Three 3kW handwashers

[oo7]

Hi guys.

Pardon my novice sounding questions here but I'm a newly qualified electrician and I would like to get some of the more unusual work I'm asked to do double checked by more experienced electricians before quoting.


The job is to install 3x 3kW hand washers. The engineer who I meet to go over the job with (it's for commercial purpose and he claims to have held a 16th edit) was telling me that he would have just taken a 13a spur off the ring for each unit. Now I'm against this (good practice to stick anything over 2.2kW on a separate circuit) and want to install a new 30a radial circuit to host all three hand washers. My install would run...

30A 3036 fuse at CU (Old installation)
30mA RCD
4mm T&E (yet to do cable calculation but buried in none insulated wall)
2x sfcu 1x fcu with pull cord switch (one unit to be fitted in zone 2 of bathroom)
2.5mm flex to each unit.

Link to manufactures instructions…
http://www.tritonshowers.co.uk/media/13910/t30i_handwash.pdf


So what do you think? I don't think I've overlooked anything, the length of the radial cicuit would be around 20m max, very unlikely that more than two units used at one time. Again pardon the simplicity of the question but like I said I want to be sure before I put in a quote.

Many thanks!

 

[ricicle]

3 x 3kw = ? amps

0.725 rule ?

 

[oo7]

Near on 40A. But these water heaters, by my understanding, only demand every so often so can apply diversty?

But you've got me racking my brains over the ".725" rule. That one seems to have slipped my mind so please refresh.

 

[Porque223]

http://www.electriciansforums.co.uk...egulations/62068-bs-3036-fuses-ring-main.html

 

[EFLImpudence]

Near on 40A. But these water heaters, by my understanding, only demand every so often so can apply diversty?

That link is not very helpful - dealing with a ring circuit.

It is nothing to do with the load but the fuse.
BS3036s have a fusing factor of 2.

When BS3036s are used the CCC of the cable must be derated by a factor of 0.725.
Therefore a CCC of at least 41.4A (30 / 0.725) is required.

That is why cooker circuits used to be (and still are) 30A fuse and 6mm² cable.

 

[JohnW2]

When BS3036s are used the CCC of the cable must be derated by a factor of 0.725.

Indeed, and if I may add a little explanation, since this ties in to a thread I started this afternoon...

The standard CCC tables assume the use of a Type B MCB, or something with equivalent characteristics. For such a device, I2 (current required to cause operation within 1 hour) is 1.45 times In (the 'rating' of the MCB). Hence, for example, if the tables give a CCC of, say,10A, that means that the cable can safely carry 14.5A for an hour.

With a BS3036 fuse, I2 is 2 times In (as compared with 1.45 times In for the MCB). A cable protected by a 10A BS3036 therefore has to be able safely carry 20A for an hour. The tabulated CCC figure therefore has to be adjusted, by a factor of 1.45/2 (i.e. the ratio of I2/In values), which is 0.725.

Kind Regards, John

 

[oo7]

Many thanks for these replys guys.

Like I said I hadn't looked through my book at the time of the post (and still haven't), this is also the first time I will have had to conceder 3036 so I rely am appreciating the prompts.

 

[JohnW2]

Many thanks for these replys guys. Like I said I hadn't looked through my book at the time of the post (and still haven't), this is also the first time I will have had to conceder 3036 so I rely am appreciating the prompts.

You're welcome. No-one has so far commented on what you said about diversity. Yes, you can apply diversity, and if you use the OSG figures for instantaneous water heaters (I assume that's what you're talking about), and assume 13A per device, the total after diversity would be 13+13+(13/4), namely 29.25A - less than 39A, but still a lot.

... and, although no-one else has said this, yes, I'm sure you're right in thinking that to run these heaters on three spurs off a ring is just plain crazy (aka extremely poor design practice) - not the least because it would leave virtually none of the design current for the ring itself!

Kind Regards, John.

 

[Porque223]

In any case it is good practice to have the engineer and you totally detail your reasoning to each other, premises and conclusions and steps in between.

 

[OwainDIYer]

Can you sensibly apply diversity?

If this is a retail shop where people go for their tea-n-pee breaks at different times, probably, but if it's a factory or even the sort of office where people tend to go for breaks together, then all the heaters will be running at the same time. In larger establishments the heaters may be running almost continuously anyway.

They are pretty feeble for handwashing and useless for washing-up or other purposes anyway and if all the outlets are sufficiently close together then a wall hung cistern heater will give better flow and have a lower peak electrical loading

http://www.tlc-direct.co.uk/Main_Index/Water_Heating_Index/ZIP_RCH/index.html

 

[333rocky333]

2.5mm flex to each unit.

Whys that

 

[securespark]

Depending on the type of commercial installation, I would be sorely tempted to do away with diversity and put the heaters on a 40A breaker (board permitting).

Although there has to be a better solution (pun not intended), but it may cost more on the plumbing side to install.

 

[JohnW2]

Can you sensibly apply diversity? If this is a retail shop where people go for their tea-n-pee breaks at different times, probably, but if it's a factory or even the sort of office where people tend to go for breaks together, then all the heaters will be running at the same time. In larger establishments the heaters may be running almost continuously anyway.

That's obviously a judgement the designer has to make - as you say, usage will vary dramatically in different circumstances. However, even in heavily used facilities, I doubt that the heaters would be running anything like continuously - probably only for a relatively modest proportion of time that each person stands in front of the basin. For an average total current of ≤32A with 3x13A heaters only requires that the duty cycle is less than about 82%, which I strongly suspect it always would be.

Also, we're only talking about three heaters, one of which will apparently be in a 'bathroom', so it's no factory. Furthermore, with only three heaters, the OSG is not very generous about diversity - only allowing a reduction (75%) for the third unit.

Having said that all, if it were me, I'd probably be conservative and not rely on diversity!

Kind Regards, John

 

[Porque223]

How close would the bottom line costs for each way have to be before you go the conservative route?

 

[oo7]

The use of each system will be very limited.
Two of the units are in one building, which has a shower room with toilet and basin and the other room is the kitchen area with sink. These two unit serve one person at any time as it's a security guard house.
The third room is another kitchen area serving a maximum of 5 people at any one time.

I think however a 40A 3036 (not checked availability yet) and appropriate cable is a better route.


And the flex is because the wall in the bathroom is tiled making chasing cable behind the unit a pain. Although I could make an addition on the quote to do the work removing tiles if requested (should have asked the question there and then, dough! Like I said still learning.)


As for pricing, it's not going to make a massive difference to go down an install that doesn't "rely" on diversity because of the relatively short run.