Underground cable size needed?

[Lectrician]

And so doesn't comply, even without considering the final circuits...

 

[JohnW2]

And so doesn't comply, even without considering the final circuits...

If you mean the 25mm², then yes - but I, for one (and perhaps also you, judging by the strength of your previous comment) was more than a little surprised by how close it got. Once the OP has determined what the real design current is, I rather suspect that 25mm² will prove to be more than adequate.

Kind Regards, John

 

[ban-all-sheds]

I had assumed that he meant what he wrote - i.e. 3 x 32A 3-phase (i.e. 'red') sockets - hence 96A per phase when fully loaded.

Well, that's the thing, isn't it.

Does "100A" mean 100A per phase, or 100 "three-phase" amps?

Is the total load 69kW, or 40kW? Because if the latter, ricicle is twicicle as nicicle.

 

[JohnW2]

I had assumed that he meant what he wrote - i.e. 3 x 32A 3-phase (i.e. 'red') sockets - hence 96A per phase when fully loaded.

Well, that's the thing, isn't it. Does "100A" mean 100A per phase, or 100 "three-phase" amps?

Obviously an important question - but wouldn't the latter be a very unusual way to be specifying a 3-phase supply? Having said that, as you've already observed, it's very odd that we're being asked the question, anyway - so I guess that 'unusual terminolgy' would not be impossible. As you have observed, if the designer cannot even work out cables sizes, he shouldn't be designing.

Kind Regards, John

 

[ban-all-sheds]

as you've already observed, it's very odd that we're being asked the question, anyway - so I guess that 'unusual terminolgy' would not be impossible.

I don't know what the usual terminology is.

3-phase is 400V, so a 40kW load is 100A. Fire up a calculator and ask it to work out the cable size for a 40kW load, 400V 3-phase supply, 100m, lighting voltage drop, what do you get?

Do it by hand using 4D4B, what do you get?

As you have observed, if the designer cannot even work out cables sizes, he shouldn't be designing.

Well it seems as if he can't. and therefore it seems as if he shouldn't, but perception can be flawed, and what I actually asked was 'Why has he taken it upon himself to design "the following" when he isn't able to work out cable sizes?'.

On reflection I should have written 'Why has he taken it upon himself to design "the following" if he isn't able to work out cable sizes?'.

 

[JohnW2]

I don't know what the usual terminology is.

Maybe it varies. FWIW, it is very clearly documented that what I have here is a "3-phase 80A supply", and equally clear that what I have is 80A per phase.

Kind Regards, John

 

[ricicle]

In our world if you require a TP 100A supply then that is 100A per phase.

 

[Lectrician]

If you talk in amps, it's usually per phase - A 63amp TP submain would be 63amp per phase.

If you talk in KVA, it's usually total. A 55KVA supply being 3 x 80amps.

People seem to pluck figures out of the air and then ask for a cable size. Quite often they are rediculously high figures based on the incomer of a fuseboard or something, with no account for the actual anticipated current and future loading.

 

[Lectrician]

And so doesn't comply, even without considering the final circuits...

If you mean the 25mm², then yes - but I, for one (and perhaps also you, judging by the strength of your previous comment) was more than a little surprised by how close it got. Once the OP has determined what the real design current is, I rather suspect that 25mm² will prove to be more than adequate.

Kind Regards, John

My initial comments were based on instinct, not being near a regs book or calcing software.

25mm is taking the ****.
35mm is pushing your luck (doesn't comply for lighting, just about does for non lighting).

Neither takes into account final circuits, which in a barn, are often long in themselves.

A 50mm would do nicely for short final circuits, and no further submains after it. a 70mm would be ideal.

You need to know the EXACT length and an accurate PROPOSED loading to work it out accurately.

Reducing it to 63mps and 90m makes a 35mm compliant, assuming small and short final circuits.

 

[GNServices]

People seem to pluck figures out of the air and then ask for a cable size. Quite often they are rediculously high figures based on the incomer of a fuseboard or something, with no account for the actual anticipated current and future loading.

Okay, I have came to some sort of sense of his thinking, "Lectrician" what you said seems the obvious here so Im going on the basis that he genuinely didnt know what amp rating he needed,

Im thinking hes either done one of the following,

32amp socket x 3 phases = 96amp, making him want cable rating 100amp (wrong)

or

have 3 x 32amp sockets = 96amp, where he must have not though to consider diversity (i think is the word?), I am pretty sure he will NOT use 100amp constant at any point on the planet,

Please disregard all previous said,

what size and type of cable for 100m buried (copper or aluminum core) for the supply of 1 x 32amp and 1 x 16amp three phase sockets, (or does anyone know a good calculator)

Thanks

 

[JohnW2]

In our world if you require a TP 100A supply then that is 100A per phase.

As I said, in my world as well, but BAS's comments made me wonder whether there were some who used terminology differently. As I implied, it certainly would seem very odd (and, I would have thought very confusing) to me to describe as "TP 100A" a supply which was 33.3A per phase!

Kind Regards, John

 

[JohnW2]

If you talk in amps, it's usually per phase - A 63amp TP submain would be 63amp per phase. If you talk in KVA, it's usually total. A 55KVA supply being 3 x 80amps.

That's what I'd always thought. Thanks for confirming.

People seem to pluck figures out of the air and then ask for a cable size. Quite often they are rediculously high figures based on the incomer of a fuseboard or something, with no account for the actual anticipated current and future loading.

Quite so. As I said, I suspect that once the actual current requirements are known, a relatively 'modest' (maybe 25mm², maybe even smaller) cable may well suffice.

Kind Regards, John

 

[JohnW2]

If you mean the 25mm², then yes - but I, for one (and perhaps also you, judging by the strength of your previous comment) was more than a little surprised by how close it got.

My initial comments were based on instinct, not being near a regs book or calcing software. ... 25mm is taking the p**s.

Exactly. As above, that was also very very point. My initial instinct/gut feeling was also that 25mm² was 'way out of court' - which is why I was a little surprised when I did the sums and discovered how relatively 'close' it actually was.

35mm is pushing your luck (doesn't comply for lighting, just about does for non lighting). ... Neither takes into account final circuits, which in a barn, are often long in themselves. ... A 50mm would do nicely for short final circuits, and no further submains after it. a 70mm would be ideal. ... You need to know the EXACT length and an accurate PROPOSED loading to work it out accurately.

Quite so, and as I said:

Once the OP has determined what the real design current is, I rather suspect that 25mm² will prove to be more than adequate.

Kind Regards, John

 

[Lectrician]

People seem to pluck figures out of the air and then ask for a cable size. Quite often they are rediculously high figures based on the incomer of a fuseboard or something, with no account for the actual anticipated current and future loading.

Okay, I have came to some sort of sense of his thinking, "Lectrician" what you said seems the obvious here so Im going on the basis that he genuinely didnt know what amp rating he needed,

Im thinking hes either done one of the following,

32amp socket x 3 phases = 96amp, making him want cable rating 100amp (wrong)

or

have 3 x 32amp sockets = 96amp, where he must have not though to consider diversity (i think is the word?), I am pretty sure he will NOT use 100amp constant at any point on the planet,

Please disregard all previous said,

what size and type of cable for 100m buried (copper or aluminum core) for the supply of 1 x 32amp and 1 x 16amp three phase sockets, (or does anyone know a good calculator)

Thanks

OK.

Assuming the cable starts at the origin, and not from the end of another submain, and assuming the 32 and 16amp sockets are close to the DB at the far end, then as below:

Assuming NO lighting, 50amp protection at origin (allows for the 32 and 16 to be both run to the max):
Copper: 16mm
Alu: 35mm

If you need lighting too, then it is still 35mm (50mm Alu), as 25mm drops just a couple volts too much for the 3% drop allowed for lighting.

100m sounds a very "rounded" figure. Distance matters. Could it be less (or more)?

Aluminium needs to be installed and terminated by someone with some experience (or the knowledge, or willingness to research at least!). The cost saving would not be huge going for Alu, and in fact, may cost more depending on the rates the wholesalers get.

 

[JohnW2]

If you need lighting too, then it is still 35mm (50mm Alu), as 25mm drops just a couple volts too much for the 3% drop allowed for lighting.

Thinking alound .... As I see it, the situation with lighting can actually be a bit 'worse' than one might at first think, unless one is able to balance the lighting loads across the 3 phases.

Take 25mm² copper as an example. For 2-wire single-phase the Tables give a VD of 1.75 mV/A/m, hence 0.875 mV/A/m for each of the conductors. For a perfectly balanced 3-phase load that means that the VD in each of the three phase conductors will be 0.875 mV/A/m, which would translate to a drop of ~1.51 mV/A/m in the phase-phase voltage at the load - which is, indeed, virtually the same as the tabulated 3-phase VD (1.50 mV/A/m).

If one installs lighting (or any other single-phase load) on just one phase (all other loads being balanced across the three phases), then that lighting will presumably 'enjoy' a VD of 1.75 mV/A/m, not 1.5 mV/A/m, won't it?

But I then hit a bit of confusion, so maybe you can help me? ... if one connects three identical lighting loads between each phase and neutral, the net neutral current due to them would presumably be zero, hence zero VD in the neutral conductor. That would seem to mean that each light would only 'see' a VD of 0.875 mV/A/m (the drop in the phase conductor). Is that right? If so, it would mean that, whilst one single-phase load between one phase an N would experience an appreciably higher VD than the P-P VD (for balanced loads) (which is what I would have expected), three single-phase loads, each connected phase-N (different phases) would each experience a considerably lower VD than the P-P VD (which rather surprises me). That sounds wrong to me - so is there a flaw in my reasoning?

Kind Regards, John