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#129140 - 08/09/04 04:13 PM Cable sizing long supply line
Belize Offline
Junior Member

Registered: 08/08/04
Posts: 3
Loc: belize
Hi all, here is a little application problem which I am hoping some of you may be able to help with. My Electrical knowledge is far from comprehensive so feel free to keep it simple!!!

Unfortunately, down here in Belize, (110V single phase) sound Electrical know-how is difficult to find. As a result with any help you may be able to give, I can try and grasp the full extent of the situation I have before me.

My “problem” is based around needing to supply 2 large Houses (under construction) with mains power. The nearest house is 600' from a dedicated 25kVA transformer - the furthest 900'. In between the two is a Utility Building housing a Laundry room - 1 Washer and 1 Gas/Electric Dryer. The two houses are split into independent self contained upper and lower level units. Without Aircon, electric cooking, Hot water heating, or any electric motors other than the odd power tool now and again; the max peak load I would estimate for the whole site is unlikely to be higher than 50A.

I am currently in the process of sizing the Cable required to bring the Mains electricity to the site. As you might guess, my main concern is that of Voltage drop along that length of line and disparities in the Voltage between the two houses. The Electric company says it can boost my voltage supply up to 130V and supply a up to 125A from my transformer.

My issues are:

1) On a 110V supply, at what voltage drop level would I start to noticeably experience appliance problems, lights dimming, etc. I believe 2% is generally used but is that practical?/reasonable?/necessary?

2) With the advice of the Electric company (whose advice is not consistently good!) I am anticipating using copper 00 cable which by my calcs using the online calculators leads to a VD of 8.7V at the furthest house equating to 7.3%

3) The Electric company only considers the one length 900' and NOT the return so say the VD will be about “4V” and therefore fine!

4) I originally wanted to take the supply to the centrally located Utility Building and supply each house from there equalizing the Voltage somewhat but this could now add to the problem by adding a longer line plus the cost of 00 Cable is high here in such quantities. As an addition, can I step up the Voltage in a line that has already been stepped down?

5) I guess what I would like to know is how sensitive to supply voltages are Lights/Fans/Fridges? My current thought, open to amendment is to run a 00 spine through the site, branching off from it to supply the two apartments in House 1, then the Utility Building and then the apartments in House 2. I realise that in doing so I will have to accept that House 1 and 2 will have different supply Voltages, which is fine as long as is functionally negligible

6) If the above is feasible what size cable can I branch off the 00 spine and which would be the best way to branch it?

7) Does the Neutral Line HAVE to be the same size as the Hots? If not how small can I reasonably go?

I realize I am probably not using the correct terminology in much of this so please excuse my ignorance. I am happy for anyone to point out the errors of my thinking or approach and very grateful to receive any constructive suggestions from you electrical bods!

Thank you

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#129141 - 08/09/04 06:56 PM Re: Cable sizing long supply line
Bjarney Offline
Moderator

Registered: 04/10/02
Posts: 2561
Loc: West-Southern Inner-Northeast ...
 
A problem with raising taps on distribution transformers to compensate for secondary-cable voltage drop is simply poor voltage regulation. During light-load periods, 130V is murder on lamps and consumer electronics, where 110V or less during heavier-load periods is tough on motors. {Don’t forget to consider the significant additional load of motor starting, if applicable.} Flicker from poor voltage regulation can be bothersome to intolerable for some. [In North America, utilities and appliance producers have sort of a “treaty” agreeing on a range of around 110-125V.] The best solution is to size the cable for acceptable voltage drop over the range of anticipated loads, although secondary cables can get quite large.

Consider using an online voltage-drop calculator like those at www.electrician.com

Remember that if 120V loads are split by using 3-wire 120/240V servce, that will effectively lower “120V” voltage drop by one-half, if a reduced neutral is not used.

Another fix may be to move the distribution transformer {extending the primary cables} to a location closer to half way between the two residences, but that may not be acceptable to the utility.

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#129142 - 08/09/04 10:43 PM Re: Cable sizing long supply line
Belize Offline
Junior Member

Registered: 08/08/04
Posts: 3
Loc: belize
Bjarney,

Thanks for your suggestions.

I am in fact in the process of trying to get the Utility to relocate the transformer closer to the site - only 200ft closer (the closest available possibility, but closer nonetheless)

Are there any "gadgets" for "smoothing" the supply regardless of the loads that are being applied?

If I bring a 2/0 cable into the site can I then distribute it to the 5 locations (4 units + 1 Utility, each now with an effectively much reduced load) with a singnificantly smaller cable e.g 4AWG?

Can you explain "splitting the 120V loads with a 3 wire system" a little more please? Is that the same as balancing the loads across each of the the two 120V supply line "phases"?

thanks

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#129143 - 08/10/04 03:43 AM Re: Cable sizing long supply line
Ron Offline
Member

Registered: 03/13/02
Posts: 577
Loc: White Plains, NY
The 3-wire system referred to, utilizes a shared neutral between two phase conductors (where the phase conductors are 180 degrees out of phase with respect to each other), 120/240V.
Since the neutral only carries unbalanced current, the voltage drop due to the neutral will be less than two dedicated separate 120V circuits.
_________________________
Ron

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#129144 - 08/10/04 11:12 AM Re: Cable sizing long supply line
Bjarney Offline
Moderator

Registered: 04/10/02
Posts: 2561
Loc: West-Southern Inner-Northeast ...
 
Refer to first illustration at www.bmillerengineering.com/elecsys.htm

Voltage drop calculations for the described are an iterative process—step-by-step; trial-and-error for various combinations of load. Pay a power EE to do that, or study the textbooks, set up a spreadsheet and do it yourself.




[This message has been edited by Bjarney (edited 08-10-2004).]

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#129145 - 08/10/04 08:46 PM Re: Cable sizing long supply line
Belize Offline
Junior Member

Registered: 08/08/04
Posts: 3
Loc: belize
Thanks guys for the answers to the questions you were able to answer.

Away from the hustle and bustle of the so called first world it is interesting to see how, theories, design rules and code requirements can be "massaged" into reasonably safe working systems.

If anyone else has any comments or suggestions as to how best to arrange the site supply I would be most interested to hear it!

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