Instantaneous water heater calcs

[Soulsurfer]

Hi all, been a while since working out calcs for size of a TP Mcb and cable for something like this and am a bit tired and ill so thought I'd take some stick and ask !

Item is a Zip DBX 27 and 3 phase at 27kW, supplied by a 3 core Swa is what client has asked for and seperate Earth alongside from D/B up into void above suspended ceiling / tiles

my mate is the Zip installer & plumber and had paperwork indicating 39Amps.

I take it, it will also need something like a 3 phase rotary or similar isolator at fitting position, and would a decent size SY flex be ok from isolator to heater ? It's going on wall in food prep area in a chip shop to replace a failing gas boiler.

Thanks,

 

[davesparks]

In a chip shop you'd probably want a heat and oil and water resistant flex rather than SY which is none of the above. As there is not likely to be a need for EMI shielding I doubt that SY is really needed?

If the client is specifying the type of cable and need for a bonding conductor (why does a final cut need a bonding conductor run with it?) then why haven't they also specified the size?

 

[Soulsurfer]

True enough with the SY, as its right between 2 large sinks etc.. So water might not be too great with the braid. Client said Zip and DNO specified the armoured. No other details I just got Zip ratings from plumber mate who was asked to quote the wet side. 27kW just seemed massive for a bit of hot water. Surely could have just run 4 core to cover the earthing.

As far as oil etc there is a tiled wall blocking out the area from front of shop 3/4 across & just a small walkway through to fryers and customer facing areas.

 

[Leesparkykent]

True enough with the SY, as its right between 2 large sinks etc.. So water might not be too great with the braid. Client said Zip and DNO specified the armoured. No other details I just got Zip ratings from plumber mate who was asked to quote the wet side. 27kW just seemed massive for a bit of hot water. Surely could have just run 4 core to cover the earthing.

As far as oil etc there is a tiled wall blocking out the area from front of shop 3/4 across & just a small walkway through to fryers and customer facing areas.

Why would the DNO be involved with a circuit for a water heater in a chip shop?

 

[davesparks]

Which leads to the next question of what on earth the DNO have to do with the design of the circuit? If zip are specifying the cable type then I'd ask them for the full set of calcs to be done to establish the size etc aswell.

So it's a 27kW resistive load, simple calculation will give you the design current and cable size can be based on that. I suspect you'll be looking at a 40A supply, but that's a bit of an educated guess. Did they give you a voltage to go with the power rating? Without it you are ****ing in the wind a little bit!

I would be concerned about wanting a separate g/y to be run with this supply cable unless there is a need for a main bond at that location? If they think it will be needed as a CPC then they will be wrong, a separate conductor strapped to the outside of a cable will do very little in the event of a fault and some people have suggested that it is technically non-compliant as an installation design. Plus aging that extra conductor would require an additional hole to be made in the enclosure of the IP rated enclosure thus adding another potential point where the IP rating could be compromised (I'm not suggesting that you would allow that to happen, just saying the potential is there)
As you say an additional core could be used, but in reality the armour will be suitable to be the cpc.

 

[Soulsurfer]

Sorry chaps, typed out a long response after hearing from the leaseholder / client & somehow deleted it all by hitting reply tab on phone ! 400V rated & supplied 100A 3 phase supply in. DNO has their own building services dept so apparently quoted by looking inside Db cover and up at ceiling tiles and quoted now in document form. After quick look said she's looking at around 2 grand with Zip unit supplied. The existing spare MCB to be used which is TP 32A B curve, seperate Cpc specified and Swa but. I size on quote. £2,430.95. My plumber mate says the Zip unit is around £800.

 

[Martin Kerr]

Best of luck keeping that breaker in
ps 10 mm must have got very expensive lately ,or thats some size of chip shop

 

[davesparks]

Sorry chaps, typed out a long response after hearing from the leaseholder / client & somehow deleted it all by hitting reply tab on phone ! 400V rated & supplied 100A 3 phase supply in. DNO has their own building services dept so apparently quoted by looking inside Db cover and up at ceiling tiles and quoted now in document form. After quick look said she's looking at around 2 grand with Zip unit supplied. The existing spare MCB to be used which is TP 32A B curve, seperate Cpc specified and Swa but. I size on quote. £2,430.95. My plumber mate says the Zip unit is around £800.

Ok, so it's the DNOs pet contracting business who want to launch a bit of SWA over the ceiling tiles with a useless bit of g/y strapped to it. It isn't actually the DNO doing it though, just their contracting business which conveniently uses its name to make it seem like the 'electricity board' are doing the work.

I'd be very wary of putting it on a 32 B type!

If they're going £2.4k then go in for around £2k and happy days!

 

[Soulsurfer]

Yes guys I think they get the whole " we are the electricity supply board " couldn't be better spiel ! and rating on Zip @ 39A seemed way under but.. Was thinking Swa 10mm minimum & trying to find a TP 40A MCB to fit the polestar board should be fun too & what do you think as far as a C curve for any lift off current ? It is 27k after all. Then it's the appropriate isolator. Think they just throw figures out there & think it's a given to get the publics work. Ps chip shop is teeny too !

 

[Soulsurfer]

Just a quick one as I'm full of bug / flu likely & braindead now !
27kW / 400 gives 67.5A am I then route 3'ing at / 1.732 ?
Leaving me 38.97A ? Or am I being .....

 

[Soulsurfer]

https://www.jec.co.uk/about-us/our-business/building-services/

 

[davesparks]

Just a quick one as I'm full of bug / flu likely & braindead now !
27kW / 400 gives 67.5A am I then route 3'ing at / 1.732 ?
Leaving me 38.97A ? Or am I being .....

400V really? Is this installation in the middle of a textbook?
Check the rating of the Zip unit, it will almost certainly be 27kW at 415V not 400V!
And your incoming supply is unlikely to be 400V either considering it will be 433V at the substation.

Polestar mcbs shouldn't be too hard to get, most wholesalers seem to have them knocking around.

 

[Leesparkykent]

The manufacturers instructions also say it needs to be on an RCD.

 

[Soulsurfer]

Just going off the Zip paperwork Dave, fair enough point as 230V over here is never anywhere near it either!

mostly few volts higher. Thanks for taking the time out to help.

 

[Soulsurfer]

Lee, I mentioned that to the client & plumber but had no definite info, was calling Zip agent today anyway. But thanks, so manager at JEC / DNO who quoted was using a 32 TP MCB & no RCD irate: