Most effiecient route to supply caravan points.

[Calebp43]

Attatched I have something i made on paint which shows multiple caravan points needing a 32 amp supply. The 3 phase board is there in the bottom right.
I was wondering what would be the best way to supply each point using all 3 phases and taking into consideration length of cable and the size. My thoughts are that a 25mm cable should be used to supply them but im really stuck on which route would be the best.
Another question I have is can I earth the metal sheath of the armour at the main board which is PME and then loop the earths inbetween each point? Or does this go against a reg somewhere because then in each point you have PME on the metal sheathing of the armour but TT going into the caravan points?
If anyone can show me using paint what you think the best route would be I would be greatful. Thanks

 

[Rockingit]

I'm not going to do the maths for you, that's down to you, but personally I'd probably start with three lollipop RFC's.....

 

[littlespark]

Can’t do the drawing just now, but similar to above, but full rings, not lollipops… and in the same rows, but have rows 1 and 4 on L1, 2 and 5 on L2 and 3 and 6 on L3

You need to link the earthed armours…. To protect the cables. Then think about TT

Routes may have to be defined by existing roads and buried services.

 

[Rockingit]

Can’t do the drawing just now, but similar to above, but full rings, not lollipops… and in the same rows, but have rows 1 and 4 on L1, 2 and 5 on L2 and 3 and 6 on L3

You need to link the earthed armours…. To protect the cables. Then think about TT

Routes may have to be defined by existing roads and buried services.

My logic behind lollipop in that scenario is that it will almost certainly be cheaper and an easier termination wrangling to bring one eg. 70mm cable down rather than 2 x 50's (or whatever the calcs are). Either way, each CPD is unlikely to accept two conductors of any size so at the panel there will be some sort of 1 to 2 split (so effectively a lollipop anyway) - my method just puts that joint downstream and saves cable.

 

[Calebp43]

My logic behind lollipop in that scenario is that it will almost certainly be cheaper and an easier termination wrangling to bring one eg. 70mm cable down rather than 2 x 50's (or whatever the calcs are). Either way, each CPD is unlikely to accept two conductors of any size so at the panel there will be some sort of 1 to 2 split (so effectively a lollipop anyway) - my method just puts that joint downstream and saves cable.

But with the lollipop method there will be one caravan point with three cables going too it and then that one cable will have all the load? Or have I got something wrong with my logic there? Sorry I am an apprentice still (don't worry I'm not doing this job alone my boss has already decided how he's gonna do it I'm just curious how others do things)

 

[Rockingit]

But with the lollipop method there will be one caravan point with three cables going too it and then that one cable will have all the load? Or have I got something wrong with my logic there? Sorry I am an apprentice still (don't worry I'm not doing this job alone my boss has already decided how he's gonna do it I'm just curious how others do things)

Yes, so you'd either have a panel with options to lug onto the incoming isolator or have a small box with 3 small buss strips or bolts to do the same. However.... wrangling short lengths of larger conductors can be 'fun'.

 

[Deleted account]

Curious to know where 25 mm came from. Plucked out of the air or consideration after cable calcs?

 

[Calebp43]

Curious to know where 25 mm came from. Plucked out of the air or consideration after cable calcs?

It's the cable size that my boss was talking about using.

 

[Calebp43]

Yes, so you'd either have a panel with options to lug onto the incoming isolator or have a small box with 3 small buss strips or bolts to do the same. However.... wrangling short lengths of larger conductors can be 'fun'.

Okay I see. So like a 5 core from the board which consists of 2 line legs, 2 neutral legs and an earth brought into a small box with the two legs coming out of those?

 

[Rockingit]

Okay I see. So like a 5 core from the board which consists of 2 line legs, 2 neutral legs and an earth brought into a small box with the two legs coming out of those?

No, not like that. One larger csa 'feeder' cable that connects onto the two cables of the ring. Imagine if you will a conventional domestic RFC wired in 2.5mm, but instead of both the lives going back to the breaker instead they attach to a single length of 4mm. Hence the term 'lollipop' because that's what it looks like - a ring with a straight bit.

(NB.... it could be done with a 5c cable of smaller csa using the principle of parallel conductors but that's a whole other story)

 

[Calebp43]

No, not like that. One larger csa 'feeder' cable that connects onto the two cables of the ring. Imagine if you will a conventional domestic RFC wired in 2.5mm, but instead of both the lives going back to the breaker instead they attach to a single length of 4mm. Hence the term 'lollipop' because that's what it looks like - a ring with a straight bit.

(NB.... it could be done with a 5c cable of smaller csa using the principle of parallel conductors but that's a whole other story)

Really do appreciate you taking the time to explain this to me. First time hearing of a lollipop circuit. I'm so glad I found this electrical forums website

 

[pc1966]

What sort of diversity is assumed?

I.e. Each caravan gets a 32A supply (presumably DP RCBO as TT) but there are 30 points so I guess you are not expecting all to be used to max at once to run 960A single phase equivalent = 320A 3-phase.

Just thinking of how to size the cable. If you assume 100A per phase (i.e. around 30% usage) then 25mm is reasonable, and with a RFC-like arrangement then you have about 400m total length, so 200m of those in parallel so assuming a reasonably uniform distribution of loads then equivalent to about 100m of 50mm carrying 100A which would meet volt drop and CCC limits.

I would agree with @littlespark that taking them all the way back to the DB would make sense so you don't have more than 2*25mm at any of the 32A points. I have never uses such outlets but assume a RCBO might take that at its input?

2C 25mm with the armours linked would be fine for distribution but you need to check sub-main ADS. R1 = 0.727 ohm/km and R2 = 3.7 ohm/km so for 400m r1+r2 = 1.77 ohm and so worst R1+R2 = 0.45 ohms. So you won't meet ADS on any 100A fuse/MCCB.

Probably would need a delay 100mA RCD to meet earth fault disconnection here, ideally one per loop so you only have 1/3 customers crying if a cable is damaged!

 

[pc1966]

Just to add, probably you should go with 300mA delay RCDs as potentially a lot of leakage from 10 installations, each of which could be as high as 15mA and still in-spec for its supplying RCBO.

Also you could look at the same idea in 16mm / 63A, etc, maybe even having 6 rings with out and return interleaved (same channel dug in ground but two SWA) depending on assumed diversity, etc.

 

[littlespark]

I wouldn’t think the buried armour cables needs RCBO protection, just the final connection to the caravan…. And most factory built caravans have rcd protection on their final circuits…. So there’s a double up there

 

[pc1966]

I wouldn’t think the buried armour cables needs RCBO protection,

It is more about meeting ADS without having 3C cable linking armour segments. You don't get delay RCBOs (other than fancy and expensive MCCB things) so probably MCB/MCCB and separate delay RCD.

just the final connection to the caravan…. And most factory built caravans have rcd protection on their final circuits…. So there’s a double up there

I thought the regs required RCD DP on any caravan feeds?

 

[littlespark]

I thought the regs required RCD DP on any caravan feeds?

Yes it does, but the caravans also have RCDs in their consumer units. Doubled up rcd by design.

Most caravans only have a bit of artic flex between it and the hookup, but I suppose a 32A supply might be armoured, and hard wired in.

OP, are these “caravans” or bigger lodge type double units with washing machines, dishwashers etc supplied?
Nothing to do with the post, just interest due to my previous employment on a caravan site.

 

[pc1966]

Given how often (or not) they are tested, having two 30mA RCD in series is not a bad idea...

 

[littlespark]

Given how often (or not) they are tested, having two 30mA RCD in series is not a bad idea...

But it’s always the one outside in the rain that trips…