Sub main query

[JonathanA]

Many thanks to all who have taken the trouble to reply!

I'll put my hands up to the criticism - I have done the job all back to front.

In my defence, I was under some pressure to get the cable laid so that the trench could be backfilled. It was blocking access to the site.

Still I am where I am now and piecing together all of your advice, I don't think it is a complete disaster.

1. John is [probably] correct in saying that the load won't draw anything like 80A or even 63A. That said, given that the cable is there, I might as well protect it using the highest rated protective device possible.

2. I understood the maximum rating of the cable is what it can carry under test conditions in a lab. In any real life installation, this depends on the ability of the cable to dissipate heat which will be much less, for example, when it is ducted underground.

3. To answer RF Lighting and Lectricians question, I checked the Voltage Drop using TLC's online calculator. It recommended a minimum 16mm 3C cable for a 15kW load. Checking this again, it gives a VD of 2.1% at 65.2A with a maximum cable load of 66.8A.

4. Whilst limiting the load to 50A may ensure that the voltage drop is allowable, it is easy to conceive of the studio requiring more than this. As the load increases, I would rather be warned by the lights dimming than by the protective device tripping - assuming of course that the cable isn't smoking at the same time!!

So, how about this for the sub-main with the existing cable:

Garage/supply end: 80A MCCB
Studio end: 63A RCBO 30mA

As ever, your thoughts and advice would be much appreciated.

Regards,

J

 

[Taylortwocities]

Is it worth us taking a look at your assumptions as to the design current load?

Many houses don't run at a 50A diversified load.

 

[RF Lighting]

What sort of studio are you trying to supply?

An upfront rcbo at the studio end is a really bad idea, and you'd really struggle to convince anyone it complies with regs.

 

[JohnW2]

2. I understood the maximum rating of the cable is what it can carry under test conditions in a lab. In any real life installation, this depends on the ability of the cable to dissipate heat which will be much less, for example, when it is ducted underground.

No. The 91A figure is the current that 16mm² XPLE SWA can safely carry when buried directly in ground or buried in ducting. With other installation scenarios, the maximum current would/could be different.

So, how about this for the sub-main with the existing cable: Garage/supply end: 80A MCCB ... Studio end: 63A RCBO 30mA

Current-wise, that sounds fine to me, but many probably would/will question the wisdom (or compliance) of having an RCBO protecting the entire studio 'installation', if that is what you are proposing. The 80A upstream protection is adequate for the cable, and, as eric has said, a 63A max load would ensure that you remained within the VD guidance even for lighting (3%). How easily you would find an 80A MCB is a different matter - anything over 50A seems pretty rare, at least in terms of 'stock items', although one sees a few 63A ones on offer.

Kind Regards, John

 

[deadshort]

Do you mean 80a MCCB ? they are readily available.


Regards,

DS

 

[RF Lighting]

You can't use XPLE figures unless the devices it's terminated onto are rated for 90°C connections.

 

[ericmark]

I was brought up with the keep it simple silly (KISS) idea and to use a moulded breaker and a RCBO of 63A is expensive compared with a 160A switched fuse unit and a RCD in the studio/workshop.

I also consider fuse feeding fuse works better than fuse/breaker mix and £5 for a fuse is nothing compared with £50 for a moulded circuit breaker. Internet search I did find a D80 MCB (DZ47-100H D80) at £7.54 so it would seem you can get MCB's at 80A at a reasonable price but as to if you want to use a FHOC circuit breaker from ebay is another question?

But it's really now up to you. Nothing wrong with your idea to my mind. Not what I would do but nothing wrong. I have many times been limited by my supplier most firms do not allow you to buy from unknown suppliers on the internet so you have to cut your cloth to match what is available.

 

[JohnW2]

Do you mean 80a MCCB ? they are readily available.

Ah! No, my comment related to what I wrote (MCB). Since the OP said that the intention was to fit it "in a spare way in the garage CU" I had assumed that he actually meant MCB. As you say, 80A MCCBs are readily available - but I wonder if there is space in the OP's CU to fit one.

Kind Regards, John

 

[JonathanA]

Halleluliah!

Thanks for all your responses.

Yes, I'd need an MCCB for 80A which a shame because they are larger and require a larger enclosure. Alternative is the 80A switch fuse. Either way, I need an extra box.

If I stuck to a 63A MCB at the supply end, which is the largest MCB available, then I could run it off the existing garage unit which would look much better. I might still do this.

Thanks again to one and all.

Regards,

J

 

[JohnW2]

You can't use XPLE figures unless the devices it's terminated onto are rated for 90°C connections.

I see. In that case, what figures does one use? It would presumably be inappropriate to use 70°C thermoplastic figures (which would give 78A for buried 16mm² SWA), since they relate to a different material.

Kind Regards, John

 

[deadshort]

Do you mean 80a MCCB ? they are readily available.

Ah! No, my comment related to what I wrote (MCB). Since the OP said that the intention was to fit it "in a spare way in the garage CU" I had assumed that he actually meant MCB. As you say, 80A MCCBs are readily available - but I wonder if there is space in the OP's CU to fit one.

Kind Regards, John

The OP has suggested a 80a MCCB at the garage end and it would be a DB and not a CU. Or as the OP suggested split tails.


Kind regards,

DS

 

[JohnW2]

The OP has suggested a 80a MCCB at the garage end and it would be a DB and not a CU. Or as the OP suggested split tails.

Apologies - I must have made the mistake of assuming that when the OP wrote CU, he meant CU ...

...via an 80A switch fuse OR fit an 80A MCCB to a spare way in the existing garage CU.

As I said, since he talked about "in a spare way in the CU", I though that he actually meant MCB (and thought he could get an 80A one).

Kind Regards, John

 

[JohnW2]

Yes, I'd need an MCCB for 80A which a shame because they are larger and require a larger enclosure. Alternative is the 80A switch fuse. Either way, I need an extra box.

Indeed. As you probably know, switch fuses can be pretty small (no bigger than a double socket), in particular ... (click here) (which appears to be a copy of the MEM 800KMF)

If I stuck to a 63A MCB at the supply end, which is the largest MCB available, then I could run it off the existing garage unit which would look much better. I might still do this.

That would obviously be easier for you. However, it does not feel very nice to derive such a large distribution circuit from an MCB in a CU. High-rated MCBs in CUs are usually only for pretty short-term loads (notably showers). I would personally be much more comfortable with the suply being separate from the CU (i.e. a separate switch-fuse)

Kind Regards, John

 

[deadshort]

The maximum size of mcb i have installed in a CU is 50A ?
I assume due to the 100a max on the main switch.

Regards,

DS

 

[JohnW2]

The maximum size of mcb i have installed in a CU is 50A ?

Same here, although some manufacturer's (e.g. Hager) do do 63A ones.

I assume due to the 100a max on the main switch.

Indeed. As I've just written to the OP, I really don't think that a CU is a place to be originating a 63A (or even 50A, come to that) distribution circuit.

Kind Regards, John