The gate to Aston Lane is about 350m from the house, via the access track running through the adjacent field. My intention has always been to have an automated gate there (though not from day 1, depending on the budget) but that means:
- There needs to be a power supply sufficient to operate the gate and any other equipment located nearby.
- There needs to be a data connection of some sort, so the gate can be opened and closed remotely and for an intercom so visitors can announce their presence. Maybe also a camera.
Option 1 – Run Cables from the House
One option is to dig a 350m trench and run a power and data cable in that. The data cable would have to be fibre instead of copper, since there’s a 100m length limit on copper Ethernet cables, but that’s not so bad. In many ways it’s the power supply that’s more problematic.
Option 1 – Data Cable
Fibre optic cables are a standard solution for medium-distance data transmission. An easy option is to buy a pre-terminated armoured fibre cable of the right length and bury that in a trench. The fibre is immune from electrical interference so it can go fairly close to a mains electrical cable without any risk of the data transfer being corrupted.
From a supplier like Cable Monkey the cost would be roughly £370 for 300m of OM2 fibre (OK for 1Gb/s at up to 550m) protected with CST (Corrugated Steel Tape) armour plus roughly another £100 for a pair of media converters to go from fibre to copper Ethernet. That’s actually quite expensive! It might be cheaper to have the fibre terminated on-site instead.
Option 1 – Power Cable
The problem with the power cable is the voltage drop under load, due to the resistance of the cable, and the wiring regulations say you need to use a big enough cable to limit the voltage drop to 5% under maximum load. For any substantial load you need a cable with a decent cross-sectional area, and long cables containing a lot of copper can be really expensive.
For example, a 6A load (1380W @ 230V) supplied over 300m of underground SWA cable requires a 10mm^2 cable (based on the voltage drop calculator provided by TLC Direct) and that size of cable is £325 per 100m; call it £1,000 for the 300m run.
The trick is to reduce the maximum current as much as possible. At 3A (690W @ 230V) a 4mm^2 cable is more than adequate and at 2.5A (575W @ 230V) a 2.5mm^2 cable is just OK. The 2.5mm^2 cable is a third the price of 10mm^2.
So how much current does a gate operator draw? They vary, of course, but since I will have a gate of nearly 4m width (in order to provide easy access for farm machinery) and since my preference is for an “underground” operator (hidden away and so much less likely to be damaged than the “extending arm” type) I could be looking at something like the CAME Frog Plus which comes in two variants: the FROG-PM4 draws 5.1A (1200W) and the FROG-PM6 draws 2.6A (600W) – the PM4 spins faster and hence moves the gate more quickly but as a result it draws more current.
Since the control panel and the intercom will draw a little extra power even the FROG-PM6 would mean a 3A requirement and hence the 4mm^2 cable; getting on for £500 for the cable alone. Arguably that’s not a big deal compared to roughly £3,500 for the CAME Frog Plus gate automation kit though.
TODO – Confirm that a 12′ or 13′ steel 5-bar gate really does need something like the CAME Frog Plus. A solid timber or steel gate would provide much more resistance, especially on a windy day, and would be much heavier. However, it’s not wise to specify a gate opener that is operating at its maximum limit.
Option 2 – Go Off-Grid
An alternative option is to use batteries as the power source and install a wireless data link. Domestic gates don’t get used very frequently and it’s not mad to think about powering them from a battery which gets re-charged at a fairly modest rate, most likely from a solar panel.
Option 2 – Wireless Data
Initially I was sceptical about the ease of getting a decent data connection over a 350m distance. I already have an outdoor WiFi Access Point set up (a Ubiquiti Networks UniFi UAP-Outdoor+) which will be staying long-term (with a possible upgrade to 5GHz) and while that works fine for WiFi clients like smartphones within the boundary of the site the signal quality drops off quickly with distance and a smartphone won’t maintain a WiFi connection more than about 100m from the Access Point. But then a smartphone isn’t optimised for long-distance WiFi…
Ubiquiti Networks offer a wide range of long-distance wireless data transmission solutions and their high-end devices (like the airFiber 5) will do 1.2Gb/s over a 100km distance (though presumably they need line-of-sight and very careful alignment). I don’t need 1.2Gb/s (yet) and I definitely don’t need 100km!
I found a blog post on linitx.com (my preferred supplier of Ubiquiti Networks gear) – HowTo: Building to Building PTP links using Ubiquiti Airmax products – which helped convince me there was a good chance of a NanoStation Loco M2 acting as a WiFi client of my existing outdoor WiFi Access Point over a 350m distance, and if that didn’t work it was almost certain a pair of NanoStations running back-to-back would be OK. I decided to risk £44 to find out and sure enough it works great – showing “five bars” of signal strength and about 40Mb/s connection speed when located at the gate. Actually the line-of-sight distance from the gate to the Access Point is a bit less than 350m since the track is curved.
One small complication is powering the NanoStation from something other than 230V mains. It uses non-standard 24V Power-over-Ethernet which is normally provided by a power injector (supplied with the NanoStation) which connects to the 230V mains. A workaround is use a 12V DC to 24V DC voltage converter (such as this one from eBay.co.uk) powered from a car battery and wired into a passive power injector like this one which I happened to have spare. Using this configuration I measured the NanoStation Loco M2 drawing 300mA @ 12V when idle and 330mA when transmitting or receiving at 10Mb/s. 300mA equates to 7.2Ah per day of 12V battery capacity.
Conclusion: Wireless data is perfectly feasible and a lot cheaper than a cable. It just needs power, although not a lot, and some means of mounting the radio out of harms way.
Option 2 – Off-Grid Power
Some gate automation solutions include a battery back-up as a standard feature, primarily so that the gate can still be opened in the event of a power cut. I’m sure I’ve seen some where you have two batteries and swap them periodically so that one can be removed and re-charged where there’s power available. Some gate openers are specifically advertised as “solar powered” – see e.g. this page from The Electric Gate Shop.
Option 2a – All-DC
The ideal is for everything to be powered by DC, and preferably all at either 12V or 24V rather than a mixture of the two. There are some 12V DC gate motors and some 24V DC gate motors – for example the CAME Frog (but not the CAME Frog Plus) has a 24V DC option.
In principle then it would be possible to build a system based on:
- One or two lead-acid batteries of a suitable specification to tolerate deep discharge and with a suitable ampere-hour capacity to support a couple of days of use without discharging more than about 50%.
- A solar charge controller to manage the charging of the batteries and to stop them getting discharged too much.
- A solar panel and / or a small wind turbine to re-charge the batteries, sized sufficiently to ensure the batteries are charged enough during daylight hours to offset 24 hours of use, even on a winter day.
TODO – Review the power requirements of a DC gate motor (based on a realistic duty cycle) and all the other equipment, and assess the practicality of keeping the batteries charged with a solar panel. It wouldn’t be the end of the world if the batteries had to be removed to be recharged under particularly unfriendly weather conditions, maybe a couple of times a year.
Option 2b – Battery-Powered Inverter
The bigger and more powerful gate motors all seem to want 230V. However, if they’re only drawing something like 3A at 230V that wouldn’t require a particularly large or expensive inverter to provide 230V from 12V or 24V batteries. This would still need the batteries, the charge controller and the solar panel (or wind turbine) but would also add an inverter. Need to be careful of the extra current drain of an inverter even with no load connected, so if the all-DC approach isn’t practical this wouldn’t be either.
Option 3 – Hybrid Approach
Of course it’s possible to opt for a hybrid approach – e.g. connecting a small power cable but carrying data via WiFi – and maybe using a battery to power the gate but then keeping that charged via a mains-powered trickle-charger would greatly reduce the peak current requirement (and hence the size of the cable) while also providing a battery backup if the mains were to fail.
The smallest available size of SWA cable (1.5mm^2) will carry 350W (1.52A @ 230V) over a 300m distance with exactly the maximum-permitted 5% voltage drop, and that would cost around £230 for the cable (plus the labour to dig and back-fill the trench, say another £500).
Update May 2022
After several years of not progressing with this, the need to dig a trench to install a duct to carry an Openreach Fibre-To-The-Premises (FTTP) broadband connection has put it back on the priority list.
My thinking has evolved somewhat:
- Since the trench is being dug anyway, it seems a shame to not install an electrical cable at the same time
- It would be good to have some flexibility in terms of initial and future gate motor options, so the cable should be sized at least 25% larger than the bare minimum
- The absolute maximum permitted power draw over 350m on a 4mm^2 cable is 750W, with the motor wanting at least 600W of that and the controls about another 50W, so only 100W spare
- A 4mm^2 SWA cable is about 25% cheaper than 6mm^2, whereas 10mm^2 is about 60% more expensive than 6mm^2
- 6mm^2 over 350m is good for 1kW so that feels like the one to go for
WiFi still seems the best option for the data:
- The Ubiquiti NanoStation Loco M2 is still earmarked to act as the WiFi ‘client’ on behalf of the wired Ethernet devices (the access control unit and the audio/video intercom)