Powering an Observatory

[old_eyes]

OK.

The frustrations of hauling my kit in an out of the house and polar-aligning each time is making me think about an observatory for about the fourth time.

I have a field which has pretty good horizons to east and west (west over the house), To the south I am clear above around 25 degrees, North I have a hedge that blocks quite a lot, but Polaris is clear.

East is my local town, so although that is my best horizon there is a fair bit of light pollution. North is my nearest city and therefore more pollution. South and West I am looking at hills and its reasonably dark.

The main internal debate is how to power the observatory. The logical location is about 50-60m from the nearest point of the house and another 25m to the consumer unit (long thin house).

I want to do imaging so a fair few electronic bits and bobs. SO far I have managed well for any session I have had with a 70Ah leisure battery. But with an observatory I would do more and I can fully charge the battery before a session.

Part of me says firmly "go off-grid. Batteries and PV will see you right". Part of me says "Yes but you'll want one of they fancy de-humidifiers, and you would like to run an ethernet cable back to the house".

So views please. And in particular what's the crossover point in cost likely to be between buried armoured cable and the PV panels and batteries to run happily for a full night? 10m? 50m? 100m?

Any thoughts welcome.

old_eyes

[Andy McK]

Hi Old_eyes,

I would start here first. http://www.stockport.gov.uk/2013/2994/developmentcontrol/14406/41351/partpelectricalsafety

When you get fed up of reading through "Part P"

Have a gander through this lot

http://wiki.diyfaq.org.uk/index.php?title=Taking_electricity_outside.

I'll have a quick rmage, as I saw another thread, with roughly the same questions

[old_eyes]

Hi Old_eyes,

I would start here first. http://www.stockport...lectricalsafety

When you get fed up of reading through "Part P"

Have a gander through this lot

http://wiki.diyfaq.o...ricity_outside.

I'll have a quick rmage, as I saw another thread, with roughly the same questions

Thanks for the links.

Yep I know about Part P and would certainly want a qualified electrician doing the work, but the question is what is the practicality of running an observatory off-grid and how would costs compare with getting a new circuit installed. I'm thinking (but I may be wrong) that you would end up needing a pretty expensive PV installation if you did not want to start trucking batteries back to the house to charge.

old_eyes

[old_eyes]

I found a thread here which is pretty negative on the whole topic (including a forgotten question from me).

What I am interested in is comparative costs for a practical installation mains vs PV. Anyone have a rule of thumb for costs of buried armoured cable per metre?

old_eyes

[pmlogg]

My observatory is very close to the house but I'm certain that the electrician was the main cost, and that armoured cable was not particularly expensive. Other than the add-on consumer panel and the armoured cable I bought all the rest i.e. IP rated socket boxes, boxes for the exit points of communicaton cables etc. I've certainly not regretted doing that rather than using batteries plus 12V to 230V convertiers. I followed the advice of others and put in lots of sockets - not come close to using them all yet. One of them is integrated with a timer to allow the dehumidifier to be switched on at set intervals.

[earth titan]

Part of me says firmly "go off-grid. Batteries and PV will see you right". Part of me says "Yes but you'll want one of they fancy de-humidifiers, and you would like to run an ethernet cable back to the house".

Don't forget the kettle, the dew heaters etc. etc.

[dmahon]

Part P has changed recently (much more relaxed).

I've been reading up a bit because I need to export cable a similar distance. There are a few things to consider, including:

1) Minimum size of cable required for power needed

2) Minimum size of cable required for voltage drop allowed

3) Earthing requirements

For an 85m run, you would need 10mm² cable for sockets (5% voltage drop allowed) or 16mm² cable for lighting (3% voltage drop allowed).

You also need 10mm2 cable to export earth (assuming you wish to) so I'd get get 3 core SWA.

That size cable can carry a large current, so you don't need to worry about that.

http://www.doncaster...ble_calculator/

A reel of 100m of 3 core 10mm² SWA will cost you about £400

[tony]

Was going to say 10mm isnt cheap, i needed it for my shower and it cost me nearly 60 quid.

[old_eyes]

Part P has changed recently (much more relaxed).

I've been reading up a bit because I need to export cable a similar distance. There are a few things to consider, including:

1) Minimum size of cable required for power needed

2) Minimum size of cable required for voltage drop allowed

3) Earthing requirements

For an 85m run, you would need 10mm² cable for sockets (5% voltage drop allowed) or 16mm² cable for lighting (3% voltage drop allowed).

You also need 10mm2 cable to export earth (assuming you wish to) so I'd get get 3 core SWA.

That size cable can carry a large current, so you don't need to worry about that.

http://www.doncaster...ble_calculator/

A reel of 100m of 3 core 10mm² SWA will cost you about £400

Thanks that's clear on the type of cable.

So it looks like the materials cost is not going to be a huge item. Manpower could be. Have you got as far as getting any quotes? Are you planning to do the trenching and laying yourself?

old_eyes

[old_eyes]

Don't forget the kettle, the dew heaters etc. etc.

Dew heaters I had allowed for - damn! forgot the kettle :-)

old_eyes

[old_eyes]

My observatory is very close to the house but I'm certain that the electrician was the main cost, and that armoured cable was not particularly expensive. Other than the add-on consumer panel and the armoured cable I bought all the rest i.e. IP rated socket boxes, boxes for the exit points of communicaton cables etc. I've certainly not regretted doing that rather than using batteries plus 12V to 230V convertiers. I followed the advice of others and put in lots of sockets - not come close to using them all yet. One of them is integrated with a timer to allow the dehumidifier to be switched on at set intervals.

If you don't mind me asking; what was the all up cost of connecting your observatory - just to give me a sense of materials vs manpower?

PM me if you would prefer.

Thanks

old_eyes

[dmahon]

I'm going to do most of it myself - needs someone part P to connect a new circuit though and will get them in before I start to make sure they are happy with the design.

I need to connect 3 sheds, fortunately I can go from on to the next.

100m ridgicoil conduit

100m cable

3 x garage RCD Consumer units with 16/6A MCBs

32A MCB (non RCD protected*) for house CU

Adaptable boxes and glands

Sockets/conduit

I reckon on £650 for the parts and £100 to hire a trencher, plus whatever the electrician charges to connect it up.

* should avoid two similar RCDs in series - fortunately I have a high integrity CU in the house with a spare unprotected way available. Alternatively you could split the incoming electricity with a Henley block into a second CU.

[old_eyes]

I reckon on £650 for the parts and £100 to hire a trencher, plus whatever the electrician charges to connect it up.

Very interesting. Thanks a lot.

old_eyes

[Colin A]

Just another thought, depending on how often you are out at the observatory what about a generator (a silenced one of course). Just another option to look at.

[malc-c]

Get some quotes in for having mains run into the observatory. If you go for a completely permanent set up with a warm room, PC's etc then mains is the way to go. I'm no expert but I think some of the cable size etc have been over specked. The previous owners of the house we are in had main laid in up to a shed at the top of the garden. It's 2.5mm armored cable, and is wired to a 13A switch via a spur from the consumer unit.

The advantage of having mains in an observatory means you don't have to plan any observing sessions - you can take advantage of those short periods between clouds when they happen

[dmahon]

Regulations change over time. What used to be acceptable may no longer be for a new installation (eg 4% voltage loss was fine before, now it's 5% for sockets but only 3% for lighting). That's more likely to be the cause of a large cable size than the actual amperage requirement of the cable.

If you know what amperage you need and what distance you want to run, the calculator will tell you what size cable is required (though you may need a minimum of 10mm anyway if you want to export earth and have TN-C-S earthing at the house).

(if the 3% requirement makes you go up to 16mm, you could always just install sockets and then spur some lighting off on a fused connection later, or even add a lighting sub-circuit later on - if 4% loss is acceptable in existing lighting circuits and they don't have to be redone, it can't be dangerous).

[Gina]

If you connect the house end of the cable to a 13A socket with a 13A fused plug, it counts as a temporary circuit - like an extension cable, so doesn't need Part C certification. Use an RCD though and properly rated garage comsumer unit in the obsy for you own protection. You'll still want to calculate your volt drop in the cable and use a sufficient size cable for that reason. It doesn't matter that the cable is rated at more than 13A as that is the main overall overcurrent protection though you will be using a lower rating in the obsy for lights.

[old_eyes]

Thanks for all the advice. Time for an exploratory discussion with a friendly electrician.

I might be able to get away with your solution Gina, but it would feel uncomfortable. In general, and in most cases, I believe such regs usually capture good practice, and you need a good reason to do something else.

What the contributions are telling me is that no one has much faith in an off-grid solution using PV. I could use a generator, but somehow that doesn't sit right with me. I think mains power is the way to go unless someone wants to make a late impassioned plea for renewables.

old_eyes

[Gina]

That's fair enough and I certainly agree that if you're not experienced with electrics or don't feel capable, you're much better seeking the advice of a qualified electrician. Just make sure he/she really DOES know what they're talking about - I've come across lots of self styled electricians who were not up to the job. My mother had one, recommended by her son in law (and my brother in law), who made such a mess of the job that I ended up fixing it. It was then checked and tested by a properly qualified council approved electrician who was quite impressed. I've also done other domestic and professional wiring jobs to civil and military standards so I think I can say I know what I'm doing. (Sorry about blowing my own trumpet )

[ian_bird]

* should avoid two similar RCDs in series

Out of interest - why is it necessary to avoid that?

Thanks and Regards

Ian

[dmahon]

There should be "discrimination", so the downstream circuit doesn't take out the upstream one and so you can test them I guess.

Personally I don't see the problem, but google RCD in series and there are plenty against.

[old_eyes]

That's fair enough and I certainly agree that if you're not experienced with electrics or don't feel capable, you're much better seeking the advice of a qualified electrician. Just make sure he/she really DOES know what they're talking about - I've come across lots of self styled electricians who were not up to the job. My mother had one, recommended by her son in law (and my brother in law), who made such a mess of the job that I ended up fixing it. It was then checked and tested by a properly qualified council approved electrician who was quite impressed. I've also done other domestic and professional wiring jobs to civil and military standards so I think I can say I know what I'm doing. (Sorry about blowing my own trumpet )

Gina - yours is a trumpet worth blowing!

I have read your many threads, contributions and builds with fascination and admiration.

i have access to a couple of reliable electricians, and armed by the expertise I can gather from SGL and a lot of experience in designing and constructing both high and low voltage systems in a research past, I am confident I can ask the right questions.

It's more case of being time-poor and wanting a solid and reliable job. I am also considering restructuring part of the house electrics as I am not satisfied with the layout in a house that has been extended many times. So there is an opportunity to get it done as part of a general overhaul.,

Keep up the good work of inspiring us all to more audacious projects.

old_eyes

[Earl]

Personally I just used a caravan hookup extension leads and adapters to my old observatory.

[old_eyes]

Personally I just used a caravan hookup extension leads and adapters to my old observatory.

I could do that Earl, but it adds to the general hassle factor. Keeping it all tidy would steal time from using the obs.

old_eyes

[dmahon]

My plan is to use caravan leads as a temporary measure, but put a garage consumer unit and permanent sockets / lighting in and then wire it permanently when I get round to the trench (and have saved up again).