Power Factor when Designing Circuits

[JohnW2]

Anything with a ballast, an induction motor or a transformer will have a power factor of less than 1 ...

That's obviously true, but I still think that a PF of 0.8 would probably be unnecessarily pessimistic as a 'rule of thumb' to use for all circuits in a domestic installation, don't you?

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.

You're right - I wasn't thinking straight In fact, now that I'm hopefully thinking a bit straighter, I'm not sure exactly how 'taking PF into account' would actually have any effect on the designing of most domestic installations. AFAICS, the main importance of considering PF in circuit design is in ensuring that cable CCCs (and, as you say, the corresponding OPD In) are adequate for the loads concerned. However, apart from fluorescent lighting (and even that may change) there are few 'known' (i.e. hard-wired) non-resistive loads that could be considered at design stage - and lighting circuits are generally so over-designed (in terms of CCC) that PF is hardly an issue. ... or am I missing something?

Kind Regards, John

 

[EFLImpudence]

or am I missing something?

No, that's why

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

 

[JohnW2]

or am I missing something?

No, that's why

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

Indeed. In my partial 'defence', I would cite the fact that my initial contribution to this thread (before riveralt's oversized contribution!) was:

...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!).

... but it does rather beg the question as to how this 'domestic circuit design software' makes use of its value for PF, whatever value that may be!

Kind Regards, John

 

[EFLImpudence]

how this 'domestic circuit design software' makes use of its value for PF, whatever value that may be!

Perhaps it is an option which can be ignored.

 

[JohnW2]

how this 'domestic circuit design software' makes use of its value for PF, whatever value that may be!

Perhaps it is an option which can be ignored.

Well, it can obviously be changed to 1.0. However, we've been told that it is the 'domestic version' of the software and that the default value is 0.88. I therefore wonder what, if any, of the software's 'answers' about design of the circuits would alter if one changed the PF value.

Kind Regards, John

 

[Spark123]

I don't think it makes any odds, when specifying discharge lighting (as an example of an inductive load) you need to multiply the wattage by 1.8 and divide by volts to give design current so I imagine this will take powerfactor into account (as well as startup current)

 

[JohnW2]

I don't think it makes any odds, when specifying discharge lighting (as an example of an inductive load) you need to multiply the wattage by 1.8 and divide by volts to give design current so I imagine this will take powerfactor into account (as well as startup current)

[I'm not quite sure where your 1.8 comes from - wouldn't it be 1.25 for a PF of 0.8?] Sure, it terms of the design current, but that wouldn't actually make any difference to the practical 'design answers' (cable and OPD sizing) for the vast majority of domestic installations, would it? ... and, as I said, a lighting circuit with discharge lighting seems about the only situation in most domestic installions for which one would know enough about the possible load to do even that. To assume that a 'fully loaded' domestic sockets circuit had a PF of 0.8 would surely be crazy?

Kind Regards, John

 

[Spark123]

http://www.tlc-direct.co.uk/Book/7.12.1.htm

 

[JohnW2]

http://www.tlc-direct.co.uk/Book/7.12.1.htm

Hmmmm. Actually, more interesting is this section , which introduces the 1.8 factor (without any explanation) - and they say that even that only applies if the PF is 0.85 or greater! Whatever this 1.8 factor is about, it's clearly about a lot more than just PF (which, in itself, would require a factor of only about 1.18, not 1.8, for a PF of 85) (they mention 'control gear losses' and harmonics as otherfactors) ... but where does it come from, I wonder?

Kind Regards, John

 

[Joelc]

Seems like its a can of worms. The calculation makes no difference to the final circuit outcomes, so I presume it's to be ignored. Just seems odd to have it in there in the first place.

It seems its not worth factoring in for domestic installs. I was reading the same thing, and I couldn't get where they got the 1.8 from either.

I just need to get my diversity calculations more realistic, and the DNO to come back to be regarding upgrading the service fuse. It's only 60a and I have a 40a oven and a 32a welder, and I don't fancy it popping when I'm welding and the missus is cooking dinner.

 

[riveralt]

and the DNO to come back to me regarding upgrading the service fuse. It's only 60a and I have a 40a oven and a 32a welder, and I don't fancy it popping when I'm welding and the missus is cooking dinner.

Good luck with that - we have a 60Amp main fuse - that was in place when we moved here in 1984 - Despite new Consumer Units electric showers etc they refuse to increase the size of the main fuse - siting the fact that they will have to increased their supply cables to accommodate me.

 

[ban-all-sheds]

http://www.theiet.org/forums/forum/messageview.cfm?catid=205&threadid=44596

http://www.theiet.org/forums/forum/messageview.cfm?catid=205&threadid=38302

 

[DetlefSchmitz]

Just for information here. At a place of work I know, the room is lit with 6A worth of old florescent lighting (based on wattage, and notional PF of 1) split between 2 x 6A MCBs.

In practice the measured current is 6A on each MCB.

So the PF for this (exact) type of lighting is 0.5 .

So if you added up the lamps without PF you would be out by 100%.


For domestic installation I believe we still count 100W per lamp position (probably wrong - not done it recently), so fitting a smaller CFL will still work. If you calculate a lighting circuit and you are using a shed load of CFLs and counting then at face value (and not 100W) then you might just fall foul of the PF.

 

[JohnW2]

http://www.theiet.org/forums/forum/messageview.cfm?catid=205&threadid=44596
http://www.theiet.org/forums/forum/messageview.cfm?catid=205&threadid=38302[/QUOTE]
Thanks - so they haven't got much better a clue than I have as to where the 1.8 factor came from, other than to suggest that is is a 'very old rule of thumb which originated in the 60s' and to mention (as I said) that, per the book in TLC link, it probably takes into account 'control gear losses' and harmonics, as well as PF!! In reality, it seems to be dominated by some 'other considerations', since PF alone can only explain a relatively small proportion of the "1.8".

Is this factor mentioned in BGB or any of the associated publications (OSG or GNs)?

Kind Regards, John

 

[JohnW2]

If you calculate a lighting circuit and you are using a shed load of CFLs and counting then at face value (and not 100W) then you might just fall foul of the PF.

Theoretically true, but it would surely have to be an awfully large 'shed load of CFLs' to run into trouble with, say, a 6A MCB ... by my reckoning, with your 'correction factor' (including PF considerations, and probably other things) of 0.5, you'd be talking about some 690W's worth of CFLs - say about 46 15W ones - not very probable in a domestic environment, I would say.

Kind Regards, John