Conductors in parallel

[Mark.fardon]

"433.4 Overload protection of conductors in parallel
Where a single protective device protects two or more conductors in parallel there shall be no branch circuits or devices for isolation or switching in the parallel conductors "


I am in debate with another spark. I have installed 2x2.5mm twin & earth in parallel to supply two ovens (no hob just ovens) (long story why didnt use 4mm or 6mm) off a single 32A RCBO. The ovens are 3.5Kw each so total load of 7KW, without diversity thats a load of 30.43A. At the end of the parallel conductors I propose to install an cooker switch (isolation device) and terminate the preinstalled flex from the NEFF ovens into the load side of the switch . Installation method is clipped direct so 27A x 2 = 54A so no issues with current carrying capacity. The kitchen fitters usual spark has said they absolutely cannot be on the same OPCD (hasnt stated why) The only issue I can see is with 433.4 where branch circuits or devices for parallel switching cannot be be in the parallel conductors, but in this case would not be IN the parallel sections of the fixed wiring, only as a termination point. I can in fact seperate the parallel supply and run each oven off its own 16A RCBO but would like to know that I'm correct in stating it is perfectly acceptable if not somewhat unusual to have used parallel conductors and the arrangement above ?

 

[telectrix]

the reason you can't spur off one leg ist that you'd then have uneven loading between each leg.otherwise,
perfectly OK, but i'd personally prefer the separate 16A (or 20A) circuits, one for each oven. that way, if there's a fault on 1 circuit or oven, you still can use the other one.

 

[snowhead]

Agree with him^^
It's nothing to do with the parallel conductors, it's in case of a fault one oven tripping the Single RCBO and knocking out both ovens.
Also for maintenance / repair /waiting for spares, you'd need both ovens off

 

[Mark.fardon]

Agree with him^^
It's nothing to do with the parallel conductors, it's in case of a fault one oven tripping the Single RCBO and knocking out both ovens.
Also for maintenance / repair /waiting for spares, you'd need both ovens off

Agree with above but convenience of being able to have each oven on own circuit isn’t the question as most installers would put the two two single ovens on one circuit - I put in another circuit for the 7kw job so 3 circuits would be overkill surely ? The post was more about regs, and acceptability of parallel conductors with single opcd

 

[Julie.]

Parallel conductors are used all the time on big jobs, it is far easier to run a number of smaller cables, and the current rating is better, so savings in copper!

But, they must be the same length & size (plus over long runs cycle the cores - but that's not applicable here).

It's completely acceptable within the regs, but yes may be unusual for this application!


As an aside, you could also run both to their respective loads - so one feeding one and the other feeding the other - both fed from the same 32A rcbo, no not actually in parallel, this is because the rcbo would be providing overcurrent protection for fault only (plus additional rcd protection), and not overload protection on the cable - this is ok because they are fixed loads.

Obviously not ideal, if you have two cable runs, then two rcbo would provide better maintenance possibilities etc, but ultimately that's just the same as running a feed to a sub-board, if that cable gets damaged you impact all the circuits off that sub-board

 

[Wilko]

It’s only early so I might’ve completely missed it (no coffee yet...) but couldn’t the 2 by 2.5mm equal length conductors form an RFC from the 32A RCBO to the cooker switch? Then the 2 cooker flexes could 2 spurs off the RFC and sidestep the parallel conductor regs. But point loads are to be avoided which is why it should’ve been 6mm, sorry.

 

[Mark.fardon]

It’s only early so I might’ve completely missed it (no coffee yet...) but couldn’t the 2 by 2.5mm equal length conductors form an RFC from the 32A RCBO to the cooker switch? Then the 2 cooker flexes could 2 spurs off the RFC and sidestep the parallel conductor regs. But point loads are to be avoided which is why it should’ve been 6mm, sorry.

my original thinking was to have a dedicated RFC for the 2 ovens on the assumption/misinfo they came with a plug top like most now do ie 2kw or so. It transpired they are both a whopping 3.5kw each (what happened to low energy?)

my understanding is that 2.5mm RFC is permissible on a 32A for bs1363 accessories although the refs don’t state not permissible for non 1363 ..anyhow that’s why I then went down the parallel and not ring approach (same length size etc)

 

[davesparks]

It's not a ring if only goes to one point, a ring serves multiple points over an area.

 

[Mark.fardon]

It's not a ring if only goes to one point, a ring serves multiple points over an area.

Thanks but you’ve missed the point Dave. I know what a ring is. I know what parallel conductors are. I was checking with you experts that I had wired the circuit within the regs and with safe good working practice

 

[static zap]

I'm wondering if the not a ring situation - is that cables may be de- rating each other (due to proximity) in adverse kitchen conditions !
So get where @davesparks is coming from !

 

[Mark.fardon]

I'm wondering if the not a ring situation - is that cables may be de- rating each other (due to proximity) in adverse kitchen conditions !
So get where @davesparks is coming from !

You’ve lost me - derating because of grouping ? What adverse conditions ?

 

[static zap]

- derating because of grouping ?

Are the cables together for the whole run ?
..Ambient temperature is a personal judgment thing.

 

[Mark.fardon]

Are the cables together for the whole run ?
..Ambient temperature is a personal judgment thing.

Yep from the board to cooker housing