Ditching gas, going electric immersion only, a wee project

chris_n

Is your 10mm T&E all clipped direct or in conduit? I wouldn't run anything electrical that is getting warm in a domestic environment. I would rather increase the cable and MCB rating long before it gets to the point of thermal trips on MCB's.

JKenH

Presumably these new electric circuits for the tank heaters need to be signed off to comply with regs.

Solarchaser

chris_n said:

Is your 10mm T&E all clipped direct or in conduit? I wouldn't run anything electrical that is getting warm in a domestic environment. I would rather increase the cable and MCB rating long before it gets to the point of thermal trips on MCB's.

Neither really, but technically clipped so 64a rating if that's what you are asking? though as I said already, the cable is not warm, the mcb is.
The mcb is already 50a, and I'm not going to 63a.
The 63a rcd is warm also, so I'm moving some of the current to a different circuit.

If you have worked with cable, you will know 10mm is a pita in a domestic cu, 16mm would be ridiculous. 
The mcb is heating up over time as I'm using its max rating or very close to it for a 3 hour period.
It all makes sense.
Taking a quarter of the current draw and putting it on a different circuit will let everything run cooler.

Ken, aye maybe in theory, which regs?
When I'm fully done with it, I'll probably get a spark to run the whole house test, same as I did a few years ago when I rewired the house.
Then again, I might not.

JKenH

Solarchaser said:

Ken, aye maybe in theory, which regs?
When I'm fully done with it, I'll probably get a spark to run the whole house test, same as I did a few years ago when I rewired the house.
Then again, I might not.

I was thinking of Part P of the building regs (in England and Wales). Presumably there is some equivalent regulation in Scotland or maybe not. 

A brief explanation is given here
https://engx.theiet.org/f/wiring-and-regulations/22713/part-p---new-circuits

Heedtheadvice

No equivalent of the E&W building regs part P in Scotland. What still applies however is the British Standard otherwise known as wiring regs. That requires conformation to those wiring regs and a competent person to do the work.

Very few DIYers will be competent and would have difficulty proving such if it came down to legal case such as an insurance claim or injury investigation. The onus is thus on the DIYer.

Non of that implies there should not be proper design, test and record keeping!

Heedtheadvice

I would also point out fir @Solarchaser and anyone else working out current that the correct approach is.

Start from the resistance (or rather more correctly impedance as it is AC but not really relevent for a resistive heater) rather than power which is not constant!

The resistance will vary only slightly with temperature.

Then one can work out the current from ohms law where I= V/R.

As the voltage rises so does the current and so does then the power wher P=VxI or Vsquared/R.

So it can be seen that as the voltage rises so does the current and the power rises with the square of the voltage!!

4Amps per kilowatt is a rule of thumb i.e. an approximation only for 250v supply. The actual value will depend upon real values and the voltage for which the heater is designed to produce 1kW of heat (62.5ohms at 250v)

However consider that the design value could be for the nominal 230v supply then the resistance will be lower at 53ohms. That is a resulting current of 4.3Amps.

When that particular resistive load is connected to a 250v supply the current rises to 4.7amps and greater than 1kW.

If  the above example were the case 4 heaters at a nominal 3kW would be 3 x 4.7 x 4 = 56.4amps.

So we need to start with  heater resistance and then use the supply voltage. That voltage varies both with location, overall generation, loads on the network and transformer tappings.

Typically South of Scotland it can be on the high side and mine has just been reduced to an average of 248v after experiencing over 252v and my Zappi getting a bit upset with short term peak even higher than that.

Summary: suggest working on practical values rather than rule of thumb. Might be the reason a 50A breaker was a bit cosy!! Perhaps 2 x 32A breakers/circuits could be an option feeding the heaters?

Solarchaser

Hi Ken, yeah I figured you were talking part P and perhaps didn't realise its not relevant in Scotland,  Scotland tends to go with the old competent person thing, which is arguable.
For instance having a qualification in electrics and electronics... which I have from a lifetime ago, generally is seen as enough for competent person. Though there is always the possibility of facing a clip board weilding maniac who likes rules.

I'd be pretty confident I could stand my ground, there is nothing that I've done which could be seen as dangerous. 
The trip going says I should have thought more about total current, but as I always do, I'd oversized the cable and put the trip close to load capacity,  so if there is any issues, the trip goes, but there is no risk of overload.

Most DIYers get to doing DIY in the same way I have, you pay for someone to come and do work as a professional, then you look at the work afterwards (or a fault develops due to poor workmanship) and think, hmm I could have done a better and safer job than what I paid for and at a much lower cost, so in future I'll do it myself.

As an aside of sorts, the reason I rewired my house a few years ago was because it had been rewired maybe 20 years before and what the "professionals" had done was run the wires over joists and right under floor boards, and the same with metal rsj's. 
I only found this out when investigating squeeky floorboards and found it was because they were on top of socket ring mains wires, the boards were creaking.
When you find something like that, you doubt everything else in the house, so a rewire was needed.
It was a good time to run lots of 10mm everywhere for showers, and cooker rings and oven. 

When all this was done I brung in a spark with the proper job electrical tester and confirmed all was good.

Though interestingly, or perhaps not, he noted since there were alot of pot lights in the house that were double insulated and so only wired with live and neutral and since I had replaced a fair bit of the heating with plastic pipe, that the overall house earthing circuit was reduced and so I would rely on the mains "earth", which he suggested wasn't so great, and that a good house earth was preferable, hence I went and joined the pot light earth's and ran more between the radiators where there were plastic pipes.
I mention this, as on one of the threads a few months ago, I was berated for suggesting that the radiators etc made up a good portion of your earth (in relation to EPS discussion)

@Heedtheadvice you will see if you go back a couple of pages I did indeed refer to P=IV  versus the old 4a rule of thumb as that's how I came to the 50a conclusion. (Admittedly after the fact, but I'm giving myself a break on this one as its only a minor faux pas)

For reference my heaters state 3kw @240v and 2.7kw @230V so 3000/240 =12.5a and so 4 of them =50a and in fact if my voltage drops, as resistance stays the same it would be 47a, not sure if I covered all of that a couple of pages ago, but I'm sure I did the 12.5 and so 50a.

In response to your suggestion of 2x32a circuits, while I agree this would reduce the load on the 50a circuit, this wouldn't help with the heat that I've spoke about in the 63a/30ma rcd unless I move half onto another rcd as well.

My consumer unit has 2 rcd /63a circuits and 10 MCB's. 
One rcd holds the battery circuit, which is charging at roughly 30a, so I can't add another 25a onto it.
The other rcd is where the tanks are connected.
There is also a 50a MCB stand alone for the EV, the EV will charge at say 30a also, and so you can see that none of those circuits really have enough headroom to add another 25a.
I have actually given this *some* thought.

If I take 12.5a off the rcd with tanks and the MCB with tanks then the 50a drops to at least 40a (if we assume sometimes 250v) which is better.
The non rcd circuit sees 12.5a and potentially 30a for the EV but still only 43a, so again nice and safe.
I feel its the best solution all round, and it also is one of the easiest to implement, which is handy for me, I only need one run of 2.5mm cable rated at 20a (though I might run 2 for some future proofing) and it's alot easier to manipulate than 10mm.

Reed_Richards

Solarchaser said:

For reference my heaters state 3kw @240v and 2.7kw @230V

3 kW @ 240 V implies a resistance of 19.2 Ohms and a current of 12.5 A
2.7 kW @ 230 V implies a resistance of 19.6 Ohms and a current of 11.7 A

Since the resistance cannot be both 19.2 and 19.6 Ohms then the figures you have been given cannot be very accurate.  Can you not disconnect from the mains and measure the actual resistance?

The resistance of the immersion heater will tend to increase as it gets hotter so it will draw most power when the water is coldest.

My consumer unit is fitted with RCBO circuit breakers.  The one for my immersion heater, nominally 3 kW, is a B32 so 32 A rated. 
 

Heedtheadvice

Hi, I had forgotten your earlier post!!

With regard to your explanation of coming up with 50A (given your heater data) I would still suggest it requires higher as at 252V (quite possible you will agree) that is 52.5A giving a you a 'healthy'. 3.3kW each.

Just an aside-Is your 63A main rcd actually on a 63A Incomer or 100A so you might be able to uprate if required. But...

I can see your reasoning to shift some heaters to your other consumer unit  and that seems a good move (provided that it is on the same phase otherwise that might complicate how you approach safety) and you then get that extra capacity you might be considering.

At least we are now in a heat wave that might become a flood!

Solarchaser

Reed_Richards said:

Solarchaser said:

For reference my heaters state 3kw @240v and 2.7kw @230V

3 kW @ 240 V implies a resistance of 19.2 Ohms and a current of 12.5 A
2.7 kW @ 230 V implies a resistance of 19.6 Ohms and a current of 11.7 A

Since the resistance cannot be both 19.2 and 19.6 Ohms then the figures you have been given cannot be very accurate.  Can you not disconnect from the mains and measure the actual resistance?

The resistance of the immersion heater will tend to increase as it gets hotter so it will draw most power when the water is coldest.

My consumer unit is fitted with RCBO circuit breakers.  The one for my immersion heater, nominally 3 kW, is a B32 so 32 A rated. 
 

Re the resistance,  I mean, does it really matter?
Whatever it is, is whatever it is.
There will be a manufacturing tolerance which will probably be around 5% or more.
I'll measure them when I remove the tanks to make room for the 3rd tank, but really its ballpark 20 ohms, so meh.
I'd expect all 4 to be slightly different resistance anyway.
I think for my own quick reference I'll just call them 13a instead of 12a which gives 52a (as heedtheadvice leans to) and thats fine.

I personally wouldn't be happy with a 32a rcbo on a 13a draw, if that was my house I'd be changing that out for a 16a, I mean worst case scenario it could be wired with single 2.5T&E meaning the if the element went low resistance the supply cable would melt before your rcbo would trip.

Heedtheadvice I've looked back at data from the lux inverters and I'm a generally pretty stable 245V except during my 3 hours of off peak, when it dips to 240V, but thats kinda expected with my current draw on one branch of a 3 phase cable.

All 3 phases are in my house, 1 for me, 1 for the neighbour and one spare... unmetered. If I had slightly less scruples I would do all my heating from that phase, but I'm not. (Not saying I'm not tempted)

So yeah everything in one phase, and you are correct it's 100a feed to the rcd, so it would be an option to upgrade, but I'm fairly set now on my plan, just need the temperature to lift a little  and to shake off some malaise.

A heat wave you say? My 7 day prediction says minus numbers for hogmanay and new years, which isn't a bad thing as you usually need to use the back step as an extra fridge on hogmanay 🤣