting to note that the whiners are whining about one of the 5 sites I posted. I give up.

I wonder who ties their shoes for them?

This table it is a copy of IEC 60346-5-52 Table A.52-18 (52-E2) - Reduction factors for more than one circuit, cables run directly in the ground .This table it is only for ampacity [current carrying capacity] calculation purpose.

No connection with voltage drop. For single core cables the reactance will increase with the distance while resistance will decrease a bit [less proximity effect].

Dear Friend,

Based on the Electrical Conductivity for Electrical Grade Copper and Aluminium there is another Standard for deciding the cable/conductor size selection, which is as as follows:

1. For Copper the Current Density is 1.55 Amps. per MM^2

2. For Aluminium the Current Density is 0.968 Amps per MM^2

The above values are with out De-Rating Factor, Ambient Temp. Correction, Bunching of Conductors, Under-Ground or Over-Head laying etc., etc.

You have to apply the factors and decide the size of the .cable and add 10% margin for any variation in the assumed factor(s) and use the nearest higher size cable.

DHAYANANDHAN.S

Regs,

Keep on studying and asking questions. The formula you quoted is only partially correct. First, it will give you the minimum cross-sectional area of the conductors for the specified load and length.

For a single phase circuit,

CM = (2*L*K*I)/VD

For a 3-phase balanced circuit,

CM = (√3*L*K*I)/VD

Where: L is the one-way length of the circuit, I is current in amps, VD is voltage drop as a number (not a percent), and K = 0.020 for copper... If you were using voltage drop as a percent, such as 1, 2, 3, 4, or 5, then K would be 2.0.

As others have correctly pointed out, this answer is only a portion of the answer you need. There are a number of factors that can and should increase the wire size:

• Temperature limit for the conductor insulation,
• Ambient temperature the circuit runs through,
• Number of conductors in the conduit, raceway, duct, etc.,
• Type of load (such as motor, welder, lighting, or others),
• Whether it is continuous or not, and
• Size of overcurrent protection ahead of the wire (or after it, if this is a short feeder and codes allow taps with overcurrent protection after the tap).

I personally would never size a motor conductor with VD=5%.

Get out the code books that are applicable for your area and study them. Get out text books and study them. Get out manufacturers' literature and study this.

When you ask questions, give us details that relate to the question, such as application, location, purpose, source of your formulae, etc.

--John M.

This voltage drop calculation formula it is based only on resistance shown in NEC. See:

http://ecmweb.com/code-basics/dont-let-voltage-drop-get-your-system-down

This is approximately good for small area cross section of copper conductors at a certain operation temperature. The more accurate calculation has to take into consideration skin effect and proximity effect for resistance calculation at the actual operation temperature and the reactance between conductors at the actual distance.

For still more accurate calculation you need to know the power factor also.

VD=I*R*cos(fi)+I*X*sin(fi)

For more details see:

IEEE 141 ch.3.11 Calculation of voltage drops.

There is a good source of advice on cable rating in BS 7671 (the IET Wiring Regulations). There's a link - http://electrical.theiet.org/wiring-regulations/index.cfm - but you have to buy it!.

For Volt drop specifically, the ECA sheet at http://www.eca.co.uk/filelibrary/download/?FileID=2361 will help.

As to motors, the criteria are i) starting volt drop and ii) volt drop represents wasted energy. 5% = 5% additional losses.

I found a calculator also - for NEC or IEC - at http://www.cablesizer.com/iec/voltdrop/.

You could say it does what it says on the tin, but the site also computes current rating. One can be lazy!

found