12V DC to green house

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SirHumphreyAppleby

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#290202 27-Oct-2021 12:11

I am working on an aeroponics system to go in a small green house. Burying a mains cable probably isn't an option as I'd need to cross both storm water and sewer pipes at unknown depths and aerial isn't an option.

I have an adjacent outdoor power point, so I am now considering running DC power from an outdoor power supply to the green house, about 5m away. For cabling routing purposes, I am allowing 10m for calculations so I think what I am proposing is okay, but I'm hoping the experts here could give confirmation.

My current idea is to install a 48V DC power supply in a weatherproof enclosure, and use that to supply the greenhouse, then reduce the voltage to 12V at the far end. The power requirements for most of the system are negligible, but the pump I will be using is rated at 10.5A (12V DC), so having sufficient power for that main concern. It's a bladder pump so I'm not sure if startup current is as much of an issue as with other pumps, and will only be run to around 100psi vs its rated 160psi.

Do you think these parts would be suitable for such purposes?

DC-DC 36-48V to 12V DC 30A... https://www.aliexpress.com/item/4001296663814.html

1.5mm^2 direct burial cable... https://www.bunnings.co.nz/tycab-cables-1-5mm-x-20m-2-core-12v-garden-cable_p0307620

Happy to look at alternative suggestions of course.

Ge0rge

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#2801927 27-Oct-2021 12:20

Could you put a battery of some description in the greenhouse? You could use it to deal with any surge current, and trickle-charge it from your power point, possibly with a view to make it either partially or fully solar powered in future. Would allow for a thinner (cheaper) cable too. I'd use a mppt solar controller to do the charging / battery management - you'll also get geek-friendly info about charging / use etc depending on the controller, plus future proof for a panel.

My only other recommendation would be to use conduit, even if the cable is rates for direct burial. This helps protect against a random spade, and also allows you to change / replace / add in future without more digging.

sparkz25

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#2801937 27-Oct-2021 12:41

If its only 5m, I would run mains and install a weather proof power point out there, 5m trench isnt very long at all.

Would the trench be under concrete or in the dirt?

SirHumphreyAppleby

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#2801951 27-Oct-2021 13:20

Ge0rge: Could you put a battery of some description in the greenhouse? ...
My only other recommendation would be to use conduit, even if the cable is rates for direct burial.

All good suggestions. I am intending to use some sort of conduit regardless or cable type. A battery is an option if someone can point me towards a way to use one as a buffer. The pump, solenoids and actuators will only run for short periods of time, so a trickle could definitely work. The house already has solar and if necessary (pressure tank size allowing) the pump could be configured to mostly run during daylight hours, but given the power the computers and pool pumps use, it's not going to be noticed on the power bill.

sparkz25:

If its only 5m, I would run mains and install a weather proof power point out there, 5m trench isnt very long at all.

Would the trench be under concrete or in the dirt?

It's a mixture of concrete and dirt. Digging a trench is not a huge concern, the existing infrastructure is. We know there is nearby waste water, a sewer line to cross and the storm water pipes converge in that area.

Zeon

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#2801955 27-Oct-2021 13:35

At 10A you will get decent voltage drop on 1.5mm2 cable. I'm guessing it will get worse as the pump runs and the cable heats up. Here is a site with a useful calculator:

https://photovoltaic-software.com/solar-tools/voltage-drop-calculator-dc-ac

At 10.5A its showing a 2.48V drop or 20.67% on 1.5mm2 cable

Rather than the complexities of an additional buck converter, for only 10m maybe just get slightly bigger cable like 4mm2 or 6mm2? Would be cheaper than shipping in a buck converter for only 10m I would think and be less complex.

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SirHumphreyAppleby

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#2801962 27-Oct-2021 13:56

Zeon:

Rather than the complexities of an additional buck converter, for only 10m maybe just get slightly bigger cable like 4mm2 or 6mm2? Would be cheaper than shipping in a buck converter for only 10m I would think and be less complex.

Certainly simpler. So using 4/6mm^2, I could get away with just using a 12V power supply?

Selecting the worst case scenario, 1.84V drop over 10m at 100C, 4mm aluminium core.

That being the case, I have a 12V 40A power supply somewhere which should do the trick.

Now the challenge is to find a roll of inexpensive 6mm cable suitable for burial.

mdooher

Hmm, what to write...
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#2802028 27-Oct-2021 14:39

SirHumphreyAppleby:

Zeon:

Rather than the complexities of an additional buck converter, for only 10m maybe just get slightly bigger cable like 4mm2 or 6mm2? Would be cheaper than shipping in a buck converter for only 10m I would think and be less complex.

Certainly simpler. So using 4/6mm^2, I could get away with just using a 12V power supply?

Selecting the worst case scenario, 1.84V drop over 10m at 100C, 4mm aluminium core.

That being the case, I have a 12V 40A power supply somewhere which should do the trick.

Now the challenge is to find a roll of inexpensive 6mm cable suitable for burial.

get a cheap bit of 16mm 2 core neutral screen from you friendly local wholesaler. you could use single core but the 2 core will be easier for you to terminate... you can get 6mm as well which will be even easier to terminate depends on what you have in way of crimpers.

you will not need conduit of any sort.

Matthew

SirHumphreyAppleby

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#2802179 27-Oct-2021 17:51

mdooher:

get a cheap bit of 16mm 2 core neutral screen from you friendly local wholesaler.

Out of curiosity, what do you consider cheap? Cheapest I've found online is around $20 per meter. Being in Auckland, I can't just pop into the local wholesaler.

Running the calculations and sticking to a recommended 3% loss, I'd need 16mm^2 cabling. 10mm^2 would keep losses below 5%, which is probably good enough too.

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richms

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#2802690 28-Oct-2021 12:08

Use landscaping lighting cable. It has a ridge on it to ID the negative.

Voltage drop doesn't matter too much for charging as it will just take longer to get there. I would put a 9AH sla in the shed with the loads and a plugpack charger at the house with the long landscaping lighting cable out to the shed. Fuse the cable at the battery Incase someone puts a spade thru it.

Richard rich.ms