220V long, 300m cable run

lazza
lazza Solar Expert Posts: 336 ✭✭✭
in PV Installers Forum - NEC, Wiring, Installation #1
HI Forum

Apologies for being off the air for so long- Just too much stuff on to be able to post and read lately :cry:.

We've managed to bag a fairly large installation (15kVA inverters, 9kW panels) and I will be asking a few technical questions to the forum, to make sure we do it right! I've never handled something so big before, so am a little nervous.

However, the first question is fairly simple. The client wants to run a cable away from the main installation to an out-house that is 300m away. It will be coming out of the inverter at 220Vac and the maximum power that the outhouse will consume is around 3kW, say 15A. So my question is:

What would the minimum cable size be for the phase and neutral wires? Are there any golden rules when laying cables that are this long?

I know how to do voltage drop calculations, but only in DC systems... any help much appreciated.

Cheers
Larry
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Comments

  • westbranch
    westbranch Solar Expert Posts: 5,183 ✭✭✭✭
    #2
    Re: 220V long, 300m cable run

    What will the voltage going to the inverter be? Have you looked at sending High V DC and using another inverter?
     
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  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #3
    Re: 220V long, 300m cable run
    lazza wrote: »
    What would the minimum cable size be for the phase and neutral wires? Are there any golden rules when laying cables that are this long?

    I know how to do voltage drop calculations, but only in DC systems... any help much appreciated.

    Cheers
    Larry
    There are a lot of voltage drop calculators out there. One of them is at http://www.southwire.com/support/voltage-drop-calculator.htm.
    It is not my favorite, but it is one that I can always find.

    Two things to remember when using a VD calculator on a 120/240 single phase three wire circuit:

    1. The voltage drop on a 120V only circuit which is balanced by another 120V load on the other side of the neutral will be that of just one wire, not two since no (or little) current will flow in the neutral, so no VD there.
    2. A 220V only load (heater, etc.) will have a voltage drop corresponding to two wire sections, since the VD in both 120V lines will affect the load.
    The numeric voltage drop will be twice that of one side at 120V, but the voltage (240) will be twice as large too so the percentages will agree.

    If you have a really long run (and IMHO 300m is VERY long) you may end up doing better by stepping the voltage up to 480 or even 600 and stepping it back down at the other end. A grid tied inverter will work fine in that configuration, but at those voltages you really should have an electrician at least supervise the installation.

    If you double the voltage, you cut the percentage VD for the same wire size and power by a factor of 4, so it really makes a difference.
    Look at using aluminum wire/cable for a run that length. You need a larger wire size, but the cost per amp is still a lot less.

    The Southwire calculator says that for 240V, 15A, 1000' you need to use #4 copper to keep the VD to 3.59%.
    #3 aluminum will give you a VD of 4.65%.
    If you go to 480 volts for the interconnection, then "1 conductors per phase utilizing a #10 Copper conductor will limit the voltage drop to 3.28% or less when supplying 7.5 amps for 1000 feet on a 480 volt system. "

    For the difference in cost between 300m x 4 wires of #10 copper and #4 copper you can buy a couple of 5KVA transformers.

    Note that according to the NEC, you will have to run either two phase conductors and an EGC (Equipment Grounding Conductor) or two phase conductors, a neutral and and EGC.
    If the GTI requires a neutral, but just for voltage sensing, you can get away with a much smaller neutral. But you may have a problem finding direct burial cable with the combination of conductors to make that happen.
    SMA SB 3000, old BP panels.
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  • stephendv
    stephendv Solar Expert Posts: 1,571 ✭✭
    #4
    Re: 220V long, 300m cable run

    Hi Lazza,

    This is normal european 220V single phase right?

    I have 380m long run from inverter to house, and the maximum draw in the house is around 3kW too. Here's what I (as a non-qualified or trained electrician) did, with the decisions based largely on my wallet:

    - Used only 2 core 6mm2 cable, live and neutral. (Between 9 and 10 AWG). There are earth rods at the inverter and a different set of earth rods at the house, so no need to run 300m of earth cable. I think this is against some of the american advice, but seems to be what's used in a normal TT grounded system (common in Spain): http://en.wikipedia.org/wiki/Earthing_system#TT_network

    - Configured the inverter to operate at 240V. All equipment works fine at this voltage, 220V is just an agreed average between EU countries. Some places in the UK operate at close to 250V. So pushed the inverter higher to reduce vdrop.

    - Buried the cable in 40mm conduit (that red corrugated stuff at the builders merchants) along with some rope in case I needed to pull more cable through in the future. Had I checked on the cost of the conduit first then I probably would've gone with shielded cable, directly buried instead. Guess it depends on how much flexibility you need in the future whether buried conduit is a good choice or not.

    - With the 2.6kW kettle on, the measured voltage is 221V, so a 19V (8%) drop. That's quite high on paper, but since it's still 220V everything still works without any issue. Well I do notice a slight dip in the output of the LED lights when the kettle or electric oven are turned on, but not sure whether that's a delay from the inverter or due to the wiring.

    When I phoned up a local electrical supplier to compare prices of thick cable for 240V versus thinner cable + 2 x transformers. The thicker cable was far cheaper.
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  • lazza
    lazza Solar Expert Posts: 336 ✭✭✭
    #5
    Re: 220V long, 300m cable run

    Ok thanks for that. I think 6mm2 cable is very small for such a large run however- as seen in the 8% voltage drop. This guy doesnt have money problems. However as most calculation programs are suggesting 25mm2 cable to achieve a voltage drop of no more than 4%, or 50mm2 aluminium cable- it's quite a job to bury.

    We're going to try to pursuade him to put a small, separate installation in the outhouse, he'll save all the hassle of laying thick cable and the main system will perform better.

    Cheers
    Larry
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  • stephendv
    stephendv Solar Expert Posts: 1,571 ✭✭
    #6
    Re: 220V long, 300m cable run
    lazza wrote: »
    We're going to try to pursuade him to put a small, separate installation in the outhouse, he'll save all the hassle of laying thick cable and the main system will perform better.

    In that case, wouldn't transformers be the better option?
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  • lazza
    lazza Solar Expert Posts: 336 ✭✭✭
    #7
    Re: 220V long, 300m cable run

    It will be another system with its own panels, regulator and inverter--- save leeching the main system and save running cables 300m :)
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  • stephendv
    stephendv Solar Expert Posts: 1,571 ✭✭
    #8
    Re: 220V long, 300m cable run
    lazza wrote: »
    It will be another system with its own panels, regulator and inverter--- save leeching the main system and save running cables 300m :)

    The cost of 300m of cables + transformers is surely not even in the same ball park as a whole other system capable of 3kW peak power!

    Having the outhouse "leech" power from the main is one way of looking at it. The other way, is that with 2 separate systems you have no choice about how to subdivide that precious solar power when there's a deficit. One big system is more flexible in that you can decide where the power goes. E.g. if the outhouse isn't occupied then use all the panel power for the main house. Very handy when there isn't enough sun.
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  • zoneblue
    zoneblue Solar Expert Posts: 1,220 ✭✭✭✭
    #9
    Re: 220V long, 300m cable run

    How about a small gauge wire 240v pair to the out building, and a small battery with its own inverter/charger. "Trickle charge" from the main system. Just a thought.

    Cable investment is a black hole, you can spend truck loads on it and gain less and less with each extra gauge. Stephenvs math stacks up because while 8% sounds a lot, the "kettle" only runs a few minutes a day, the rest of the time the loss will be less then 2%.
    1.8kWp CSUN, 10kWh AGM, Midnite Classic 150, Outback VFX3024E,
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  • lazza
    lazza Solar Expert Posts: 336 ✭✭✭
    #10
    Re: 220V long, 300m cable run

    He's decided to put in a separate system

    That way when he comes to his estate at the weekend, the outhouse inhabitants wont have consumed all his batteries and we wont be getting some very angry calls.

    Our systems are cheap (our margins are pitiful) and it made sense all round.
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