Power Factor when Designing Circuits

[Joelc]

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.

They seem to think they can upgrade to 80a without any cable upgrades, but the drive is coming up soon, so I may opt to have the service cable upgraded at the same time. However this may be more difficult as they were unsure if I am on a shared supply, so I'm waiting for them to do survey and quote.

 

[DetlefSchmitz]

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

Probably true. Though I posted this as information to the OP, as a guide to when it might be relevant, rather than, yet again, to be informed by you that I am posting quibbling or irrelevant information.

I am sorry to say this, but I am growing tired of being accused of not being able to read an MK data sheet, or not posting views which match your approved view of the world.

I almost , but not quite, wish that you had tried out Bas's experiment with the kettle element, since you told me I was talking rubbish about a temperature rise.

End of rant.

In my experience - the DNO upgraded my fuse from 60A to 100A because they liked my tidy installation, and I made them tea.

 

[viewer]

Question. Not really sure if it is connected to original question.
I can understand the power factor related to electric motors, and CFLs have been mentioned - which is not as clear to me-, but what other equipment? Induction cookers can be quite large machines - is it relevant here?
Mostly I use this site as a learning resource (as if you hadn't guessed).

 

[JohnW2]

Probably true. Though I posted this as information to the OP, as a guide to when it might be relevant, rather than, yet again, to be informed by you that I am posting quibbling or irrelevant information.

I'm sorry that you feel like that, and apologise if that's how it's come across, but I really wasn't being in any way critical - as I said, what you said was true, but probably not relevant to the OP's task. On the contrary, just as you say of yourself, I was attempting to put things into context for the OP, in relation to the actual question he had asked.

The OP asked what PF figure to put into his software in order to design a domestic re-wire. Virtually eveyone has told him that he does not need to take account of PF for domestic installation design (i.e. that he should use PF=1.0) and I thought he might get confused by the fact that you appeared to be suggesting that a 'correction factor' of 0.5 might be appropriate for some circuits. That's all.

Kind Regards, John

 

[JohnW2]

Question. Not really sure if it is connected to original question. I can understand the power factor related to electric motors, and CFLs have been mentioned - which is not as clear to me-, but what other equipment? Induction cookers can be quite large machines - is it relevant here? Mostly I use this site as a learning resource (as if you hadn't guessed).

Any load other than a simple passive resistive one (like a heating element) is likely to have a PF less than 1.0. As well as obvious inductive loads such as motors and things with inductive ballasts (like traditional fluorescent lights), I think one of the greatest type of culprits are switched-mode power supplies (SMPSUs), which are becoming increasingly prevalent in this world. 'PF-corrected' SWPSUs exist, but I don't know how common they are, and I have a feeling that there are some problems associated with them. I would suspect that, as you say, an induction hob would probably have a low PF.

Kind Regards, John

 

[ban-all-sheds]

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".

According to this:

the 3rd harmonic component of discharge lamps can be of the order of 25%, so toss in some control gear losses and you're heading towards the 1.8.

But on a quick skim-through of the chapter in that book, it seems to me that 3rd (and other odd) harmonic currents are only of concern in 3-phase environments (where they affect the neutral current) and large cables (where the reactive losses become significant).

I imagine that the 1.8 came into use for electricians doing industrial/commercial work where there was a lot of discharge lighting and it was a 3-phase installation, and it would have erred on the side of caution, i.e. known to produce a safe result without having to do detailed calculations. They would have been saved for really large loads where a simple ROT would give you an expensively oversized cable.

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

No, but see Appendix 4, paras 5.5, 5.6, 6.2, and the above book Chapter 11 and 5.5

 

[JohnW2]

According to this: ... the 3rd harmonic component of discharge lamps can be of the order of 25%, so toss in some control gear losses and you're heading towards the 1.8.

Thanks. I'll have a look at my copy! Mind you, 25% is a lot less than 80%.

But on a quick skim-through of the chapter in that book, it seems to me that 3rd (and other odd) harmonic currents are only of concern in 3-phase environments (where they affect the neutral current) and large cables (where the reactive losses become significant).

I'm glad you said that, because I've been trying to fathom out what is the significance of 3rd harmonic currents in a single phase situation - without too much success so far! AFAICS, in a single-phase system, 'current is current', regardless of the shape of the waveform (and hence the nature of the components of that waveform).

I imagine that the 1.8 came into use for electricians doing industrial/commercial work where there was a lot of discharge lighting and it was a 3-phase installation, and it would have erred on the side of caution, i.e. known to produce a safe result without having to do detailed calculations. They would have been saved for really large loads where a simple ROT would give you an expensively oversized cable.

That makes sense.

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

No, but see Appendix 4, paras 5.5, 5.6, 6.2, and the above book Chapter 11 and 5.5

Thanks, again. I'll do some reading and report back. Whatever, I suspect that we are probably mainly agreed that the 1.8 factor (or, indeed, any 'factor) is probably not appropriate for most/all single-phase domestic installs.

Kind Regards, John

 

[plugwash]

But on a quick skim-through of the chapter in that book, it seems to me that 3rd (and other odd) harmonic currents are only of concern in 3-phase environments (where they affect the neutral current) and large cables (where the reactive losses become significant).

All harmonics are a "concern" in that they reduce the power factor but the third harmonic is especially bad in three phase systems because it sums rather than cancelling in the neutral. So if the third harmonic for each phase was 25% of the phase current then the third harmonic in the neutral would be 75% of the phase current.

The 9th harmonic would also sum in the neutral but I belive that is typically much weaker than the 3rd harmonic.

 

[JohnW2]

All harmonics are a "concern" in that they reduce the power factor but the third harmonic is especially bad in three phase systems because it sums rather than cancelling in the neutral. So if the third harmonic for each phase was 25% of the phase current then the third harmonic in the neutral would be 75% of the phase current.

I understand that but, as I said, I have so far not managed to think of a reason why 3rd harmonics would be a 'concern' (or, really, even relevant to design calculations) in a single-phase circuit. Am I missing something?

Kind Regards, John

 

[ban-all-sheds]

They would be of concern in a single-phase high current circuit (100's or 1000's of A) because of significant inductive reactance. 150Hz, remember.

I don't think anybody would be able to show that in a normal domestic installation one needs to bother applying any factor to any discharge lighting.

 

[JohnW2]

They would be of concern in a single-phase high current circuit (100's or 1000's of A) because of significant inductive reactance. 150Hz, remember.

Sure, but the BS7671 tables don't even bother giving reactance until they get to 25mm² (when it is just ~10% of total impedance, at 50Hz). By 185mm², it's around 50% of impedance (at 50Hz) and by 400mm it has risen to around 80%, so clearly would be a dominant component of impedance at 150Hz with the larger cable sizes, hence very relevant.

I don't think anybody would be able to show that in a normal domestic installation one needs to bother applying any factor to any discharge lighting.

Quite. I don't think you'll find domestic lighting circuits wired in 25mm², let alone 400mm² cable At 1/1.5mm², I would imagine that the reactive component is so small that multiplying it by 3 (for 150Hz) would be of no practical significance at all.

Kind Regards, John

 

[Spark123]

Discharge lighting with magnetic ballasts pull more current than what they say on the tube, for example my garage light consists of two 55w fluorescent tubes. If I stick an ammeter inline they are drawing 0.64A which is around 150w. Even in domestic installations I still take into account the difference

 

[JohnW2]

Discharge lighting with magnetic ballasts pull more current than what they say on the tube, for example my garage light consists of two 55w fluorescent tubes. If I stick an ammeter inline they are drawing 0.64A which is around 150w.

Sure, but that probably is primarily PF (plus a little bit of 'losses').

Even in domestic installations I still take into account the difference

...but, as I've been asking (by implication), what does that actually mean in practice? It's irrelevant in terms of cable sizing, since the CCC of cables used for domestic lighting circuits is invariably far more than is required. About the only relevance I can think of is (using your figures) remembering not to put more than about 18 55w fluorescents on one 6A lighting circuit - but that's hardly a situation one is likley to be contemplating Am I missing some other way in which one should 'take into account the difference'?

Kind Regards, John

 

[Spark123]

Not so much having 18 fluoros on a circuit, just remembering that a few of them on top of the rest of the lighting circuit may just push it over, not applying diversity of course.

 

[JohnW2]

Not so much having 18 fluoros on a circuit, just remembering that a few of them on top of the rest of the lighting circuit may just push it over, not applying diversity of course.

Yes, I suppose one has to keep that in the back of one's mind - but, particularly with the gradual move to CFLs and LEDs, I would think that overloading a "1380W" domestic lighting circuit (even ifthere are a few fluoros on it) is rapidly becoming a thing of the past.

Kind Regards, John