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Author Topic: 2.5 mm sq rigid cable  (Read 9406 times)
dodgy rog
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« on: March 18, 2008, 07:18:57 PM »

hello, if i use some off the existing rigid cable that my house is wired with to pass 24V from traction battery fed bye pv array,
i think the longest run would be say 12m , will i experience too much voltage drop;
and what is the max length run you can safely,efficiently do with 2.5mmsq

i have deiced to stick with 24V dc because i did some calculations and it seems i would use 1412wh/pd  220vac if i put everything through an inverter,                 or 984wh /pd at 12Vdc, so it seems i can save myself the cost of a big inverter and more pv panels ,windmills and use 24 to 12V converters when needed.

p.s your forum is exactly what people starting out like me need fantastic,more power to you all
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wyleu
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« Reply #1 on: March 18, 2008, 08:47:30 PM »

The voltage drop in a cable is calculated by ohms law, and isn't just dependant on the voltage, it's the number of electrons flowing that makes the difference.

Ohms law is V = IR

where v is the voltage across a resistance in ohms and I will be the current flowing so, if you have a 24 volt supply and you have 24 metres of cable with a reistance of roughly .1 ohm per metre you will have a current of:

   V = I * R

  24 volts =  (.1 ohm * 24 ) * current in amps

   24/ .24   =   100 Amps....

So as a starting point a direct short at the end of your cable wil allow 100 ams to flow and this will probably do your cable considerable damage cos of the other great electrical rule


   W (power) =  V (volts *  I (Amps)

   W = 24 Volts * 100 Amps    = 2400 Watts = 2.4kw

  2.4 kilowatts of power to be dissapated in 24 metres of cable or 100 watts a metre. This wouldn't be good.

Now actually the odds of your voltage source sustaining 24Volts under that sort of load is minimal but it gives you an idea of the sort of fault current that could flow if there were no fuses to protect the cable. You should design for this eventuality and the fuse you select should limit the current to an acceptable value. At 240 Volts mains the degredation of cables of various types are all carefully calculated for you but the situations for DC are less so, but the need for a fuse is demonstrated if only to show that if you don't want to relay the cable after the inevitable short you should have a suitable fuse in line at least, and that sets your maximum current flow. The fuse will blow in a pre prescribed time and the cable is protected.

Now your PV array will self protect pretty well, you actually measure the short circuit current (Isc) as part of the testing but you must also consider a short at the PV end with the batteries behind it. This is far more likely to get to the 100 Amps mentioned above and that will be a much more likely limiting factor. Remember that things don't only fail one way. Your cables don't care which way the electrons rush.

So what is a suitable value for the current?

Well batteries driven directly from the array are going to be a fairly variable load and the likelyhood of much energy transferring reliably in such a situation ain't very likely and some form of controller would be probably be required to match it up nicely. ( Again anyone who has a more experienced eye then correct me here) Quite how such a device behaves will decide how much current will actually flow whilst charging and depending on the optimum performance of such a device will set the ohms law losses you will get in the cable.

There is a cable loss calculator somewhere on the site and it might still be somewhere if the Navitron quantum leaps haven't shaken up the mothership too much. But to do this sort of calculation reliably requires a bit more control and the controlling device would be the key. Environmentally to re-use cable is probably going to earn you more Karma points, and perhaps you could parallel it up if you have a fair bit but On the very few occasions that I've tried such an arrangement ( It's good enough for rock and roll and the band goes on in five minutes) I've been surprised how one cable has hogged the current, which is probably an issue more to do with connectors than cable characteristics, so at the end of the day it's a design and resource issue and if you predict with calculation and confirm with measurement you are at least on the way to demonstrating an first order understanding of your system.
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billi
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« Reply #2 on: March 18, 2008, 09:08:50 PM »

dodgy rog

how did you figure
Quote
i would use 1412wh/pd  220vac if i put everything through an inverter,            or 984wh /pd at 12Vdc
 that ?.....

don't get me wrong its very important i think to build up a own system and not rely on two many other people ( i would say off grid living means as well help your self and investments are for the future  ...)

the watt hours should be the same  the ampere hours are perhaps diffrent....


you should go the way you think it will be best for you and grow on that idea....  and built it up as you go

for me now it is like a spaceshuttle.... Tongue Grin   started a year ago (investing was started 3-4 years ago) and now i have a space shuttle equipment   ( i had a dream at the age of 14 teen   to be a artist one day)  looks like  that i will be an  electrical engineer soon 25 years later....  Tongue


like said earlier it depends what way your live, living is moving  and i do understand that there is losses when you use inverters, but how much more money do you have to spent on all the dc pumps, perhaps washingmaschine, gasdriven fridge, 24volt to 12 volt converters, 12 volt lights.... 12 volt tv, music, dvd ?

 i would guess an additional of 300 -500 pounds of pv  would compensate your  calculation  between ac and dc  ....

but like i said upfront its a lonely road we drive .... so  a punctured tyer has to be repaired by ourselfs or its a luxury we cannot afford....


Ps   i think rigid cable is shJite cable  for pv   or you take a tracker charger  to have your  pv at a higher voltage.. to reduce loss  

but the morningstar ivan sells have a good website  to find out about cables....


PS again : people and including myself adviced you to go for a 24 volt system ....  i am not sure now anymore because most dc equippment is 12 volt and 24 volt is more rare to find....
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1.6 kw and 2.4 kw   PV array  , Outback MX 60 and FM80 charge controller  ,24 volt 1600 AH Battery ,6 Kw Victron inverter charger, 1.1 kw high head hydro turbine as a back up generator , 5 kw woodburner, 36 solar tubes with 360 l water tank, 1.6 kw  windturbine
northern installer
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« Reply #3 on: March 18, 2008, 09:28:30 PM »

Important to use carefully matched lengths of cable if you are going to parallel them,fuse protection becomes an issue if there is a mismatch
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Darren8512
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« Reply #4 on: March 18, 2008, 11:45:11 PM »

 dodgy rog

 Not sure if this might help?

http://www.unlimited-power.co.uk/Cable_sizing_chart.html

Darren
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dodgy rog
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« Reply #5 on: March 19, 2008, 07:14:06 PM »

thanks to all,

Billi, yes you are right i got a bit confused there, just went through all my calculations (scraps of paper) and what i should have written was maybe,    49.8 ah/pd @ 24v    and  70.6ah/pd @ 220v

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billi
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« Reply #6 on: March 19, 2008, 07:54:35 PM »

rog

i think your  watthours per day calculation was  better then the ahhours a day  Shocked Roll Eyes

i didnot mean to confuse  with my last post , i just wanted to question the result that an inverter idea is as inefficient as a direct DC idea for a house.....   your figures say that with an inverter  instead of direct dc usage you need about 30-40 % more electricity ? 

i doubt that , but i am not certain either...

all the best billi
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1.6 kw and 2.4 kw   PV array  , Outback MX 60 and FM80 charge controller  ,24 volt 1600 AH Battery ,6 Kw Victron inverter charger, 1.1 kw high head hydro turbine as a back up generator , 5 kw woodburner, 36 solar tubes with 360 l water tank, 1.6 kw  windturbine
peter999
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« Reply #7 on: March 19, 2008, 11:27:54 PM »


  buy a good 24v pure sine wave inverter ( max 3kw) a automatic or manual transfer box use your existing house wiring. Would seem to be the simplest and problely cheapest way to go long term

good set of wet lead acid batters wired in pairs to give 24v with a 60% discharge capacity of say 15kw
500watts of pv 1kw wind turbine 5kv diesel genny(bio diesel/wvo) or even better lister 30 tube solar hot water system south facing

job done change from 10000 !!!!

that all there is to it (i joking)

If your doing this to save money  wait another 10 years!!

my humble opion Peter
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dodgy rog
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« Reply #8 on: March 26, 2008, 08:42:45 PM »

thanx too everyone

pete, i have no choose,no chance of Maine's power here, i love the idea of keeping it simple and doing everything on inverter,but struggle too get to grips with the losses and extra outlay on more panels and batteries

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