3 main wires to one light?


Postby garfy787 » Wed Nov 16, 2011 11:26 am

I have removed my strip lights from the garage to find 3 main wires go into one, then 2 main wires out one to each other seperate strip light. Can anyone tell me why 3 main wires go to first light? I am repalcing only 2 lights, do i need to run all wires into first light then take one lot out for second light?
When i say main wire i mean one grey wire with 3 wires within it, red, black and earth, therefore in total i have 3 red, 3 black and 3 earth. Hope this makes sense!!
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Postby ericmark » Thu Nov 17, 2011 5:36 am

The standard method to wire in a house is to daisy chain the supply from one ceiling rose to next and to take a cable from each ceiling rose to the switch. This means each lamp apart from last will have three cables connected. The last lamp will have two cables. Forgetting earths which clearly all connect together each lamp except last one will have:-
- Line feed in (red or brown)
- Line feed to switch (red or brown)
- Line feed to next lamp (red or brown) [not in last one]
- Neutral feed in (Black or Blue)
- Neutral feed to next lamp (Black or Blue) [not in last one]
- Line return from switch (In theroy this should be red or brown but in practice will be black or blue and should have red or brown sleeves or tape on the wire. This is the wire which causes many problems as many DIY people think it's a neutral and get it mixed up)

This method is not always used and with a garage you may find it goes to switch to switch with feed. Where units are used which don't allow one to use them as a connection box as for example extra low voltage transformers electricians may use the switch box to do the loop connection. But looping from switch to switch means there is more cable in the circuit and it can mean problems in keeping the loop impedance and volt drop within limits. Also it means a permanent line is available for any emergency fittings.

With commercial premises it is common to use plug in ceiling roses to feed each florescent lamp fitting, these then like with the house loop the feed from one rose to next and allow one to unplug a single lamp to work on it without needing to switch off the rest. The Klik PCR2000 6A 3 Pin Plug In Ceiling Rose is used a lot but the Maestro Ceiling Master has a hook built in and is better where then lamp is being hung from jack chain. There are a few versions including emergency types which have four connections as a permanent line is required for emergency lamps.

You will find more including diagrams in the projects section.
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