How to Cable Calculate Voltage drops and cable sizes

Submitted: Tuesday, Aug 01, 2006 at 22:07
ThreadID: 122823 Views:3750 Replies:7 FollowUps:3
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I am currently staying with a friend in Broome who is an expert in electrics and solar and in fact runs his entire home on power supplied from solar panels.
He has given me the following simply formula that enables people to calculate what the voltage drop of a cable will be for any given cable length and current draw. The conversion factor of 0.017 is close enough to what is used as the world standard that any difference in our calculations is not worth mentioning.
In his extensive work in this field over many years he has found that the theoretical calculations are within a per cent or two of the results of actual tests.

The formulae is as follows

D = L x I x 0.017 / C

Voltage Drop (D) = Total Length of Conductor in Metres (L) x Amps (I) x 0.017 divided by cross sectional area of cable (C) in square millimeters. The result is the actual voltage drop (in volts).

NOTE: Length of Conductor is the combined distance from battery to appliance and return to the battery via the earth.

This formula can be used to calculate the cable size for any AC or DC application, and as almost every cable is made up of copper wires, there is no need to mention other conductors (such as aluminium) that may be used for specialialised projects.

i.e. As this is a caravan forum I have used caravan appliances to calculate cable voltage drop - this is an example.

In a caravan fitted with a 12-volt fridge, the fridge tends to be responsible for 65-75% of the total power used. Fridges are also surprisingly voltage sensitive. I have prepared the following example of voltage drops between a battery and fridge, in a hypothetical situation.

If the fridge is 10 metres from the battery (and that 10 metres is made up of six metres to the Regulator, and then four metres to fridge (which is more than in most caravans but I know of one caravan where that distance is 13 metres), because twin cable must be used, the total conductor length is 20 metres.

This example is for a fridge using 6.0 amps when running (but would use about twice for a second or so each time it cycles on.

Using Cable size 6.0 sq mm for entire length.

D = L x I x 0.017 / C

D = 20 x 6 x 0.017 / 6
D = 0.34 volts. (NOTE: This is cable voltage drop only, it does not take into account any other voltage drops in connections etc.)

If you then add in a water pump (say 6 amps), TV & DVD player (say 6 amps) and some lights (say 6 amps) you then have the following. Total current draw is now 24 amps.

Calculating using 6 sq mm cable all the way.
Battery to Regulator
D = 12 x 24 x 0.017 / 6
D = 0.82 volts.
Regulator to Fridge using 6 sq mm
D = 8 x 6 x 0.017 /6
D = 0.13 volts
Total Voltage drop at fridge = 0.95 volts.

If the fridge was running and a pump was started then the start up current draw of the pump is roughly double the running current (total 30 amps) so for a short time the voltage drop would then be 1.02 volts giving a momentary total cable voltage drop of 1.15 volts.

This is why a fridge can stop running when a pump is turned on.

If we now increase the cable size to 16 sq mm (Battery cable size) between battery and Regulator the voltage drop will be much different.

Battery to Regulator
D = 12 x 24 x 0.017 / 16
D = 0.3 volts.

Regulator to Fridge using 6 sq mm
D = 8 x 6 x 0.017 /6
D = 0.13 volts

Total drop between battery & fridge now = 0.43 volts which, whilst far from optimum, is a lot better than 0.95 volts with 6 sq mm cable all the way if several appliances are used at the same time.

My contact suggests that fridges be wired directly from the solar regulator, using at least 8 sq mm cable (auto sparkies know this as ‘8-gauge’) for distances greater than a couple of metres or so. This may seem overkill, but even 0.1 volt makes a big difference in fridge performance, especially when battery voltage is low.

My direct measurements confirm the above.

My contact says he is surprised by suggestions any of this is complex or esoteric. He says it is totally basic stuff that any electrician or auto electrician will be well aware of.
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Reply By: Bushtracker - Tuesday, Aug 01, 2006 at 22:33

Tuesday, Aug 01, 2006 at 22:33
16mm cable? LUDICROUS! That is Jumper Lead size, you can’t even bend it around corners or get it into the walls!!! 8mm that we have used for years between batteries and solar regulator surpasses what anyone else uses in the Industry. Lights flicker in busses, yachts, aircraft, it is the nature of a DC systems even in $50 million dollar yachts and aircraft. Even if we double the size of the cable, there will still be about .3 of a volt loss due to the internal connections. And the connections of the equipment are not large enough to run any bigger...!!

Paper work engineers? At M.I.T. They have proven that pelicans cannot take off from the water, but if you have a look it happens. They may not have tried to put it into Production like we do... We do it all the time and play with it all the time in Production. Brian, once again, you always want to tell us our business? You collect horse pucky from anywhere you can because you do not know what is practical or possible when put into real life. I will give a personal invite and you may have a few truckloads of horse pucky free of charge. You have been unnecessarily worrying your topic for months worthlessly and now bring it to the Public Forum? 8mm that we use is plenty and they don't even make that in twin sheathed twin insulated. 10mm is so big it would cut out all of the structure in the walls... RIDICULOUS. And the equipment only has connection sizes for what we currently use.

We are already on track here at Bushtracker, and have explained it all to you too many times in private email, and you just will not let go of it... Move on...

Ranger...
AnswerID: 568977

Follow Up By:- Tuesday, Aug 01, 2006 at 23:33

Tuesday, Aug 01, 2006 at 23:33
Hi Ho Silver Neighhhhhhhhhhhh Winneeeeeeeeeeeeeeeeee lets go Silver we will meet Tonto and ambush the Blackfoot in death canyon DA D AD AD AD
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FollowupID: 846099

Follow Up By: Noosa Fox - Thursday, Aug 03, 2006 at 05:52

Thursday, Aug 03, 2006 at 05:52
Steve,
I take it from your comments above and your personal attacks on myself (which are outside the terms of use of this site) that all caravans that BTi build have 8mm cable irrespective of the distance the battery is from Regulator and appliance.

This does not appear to be consistant with Ohms Law but the way you wire up a van is something that is between you and the purchaser.

OHMS law is what everyone in the electrical world uses and if it was wrong someone surely would have picked up the fact and changed it by now.

Incidentally, do you know the difference in diameter between the 8 gauge you appear to use (regardless of distance) and 16 sq mm? A quick check with your slide rule will show it is dimensionally very small, but electrically is very large.

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Reply By: Noosa Fox - Thursday, Aug 03, 2006 at 05:25

Thursday, Aug 03, 2006 at 05:25
No where in my article above have I critisied Bushtracker or the stated that they are using the wrong sized wiring. It is purely an article letting people work out how to calculate voltage drops and cable sizing required for any application that they may wish to used it.

I cannot help it if Bushtracker management believe they know more about this topic than the electrical world in general, but I do know that what is printed above is basically what is used in TAFE colleges teaching auto electricians how to wire up motor homes etc.

Don't take everything on this forum as a peronal attack on Bushtracker.
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Follow Up By: Bushtracker - Thursday, Aug 03, 2006 at 17:48

Thursday, Aug 03, 2006 at 17:48
As usual you know it all, collecting horse pucky from where ever you can, trying to tell us how to do things for years. You know so little about putting the technical jargon you collect, into practice in the real world, that you cannot differentiate what works on paper and why it is not practical in Production. You take whatever back seat drivers you can find as Gospel and try and tell us how we should be running our business. We consult with the best Engineers in every field all the time, trying to do it better.

The paperwork Engineers at M.I.T. (Massachusetts Institute of Technology) Aeronautical and Avionics Division have proven with mathematics that it is impossible for a pelican to make a water take off… You with your obsessive behavior writing email to us should argue that one with a higher force.. At least you will be off our backs.

We will have no comments for you in the future. Other Staff have not been impressed for a long time, while I continued to humour you for the past years. Now I join them. Quote from you “personal attack on you” ?? You have called me a liar and unprofessional on the Owners Forum, negating your quoted "Terms of Use" and when I expose your obsessiveness you delete my Post? Please go delete yourself.

You have no more good will left on this end. Please go away and stop misleading people that come to us that you have concerned about nothing.

Adios.
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Reply By: Deleted User - Friday, Aug 04, 2006 at 00:35

Friday, Aug 04, 2006 at 00:35
The Shootout @ OK Corall

Score Is

CHRISTIANS 7 LIONS 2

Nero.
AnswerID: 568979

Reply By: Deleted User - Friday, Aug 04, 2006 at 00:49

Friday, Aug 04, 2006 at 00:49
I am so wicked but the DEVIL makes me do it

as well we shouldnt take life too seriously were not here for long

and only to care for it while we are here
AnswerID: 568980

Reply By: TRB60 - Sunday, Aug 06, 2006 at 04:47

Sunday, Aug 06, 2006 at 04:47
Hi.
As they say,a little knowledge is a dangerous thing.
Regards Terry Bridges.
AnswerID: 568981

Reply By: Burt & Mary - Friday, Sep 08, 2006 at 08:00

Friday, Sep 08, 2006 at 08:00
Noosa Fox shame your expert (drip under pressure) got the basic formula constant wrong.
AnswerID: 568982

Reply By: Noosa Fox - Saturday, Sep 09, 2006 at 06:40

Saturday, Sep 09, 2006 at 06:40
I would just like to say one thing about the things quoted above

"The paperwork Engineers at M.I.T. (Massachusetts Institute of Technology) Aeronautical and Avionics Division have proven with mathematics that it is impossible for a pelican to make a water take off…"

When everyone knows that Pelicans CAN fly why would anyone be silly enough to try and prove that they cannot. Obviously the MIT are not as clever as they think they are, and got their formulea wrong.

If BTi are correct in saying that voltage loss is not directly linked to the size of cable used and the length of the cable required, then they will make history and become the new authority in this field.

As it is however, I believe that BTi stance on trying to say that the rules of electricity as all the worlds experts use and agree on is wrong, is similar to the Pelican quote above. They are just trying to prove something can't work when everyone else in the world knows it does.

If I am wrong with what I have said above, apart from the minor change in the 0.17 factor being slightly different, then I will apologise. However the posting that Turist put on about the same calculations being put in the document folder would tend to back up what I have said as being correct.

I would be pleased to see someone prove me wrong by publishing a formulea that can be scientifically tested as being correct, and the above formulae wrong as I don't believe it can be done. It is one thing to say someone doesn't know what they are talking about, it is another thing to prove it with solid facts.

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