Morris Minor Owners Club

A little help needed please. I want to run electric 17 metres underground, and in working out the cable size I have come across a list of things I need to take into consideration. A couple of them I don't understand, they are-----full load power(pu)---load starting current----starting power factor. Full load current(A), I presume is the total amps likely to be used at any one time? Also do I need to consider wattage? One more query---Load type (motor or feeder.)

Les,

Work out what the maximum load you are likely to put on the far end of the cable. Amps or Watts doesn't matter because if you know one you can calculate the other, assuming that you are using nominal 230 Volts

1. Power (watts) = I (current in Amps) x V (voltage , in this case 230 volts), therefore
2. I (current) = Power (Watts) / V (voltage - 230 volts).

Use this calculator to help you, using the following settings:

1. Cable Type = SWA 3 Core XPLE (armoured cable)
2. Application = Underground
3. Circuit Type = Non-Lighting Circuit
4. Ambient Temperature = 15C (it's cool down under the ground)
5. Number of circuits = (number and separation - If only 1 circuit then separation should have no relevance)
6. Power = (n) KW, calculated using the formula above
   (Stating the obvious, 1KW = 1,000W. For example:
   • 13A (normal maximum for a single plug) = 2990W = approximately 3.0kW
   • 16A (normal value for a power circuit breaker in a garage/outhouse) = 3680W = approximately 3.7kW
   • 22A (normal for 16A power circuit plus 6A lighting circuit) = 5060W = approximately 5.1kW
   (Add some contingency to the maximum requirement that you currently forsee, for safety/future use - the cost of the next highest cable now is probably less than the cost of replacing an underspecified cable later!)
7. Voltage = 230 volts (normal for domestic electrics)
8. Length = 17 meters (in your case)

If in doubt 'go large'

That should give you an approximate value, but please remember that Part P regulations apply to domestic cabling and a 'competent person' should carry out the work in accordance with Building Regs / Electrical Regs.

Please stay safe!!

Hope that helps.

Thanks for taking the time to give that detailed reply, I shall work my way through it! As you say I will need a certificate from a competent person but I will be doing the digging!

I think that I shall forget about the garden water feature!!

Over the years i have had a few cables run to out buildings and lights, i have always tended to go large as adding a heater or a few outside lights can put you near the max rating, but the one piece of advice i got was always use ducting of some sort, if you have a cable in the ground its a pain to replace but if its ducted you can get a new cable in easier, and i have alarm wires, data and cctv cables , all shielded as the mains can mess with some cables . .. The cable i have in use at the moment is 10mm armoured XPLE hope this helps. Rob

Thanks Rob, all tips and advice gratefully received.