cable sizing / length of circuit

[Ianm]

I am planning on running armoured cable from an rcd protected fused spur out to a shower proof area housing pumps etc and weatherproof switches for a pond. The pumps do not draw much (max 3 amp) and the length is about 3M from CU to spur (internal) and 5M from spur to pump (external). This seems straightforward and doesn't need anything heavy duty. However the cable will be flagged over and in the future I will need to add some lighting and maybe even an exteral socket. The lighting will require further runs of HV cable to transformers maybe 30M away. The maximumum draw will be 16 amp as it's own radial circuit. I don't want to have to dig the flags up when I add the lighting so would I be advised to put in 4mm armoured cable now? What about the cable between the CU and the spur - would 2.5mm be OK? What is the longest run I could get away with in the garden and size of cable required and finally if the HV cable is only supply transformers for lighting does voltage drop matter anyway? ( I know this isn't the only reason for bigger cable - I'm just curious).

Lots of question - grateful for any answers.
Thanks
Ian

 

[ricicle]

Have a look at the cable sizing guide on TLC Direct, it should point you in the right direction.If you are still in need of advice then post again

 

[Ianm]

Just had a look. The website indicates that I can use 1.5 mm for the whole length (which seems light). It also states that there will be quite a voltage drop over 30M. There isn't any function that allows a calculation of voltage drop with a larger gauge cable. Does voltage drop matter if it's only being transformed down to 12v anyway? I know there is a calculation behind all this but haven't been able to track it down - can anyone let me know what it is?

 

[davy_owen_88]

Just for future reference you are never going to come into contact with 'high voltages' in a domestic situation (well unless you are unlucky enough to get struck by lightning...) Low voltage and extra low voltage are the correct terms:

ELV - <50V
LV - 50 - 1000V
HV - >1000V

Regardless of whether voltage drop will be a problem or not, it should not exceed 4% or 9.2V. If you want 16A at 30m then 2.5mm² SWA just about scrapes through but 4mm² would be better.

The calculation for voltage drop is:
VD = (mV/A/m)

You get the VD figure from the tables in the regulations and then plug in the numbers:

1.5mm² - 29
2.5mm² - 18
4mm² - 11

So for 16A on 2.5mm² SWA at 30m:

18mV / 16A / 30m = 8640mV / 1000 = 8.64V

It would be a good idea to run a duct for the cables and add a few draw lines so you can pull cable through as and when you need it. It's also a good idea because if the cable gets damaged or needs upgrading then the whole garden doesn't need to be dug up.

 

[bernardgreen]

If the pumps are mains voltage take account of the start up current when calculating the size of cable. The start up current can be many times the running current. The voltage drop along the cable during start up can mean the motor takes longer to reach running speed and thus the higher current is there for longer and this can shorten the life of the motor.

Voltage drop in the feed to a 12 volt tranformer is important, if the input voltage is reduced the ouput voltage drops by the same per centage.

This doesn't always apply to electronic ""transformers"" which are not true transformers ( coil to coil ) but are a form of crude power supply.

Ducting is a very sensible idea. Make it over size so you can add those extra services later.

 

[bernardgreen]

Davy suggested putting in more than one draw cord.

Doesn't always work as the cords sometimes twist up with each other ( as do thin very flexible cables already in the duct ) when a new cable is drawn in.

Put a single draw cord in when you lay the ducts. rot proof cord, annoying to be pulling in a new cable when halfway in the cord snaps having rotted away over the years.

When pulling in a new cable pull in a new draw cord at the same time.

 

[Ianm]

Thanks to all - very useful advise and info

 

[Ianm]

Just one final question - presumably it's OK to reduce the cable gauge as the load decreases. EG 4mm from the CU through to the switches to support the total load, but then only 1.5mm to connect to an individual transformer if the voltage drop to the end of the cable remains within 4% overall?


Thanks
Ian

 

[ricicle]

Just one final question - presumably it's OK to reduce the cable gauge as the load decreases. EG 4mm from the CU through to the switches to support the total load, but then only 1.5mm to connect to an individual transformer if the voltage drop to the end of the cable remains within 4% overall?


Thanks
Ian

Yes - as long as the correct protective devices are fitted for the smaller cables.