Wiring size for compressor

[JohnW2]

I may be very wrong, but I get the impression that these adiabatic calculations are probably not often routinely undertaken.

I suspect that you are probably correct in what you meant. However, 'often routinely' is a bit confusing.

Agreed, my wording was confusing. Just 'often' or just 'routinely' would have been adequate. Alternatively, I might have said that I doubt that many electricians undertake such calculations 'routinely'.

With routine domestic circuits, it is unlikely that it would not comply with the requirements. In a special circuit such as this thread it may be necessary. Having said that, increased csa if employed would make the likelihood of non-compliance less likely.

All true.

Kind Regards, John

 

[JohnW2]

Whereas with (2), you rightly say that one has to ensure that the loop impedance is low enough to guarantee operation of the MCB under fault conditions, in terms of the adequacy of the the CSA of the CPC (determined by an adiabatic calculation), the need is to ensure that the loop impedance is high enough for the CPC's CSA to be adequate. Combining those two, one therefore has to ensure that the loop impedance is between the minimum (for CPC) and the maximum (for MCB operation).

Not sure I follow. As I see it, there is a need for the loop impedance to be low enough that the OPD will operate before the cable overheats.

Sort of - the Zs has to be low enough for the OPD to operate within the required maximum disconnection times. Those disconnection times may (I'm not sure) be designed to 'guarantee' that the live conductors will not 'overheat' (however defined) but it certainly doesn't guarantee that the (usually lower CSA) CPC will not 'overheat' (or even melt) - that can only be determined by an adiabatic calculation.

I strongly suspect that for most domestic circuits, the loop impedance required for reliable OPD operation is a limiting factor before the heat issue comes into play - this may be a case where it matters.

I agree. As EFLI wrote, this is probably one of the reasons why adiabatic calculations are probably rarely undertaken by electricians in relation to 'normal domestic circuits'.

However (as you and EFLI also said), in some circumstances ('special circuits') it may be necessary to confirm the adequacy of the CPC by means of an adiabatic calculation (and, I would remind you, it was you who raised this issue!). The purpose of these calculations is to confirm that the CPC will survive (without melting) the (very brief) period of flow of very high fault current which the OPD will allow. For a CPC of a given CSA, the answer will depend essentially upon two things - the duration of current flow (determined by characteristics of the OPD) and the magnitude of the fault current (i.e. the PEFC). The PEFC will, in turn, be inversely proportional to the Zs of the circuit. Hence, with a given OPD and CPC of a given CSA, if the Zs is below some certain level, PEFC will rise to a level at which the CSA of the CPC is not adequate, such that the CPC could be damaged or melt before the OPD disconnects the supply.

Hence, as I said, for a given OPD and CPC, Zs has to be low enough to satisfy requirements for satisfactory operation of the OPD (required disconnection times), but also high enough to ensure that the adiabatic process within the CPC does not cause it to suffer/melt. Of course, if an adiabatic calculation indicated that the CPC was not adequate, one would not attempt to cure that by trying to somehow increase the Zs of the circuit (which might well prevent required disconnection times being achieved) but, rather, would re-design with a larger-CSA CPC.

Kind Regards, John
Edits: several typos detected and corrected!

 

[ban-all-sheds]

I am looking to order some cable to wire in an air compressor.

Your electrician will probably be able to get it cheaper.

My question is what amperage do I use in my calculations for sizing the cable? Since its only a 40amp startup surge, do I base my cable on a maximum load of 13amps?

Your electrician will decide what size cable to use.