12V Volt Drop

[Chris Electron]

Hello,

I seem to be getting a bit confused here and wonder if anyone out there could give me a bit of help.

I am wiring a new build in North London. The Architect is also the client and he is putting some pretty funky and complex kit in the build.

The house has large shutter over the windows that he is using for security and insulation. He wants to run these off a 12V leisure battery because they have to open if the mains power is down for escape purposes.

I do volt drop calculations every day of the week but I want to know whether the calculation for volt drop with 12V is the same-

For example-

Is the maximum volt drop allowed (other than lighting) still 5%?
Can you use the same mV/A/m figures that are in the regs book bs7671?

If I do this calculation-

3A x 18 (for using 2.5mm cable) x 10 metres / 1000

= 0.54

Where 5% of 12V maximum drop would be 0.6V so above calculation would pass.

Am I doing this right?!!

Also I should be using some sort of 12V circuit breakers also? It all seem a bit camper van or yachting type work! Anything else I may have missed?

Any help gratefully received.

Thankyou,

Chris.

 

[telectrix]

sounds right to me. i would be inclined to use a fuse in line with the battery + rather than a breaker.

 

[john999uk]

Re Voltage drop. I would be very wary of you specifying cable sizes and voltage drops for the 12v system. The reason I say this is that it's a safety critical system!!! I think that the local Fire Officer needs to be involved. While there may be no intention to have the shutters in use while the building is occupied it might happen. Imagine if the shutters are down and there is a fire!! Without the shutter motors being able to operate people are trapped!!!! 5% may or may not be safe but it's the motor manufacturers who can tell you not BS7671.
I would pass this back to the Architect and get it in writing what he wants done and what his design is and it has cleared building control and Fire Brigade etc.

 

[Chris Electron]

Thats really good advice thank you. It's one of these jobs where he keeps throwing things at me and I am kind of thinking that he is out of his depth.

When working out some of this volt drop stuff anyway it looks like he will need like 10mm cable. Would be interesting to see how other Architect/ designers go about this kind of thing.

Many thanks.

Re Voltage drop. I would be very wary of you specifying cable sizes and voltage drops for the 12v system. The reason I say this is that it's a safety critical system!!! I think that the local Fire Officer needs to be involved. While there may be no intention to have the shutters in use while the building is occupied it might happen. Imagine if the shutters are down and there is a fire!! Without the shutter motors being able to operate people are trapped!!!! 5% may or may not be safe but it's the motor manufacturers who can tell you not BS7671.
I would pass this back to the Architect and get it in writing what he wants done and what his design is and it has cleared building control and Fire Brigade etc.

- - - Updated - - -

Thankyou for your reply. Glad I am doing this right!

sounds right to me. i would be inclined to use a fuse in line with the battery + rather than a breaker.

 

[yellowvanman]

Dosn't the shutter motors have a spec for their working voltage range? If so, use that to work out what vaolt drop you're allowed.

 

[Darkwood]

Thick curtains provide good insulation and a baseball bat give security :wink_smile:

 

[Knobhead]

3% Volt drop is 3% whether it be 11KV or 5V. Where's the problem?

 

[Chris Electron]

This is a good point. The guys who manufacture the actuators came back to me today and just said that they can take even a small voltage but the shutters would move slowly. So maybe it doesn't matter!! Not much mentioning of 12V batteries in BS7671 and chatting to NICEIC today they didn't give me any useful help.

Dosn't the shutter motors have a spec for their working voltage range? If so, use that to work out what vaolt drop you're allowed.

 

[Robert Turnbull]

I've fitted mains shutters with a link to the fire alarm. When the alarm is activated it opens them. Locating a break glass next to it would maybe do the trick

 

[Chris Electron]

Again this sounds like a plan to me. Thankyou. The thing is he is building an arduino system which I am not an expert on at all but the way he is doing it sounds a bit boffin rather than pro electrics. This is why he is keeping everything 12V. There is a lot if space issues so he can't have batteries installed locally to each shutter. It is driving me mad now.

 

[ayjay]

read the spec

 

[guest119]

Cable would need to be fp200 or the like to maintain the supply in the event of a fire as it would be an essential service.

Remember ventilation ( in and out) at the battery charging area.

Last time I saw something similar the supply applied would wind up the spring in the actuator and then close the shutter, about 5 mins.
To open the shutter the supply is removed and the spring opens the shutter.

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Boydy

 

[Robert Turnbull]

If I remember rightly the shutter had a battery pack in with the motor for use in a fire situation. I could be wrong, this was quite a few years ago. It was wired in FP was a commercial fire alarm system