Load and heat

[Gordon lees]

This isn't a wind up, gather I'm a bit of a laughing stock already and only joined this site 6 hours ago Arranged with the client to install a 63A MCB after the RCD. Or is this going to cause an eruption of laughter amongst the keyboards?

 

[DPG]

If this is for real then please get a qualified electrician to complete the job. No offence intended but you are out of your depth I would say. Daz

 

[Octopus]

wonder what size the main fuse is.....

 

[westward10]

How is it currently connected to the rcd.

 

[The Ghost]

I think you should have used 16mm² ccc=63a in that type of cable. On AEI cables it shows 47a for 10mm² ccc. If you calc 12000 /230, well I get 52a. f.l.

 

[LankyWill]

How is it currently connected to the rcd.

Stuffed in the top of the RCD no doubt, or some rogue form of splicing..

 

[dmxtothemax]

QUOTE [the cable is omitting a slight heat when drawing this load, was just wondering if this is normal]

Yes it is normal !
cables do heat up under large loads.

 

[Gordon lees]

I think you should have used 16mm² ccc=63a in that type of cable. On AEI cables it shows 47a for 10mm² ccc. If you calc 12000 /230, well I get 52a. f.l.

I've been thinking about it, met with the client today and although the boiler is rated at 12kW (52.7A) it only draws 45.9A on maximum demand. I'll be calling the manufacturers helpline one Monday, as they advise 10mm H07RN-F cable is used. Which obviously isn't meeting the requirements of the CCC for the cable based on a 12kW power usage. A 50A MCB has been installed which, according to load measurements, will be suffice. I've seen on other forums that people have advised that this cable (10mm) is used for this very same boiler. A learning curve to say the least, kicking myself about the over current protection as I definitly should know that, and did. My only explanation is I'm in a job where a lot of the time I'm not doing electrical work, and so not " keepin it fresh " as they say. A couple of courses have been booked before I sit the city and guilds 2394 and 5 combined to brush up on electrical theory. Anyway, thanks to all for the helpful advice, and to the others, well, it must be great being perfect and never making a mistake

 

[Pete999]

What is the actual voltage? 230 is the harmonize voltage but in reality it more likely to be 240 ish = 50Amps

 

[Gordon lees]

What is the actual voltage? 230 is the harmonize voltage but in reality it more likely to be 240 ish = 50Amps

Actual measured voltage this morning was 236 volts. Would I be right in saying that the indicated maximum demand of an appliance is perhaps an slight over statements, to take into account other factors, such as volt drop

 

[Pete999]

Actual measured voltage this morning was 236 volts. Would I be right in saying that the indicated maximum demand of an appliance is perhaps an slight over statements, to take into account other factors, such as volt drop

I would say the stated max demand is an ideal figure, I doubt very much if the manufactures have accounted for VD, derating /correction factors etc, but you have to take their statement into account when you are designing the installation, correct me if I'm wrong but you seem to have just done what they suggest, without taking stock of the variables, if I'm wrong I apologise.

 

[Wilko]

Hi - often heaters are rated 12KW at 240V and 11(ish) at 230V, so depending on cable run etc 46A is possible, but perhaps not always the case? So as you say, check in with manufacturer specs to confirm. The cable is likely rated to 60C, and the one I saw was 51A at 60C conductor temp for 10mm2. Design-wise I like to live a bit further away from rated capacity for high demand long duration circuits to allow for less than perfect installation conditions and long times between inspections. Just sayin.
http://www.batt.co.uk/upload/files/4f1a_1227798371.pdf

 

[The Ghost]

When making the load calc 230v nominal voltage is always assumed irregardless of the temptation to use TRMS voltage measured.
Is the ccc of the cable rated from the view point of assumed 20 degrees. Because you mention the cable goes up from the boiler into a void. This will increase ambient temperature and will derate the cable further. As you probably know something like 6242y T&E is rated at 70 degrees operating temperature, which is pretty warm! But is regarded safe up to that temperature. Similarly HOFR is rated to 60 degrees. So a bit warm is not really a problem unless it goes over its rated temperature. As Wilko I would always go for the cable above where it just about works with no margin for derating.

 

[davesparks]

When making the load calc 230v nominal voltage is always assumed irregardless of the temptation to use TRMS voltage measured.
Is the ccc of the cable rated from the view point of assumed 20 degrees. Because you mention the cable goes up from the boiler into a void. This will increase ambient temperature and will derate the cable further. As you probably know something like 6242y T&E is rated at 70 degrees operating temperature, which is pretty warm! But is regarded safe up to that temperature. Similarly HOFR is rated to 60 degrees. So a bit warm is not really a problem unless it goes over its rated temperature. As Wilko I would always go for the cable above where it just about works with no margin for derating.

The load calculation should be done at the voltage it is rated for, you can't do the calculation at 230V if the power rating is at 240V as that would be a nonsense.

 

[davesparks]

The cable itself has 3 layers of insulation, quite thick as well. Conductors single insulation and two outer layers, obviously to withstand high external temperatures.

The cable does not have three layers of insulation, it has one layer of insulation around each conductor and then a sheath. The insulation is made of rubber (R) and the sheath of neoprene (N)
This cable has no specific resistance to high external temperatures, but does have a very tough mechanical strength and high UV resistance.
I don't really understand why you chose a flexible cable for this application when the Moreno traditional installation cables would be suitable? The extra work in providing the extra supports required, bigger physical size, greater weight, lower operating temperature, need for ferrules etc would surely make it more expensive and less suitable? Or did you not bother with any of that and just install it without correct calculations or installation methods?

As for omitting over current protection, well, I don't think I really need to express my opinion on that as I think everybody can hazard a good guess already.

 

[The Ghost]

that would be a nonsense.

BS7671 pg. 316 deems Nominal voltage is 230v thats good enough for me. As well 3rd amendment bought in Cmin so we are now talking 218.5v. All theoretical agreed. However if voltage dropped to 210v (not unusual) we are now talking 57a on a 47a cable. So in conclusion I feel safer with Uo as in the regs gives a bit more leeway on voltage variations and anticipating the unseen possibilities. So I wouldnt be too quick to dismiss it as nonsense.

 

[HandySparks]

However if voltage dropped to 210v (not unusual) we are now talking 57a on a 47a cable.

What?? This heater is, effectively, a fixed resistance type load. If you drop the supply voltage, you'll drop the load current in proportion. If it was a switched mode power supply feeding a constant power device, yes, the current would go up with reduced supply voltage.

So, as davesparks says, you need to read the spec for the device and take your calcs from there.

 

[GL4Spark]

Gordon Lees, read what the boiler spec is telling you. It will say something like 12kW @ 240v. You then need to use your p= iv v=ir to calculate resistance of load. Then put this back into your formulae for 236 volts.

You now have an accurate current demand for your circuit. Ideally you should be checking all this prior to installing the circuit and certainly don't take manufacturers word for it- it's your name on the cert!

 

[davesparks]

BS7671 pg. 316 deems Nominal voltage is 230v thats good enough for me. As well 3rd amendment bought in Cmin so we are now talking 218.5v. All theoretical agreed. However if voltage dropped to 210v (not unusual) we are now talking 57a on a 47a cable. So in conclusion I feel safer with Uo as in the regs gives a bit more leeway on voltage variations and anticipating the unseen possibilities. So I wouldnt be too quick to dismiss it as nonsense.

You clearly don't understand the very simple physics you learned at school then.
Power and current are not constant, as the voltage is reduced they will both reduce proportionally. What you are suggesting is that the power remains constant and therefore the current rises as the voltage is reduced, which would mean that if the voltage was reduced to zero then the current would rise to become infinite!
If a load has a rated power at a particular voltage then it will only achieve that power at that voltage, at a lower voltage it will achieve less power.

You also need to consider the fact that all substations in the U.K. Are fixed at a 250V output, they cannot be altered without bringing the entire country to a grinding halt!

 

[davesparks]

Actual measured voltage this morning was 236 volts. Would I be right in saying that the indicated maximum demand of an appliance is perhaps an slight over statements, to take into account other factors, such as volt drop

No, the power rating is specified at a particular voltage, it is only relevant at that voltage. At lower or higher voltages it will change proportionally.

This is basic physics we were all taught at school!