Supply for a Marquee

[danrich123]

Hi, don't bother asking stuff on here, as most of the guys on here are twonks

 

[ban-all-sheds]

Seems a strong likelihood that if you are missing the fact that if you are having to ask these questions on a DIY forum you should probably not be taking their money.

 

[danrich123]

You're a sarcy get. I'm pretty sure not every question on here is how to change a plug, and sparks can ask for abit of guidance from time to time.

 

[Taylortwocities]

Ban's reply is the same as my first impression when I read your post.

Surely you know from where and how you calculate volt drop?

Scary

 

[ban-all-sheds]

You don't understand voltage drop.

You say the marquee is semi-permanent, and then ask if the cable needs to be 450mm deep because it's temporary, as if, even if it were temporary, that would somehow make a difference to how safety should be achieved.

Presumably 60A implies more than a few festoon lights, and you don't know about earthing.

Public liability, and EAWR issues abound for you, and public, EAWR and employer liabilities for the golf club, and I'll bet you didn't tell them that you'd have to come here to ask how to do the job you agreed you could do.

 

[danrich123]

Lol, you guys should get a life. 46000 posts, go find something else to do other than shoot someone down.
Did I say I was doing the the job, no, but they asked my advice, so would try and find an answer. I just do domestic and can do a volt drop calc and if I worded the volt drop question badly then sue me, but off top of my head I don't know what size cable 120m @ 60a needs to be
Why does the post mean I don't know earthing, I only asked about TT and the marquee as I know it can't just go on a PME.
Go a have a wonk
Over n out

 

[DaftPunk]

70mm² is required for 60a over 120m

 

[plugwash]

You need to consider the volt drop of both cables.

Also you need to be careful about how you calculate the volt drop of the three phase cable. AIUI the "standard" rules for three phase cables assume there will be negligable volt drop in the neutral but if you are putting all the load on one phase that clearly will not be the case. So you'd have to treat the three phase cable as a single phase one.

Any reason for not taking the three phase to the marquee and spreading the load over the phases?

 

[Taylortwocities]

Looks like the OP has deleted his original text so no point in taking this further.

Good luck on the Screwfix forum danrich!

 

[plugwash]

Looks like the OP has deleted his original text so no point in taking this further.

Yes there is a point and that point is showing bas that he doesn't f*cking own the f*cking place.

 

[JohnW2]

AIUI the "standard" rules for three phase cables assume there will be negligable volt drop in the neutral but if you are putting all the load on one phase that clearly will not be the case. So you'd have to treat the three phase cable as a single phase one.

I've never really had any reason to think about or investigate it very deeply, but I've never fully understood what the 3-phase VD figures given in the tables in the regs (which are generally slightly (~15%) lower than the corresponding figures for single-phase) actually represent. Thinking simplistically, if a balanced 3-phase load drew 60A per phase, I would expect there to be "60A's worth" of VD in each of the three live conductors and hence, I would have thought, double that 'one conductor' figure for the reduction in each of the phase-phase voltages - but that 'logic' does not take the actual phase differences into account. I suppose my main problem may lie in my not being too sure as to exactly what is meant by 'voltage drop' in a 3-phase context. Is there a simple explanation?

Kind Regards, John

 

[OwainDIYer]

Surprised no-one's mentioned BS 7909 yet

 

[securespark]

BS7909.

Oh, shucks, late as usual....

 

[plugwash]

AIUI the "standard" rules for three phase cables assume there will be negligable volt drop in the neutral but if you are putting all the load on one phase that clearly will not be the case. So you'd have to treat the three phase cable as a single phase one.

I've never really had any reason to think about or investigate it very deeply, but I've never fully understood what the 3-phase VD figures given in the tables in the regs (which are generally slightly (~15%) lower than the corresponding figures for single-phase) actually represent. Thinking simplistically, if a balanced 3-phase load drew 60A per phase, I would expect there to be "60A's worth" of VD in each of the three live conductors and hence, I would have thought, double that 'one conductor' figure for the reduction in each of the phase-phase voltages - but that 'logic' does not take the actual phase differences into account. I suppose my main problem may lie in my not being too sure as to exactly what is meant by 'voltage drop' in a 3-phase context. Is there a simple explanation?

Well if we assume a balanced resisive load and a perfect three phase source there sill be no drop in the netural. So the P-N voltage will drop by the volt drop in the live conductor.

Again assuming a balanced load the phase angles won't change, so the P-P voltage will drop by sqrt(3) times the ammount the P-N voltage dropped by.

These assumptions would seem to tally with your statement that the three phase figures in the regs are about 15% lower than the single phase figures since sqrt(3) is about 15% less than 2.

 

[JohnW2]

... I suppose my main problem may lie in my not being too sure as to exactly what is meant by 'voltage drop' in a 3-phase context.

Well if we assume a balanced resisive load and a perfect three phase source there sill be no drop in the netural. So the P-N voltage will drop by the volt drop in the live conductor. ... Again assuming a balanced load the phase angles won't change, so the P-P voltage will drop by sqrt(3) times the ammount the P-N voltage dropped by. ... These assumptions would seem to tally with your statement that the three phase figures in the regs are about 15% lower than the single phase figures since sqrt(3) is about 15% less than 2.

Thanks. All agreed. So you're saying that when one talks about VD in a 3-phase context, that is in terms of P-P voltage?

What does this mean in terms of 'permissible' VDs, particularly for lighting loads, which presumably usually will be single-phase, connected between one phase and neutral? Is one allowed to use the 3-phase (seemingly P-P) VD figures, per tables in the regs, for one's calculations - or does one have to (which would seem much more reasonable) use the (larger) 'single phase (P-N) VDs' for this purpose?
Edit: I should have added to that question "...even if the (single-phase lighting) circuits are balanced across all three phases" - the point being that, even if the circuits are theoretically balanced, one presumably can't be sure that the load will always be balanced.

Kind Regards, John