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32A to 16A 3 phase question


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Hi There,


You could have an extension lead 500 yards long, it makes no difference so long as you have calculated the size needed.


The current rating of the cable rarely comes into it. The usual factors are "earth fault loop impedance" [so that fuses and circuit breakers will operate correctly] and "volt drop" so that motors will start correctly and contactors not drop out etc


You have to calculate the size, as most of the cable calculators online are a mile out as they go about things backwards..


1, First step is select the OCPD [over current protection device] [that is fuse etc] size and TYPE you need.

2, Calculate the max EFLI for the breaker selected.

3, Calculate the max allowable volt drop.

4, Select a cable to suit 2 and 3 above.


Obviously the cable nominal current rating has to be higher than the load rating AFTER you have applied the derating factors, [grouping, how installed, [in the ground, clipped to wall, insulation etc] ambient temperature, max conductor temperature] but in the real world, you can EASILY, and usually do, end up with a cable with a nominal current rating FAR FAR higher than the load current, purely to meet the criteria for EFLI and volt drop..



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5 hours ago, sandspider said:

Sorry to hijack the thread, but I occasionally use a 7T forest master log splitter with a 25m extension cable. Think the splitter is 2200kw. I know using it with an extension lead longer than 10m isn't ideal, but it seems to work fine, and has done for years now. Am I damaging it?

Single phase or three phase??

I would imagine you are fine, the motor would have burnt out by now otherwise.


i will calc the cable size for you later, but a lot depends on the source of the power as this could be 2 feet from the consumer unit which could be 30 feet from the local substation, or is could be a shitty overhead supply 500yards long..



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Thanks John!


It's single phase, and the extension lead is plugged into an outdoor socket about 5m from the consumer unit. But our house power supply is rural Welsh, so a shitty overhead line that runs through trees, sheep etc.

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