Power Factor when Designing Circuits

[Joelc]

Hi All,

Currently preparing for a DIY house rewire, I am now designing the circuit layouts and doing all the calculations for the new circuits before i put my notice in to the LABC.

I am working out the load on the individual circuits to calculate cable sizes etc. However - I'm not sure how to work out the correct power factor for each circuit. Obviously I can't calculate the actual until the circuits are in, so is there a set figure I should use. I had a check though the wiring regs etc, but I couldn't see anything - but I may have missed it

Thanks

 

[riveralt]

For normal domestic installations you do not need to consider power factor correction.

 

[Spark123]

Domestic premises are billed by the KWh so powerfactor isn't a major issue.

 

[JohnW2]

Domestic premises are billed by the KWh so powerfactor isn't a major issue.

It was cable sizing, not billing, that concerned the OP - but I can't say I've ever heard of anyone taking PF into account when designing normal domestic installations (not that it would even be very possible, given uncertainties about what loads might be plugged into sockets!).

Kind Regards, John

 

[Spark123]

A rule of thumb figure is 0.8, but as I said, don't worry about it.

 

[mfarrow]

There aren't many circuits within a domestic situation where loads need to be considered in any great detail, and those that do, i.e. showers, immersions, storage heaters, are mainly resistive anyway. Anything else is either plugged in or contains a substabtially greater resistive load than inductive (washing machine, for example).

I forget what the software I use puts in as a baseline, but I would go with 0.9 if you want to put something in.

 

[plugwash]

We can split the circuits one would find in a domestic property into three main categories. Socket circuits, lighting circuits and dedicated appliance circuits.

For socket circuits the significant loads are likely to be mostly resisitive. So you can generally assume a power factor of unity.

For dedicated appliance circuits again resisitive loads are likely so you can again assume a power factor of unity. The possible exception is if some large motors are in use in a garage/workshop, in this case you should consult the documentation for that equipment before designing the circuits to feed it.

For lighting circuits incandescent lights will have a power factor of basically unity. CFLs will have a lower power factor but also a lower power consumption. Large flourescent fittings may have both a high power consumption and a lower power factor but they aren't very common on domestic. Generally i'd suggest installnig a maximum of 10 conventional fittings or 20 downlights* per circuit, this allows 100W per fitting (50W per downlight) and allows some spare for additions or the occasional unusually large fitting.

So in summary you don't have to worry much about power factor domestically.

* That is not to say I think downlights are a good idea.

 

[Joelc]

Thanks all, the software I am using has .88 as a baseline so I just wanted to make sure that was correct, given some of the defaults are already iffy.

 

[plugwash]

It's probablly unnessacerally pessimistic but it isn't likely to hurt anything too much.

 

[JohnW2]

A rule of thumb figure is 0.8, but as I said, don't worry about it.

Wouldn't 0.8 be unduly pessimistic for a domestic installation? - as has been said, domestic loads are generaly either resistive or, if not, fairly low (or, at least, relatively short in duration). Over a period of time, I wouldn't have thought that the 'average' domestic PF would be appreciably below 1.0, would it?

To use such a factor would essentially mean de-rating cable CCCs by 20% - which would mean, for example, that a 2.5mm² ring final (or 20A radial) would be non-compliant if any of the cable were not 'clipped direct', and that a 4mm² 32A radial would be non-compliant with any installation method.

As you've said, I think PF is usually ignored for domestic installs. I've certainly never seen, in my limited experience, anyone taking it into account in designing a domestic installation.

Kind Regards, John

 

[riveralt]

Thanks all, the software I am using has .88 as a baseline so I just wanted to make sure that was correct, given some of the defaults are already iffy.

You do not need to include power factor correction in calculations for domestic installations!!!!

 

[flyingsparks]

Thanks all, the software I am using has .88 as a baseline so I just wanted to make sure that was correct, given some of the defaults are already iffy.

Out of interest, what software are you using?

 

[Joelc]

Thanks all, the software I am using has .88 as a baseline so I just wanted to make sure that was correct, given some of the defaults are already iffy.

Out of interest, what software are you using?

Amtech ProDesign - the domestic version. I'm not entirely sure why its factored into the software if it isn't applicable.

 

[Spark123]

A rule of thumb figure is 0.8, but as I said, don't worry about it.

Wouldn't 0.8 be unduly pessimistic for a domestic installation? - as has been said, domestic loads are generaly either resistive or, if not, fairly low (or, at least, relatively short in duration). Over a period of time, I wouldn't have thought that the 'average' domestic PF would be appreciably below 1.0, would it?

Anything with a ballast, an induction motor or a transformer will have a power factor of less than 1, some fixed items have correction but some don't. I'll have to get me clamp meter out and have a play sometime. I'm pretty sure last time I used it that it came out around there but there was very little current being drawn, probably fluoros.

To use such a factor would essentially mean de-rating cable CCCs by 20% - which would mean, for example, that a 2.5mm² ring final (or 20A radial) would be non-compliant if any of the cable were not 'clipped direct', and that a 4mm² 32A radial would be non-compliant with any installation method.

No, as long as the cable rating is bigger than the over current protection, if you want to make your over current protection larger then you'll end up in a pickle.

 

[plugwash]

To use such a factor would essentially mean de-rating cable CCCs by 20% - which would mean, for example, that a 2.5mm² ring final (or 20A radial) would be non-compliant if any of the cable were not 'clipped direct', and that a 4mm² 32A radial would be non-compliant with any installation method.

Power factor is the relationship used to convert between real power and apparent power.

With a socket circuit the design current is assumed to be equal to the breaker rating so power factor is not relavent to determining if the breaker rating is ok for the cable.