Heating element maths

[JamesF]

It's just occurred to me that many of the kettle heating elements I'm looking at give power ratings at 220V. We're not on 220V though. We're on 240V, and in fact it's a but more than 240V. Probably nearer 242V. I'll check again in a moment if I can.

But, that makes a difference from the point of view of wiring, doesn't it? Here's my thinking...

From P=IV, a 2.4kW element at 220V draws a fraction under 11A. That's probably fine for a 13A outlet.

V=IR gives the element a resistance therefore of 220/11 = 20 ohm.

But at 242V, V=IR gives I = 242/20 = 12.1A.

Drawing 12.1A from a 13A outlet may well work, but I'm thinking it's getting a bit marginal. Perhaps I should forgo using three pin plugs and wire into a switched 20A flex outlet plate.

Is my maths correct? Am I being over-cautious?

James

[JamesF]

Looks like we're actually getting 238.5V at the moment, so the current would be 11.9A.

James

[Hairybiker]

But it isn't a straight resistive load, there is a bit of inductance in it as well, (wire wound). So you would need to calculate in the lamda factor as well.
I happily run a kettle element (3KW) on a single plug, I use it for making tea

[JamesF]

But it isn't a straight resistive load, there is a bit of inductance in it as well, (wire wound). So you would need to calculate in the lamda factor as well.
I happily run a kettle element (3KW) on a single plug, I use it for making tea

But not continuously for an hour and a half

Looks like we're back to 241V now. I take your point about the inductive component though. I guess the change in phase angle doesn't have to be too much to make a fair difference to the figures.

James

[clarets7]

The target mains voltage for the UK is 230V, and can vary by +10% to -6% of that value.

[JamesF]

Yup. Ours used to hover around 247V, but I asked them to wind it down a bit when they replaced the transformer a couple of years back. You get through a lot of light bulbs at 247V, perhaps because it's near the top of their design specification. They last far longer now.

In other news, it's amazing the number of web pages that say "A purely resistive load such as a kettle element has a power factor of 1"...

James

[themadhippy]

The inductive component dont realy matter as its rather small,if you want to get realy anal then include the capacitive reactance ,the cables a capacitor after all .
And yes your correct,the resistance of the element is fixed,change the voltage and you change the amount of power it produces,and of course the current drawn,in this case at 240v youve got a 2.8kw element. To avoid loads of sums dont forget you can combine both the simple ohms law equations (V/IR and P/=IV) into 1 equation ,so R=V^2 /P

[Troutman47]

But as long as your sockets are on the ring then you could run 2x2.4kw elements drawing 20amps together and your MCB is rated at 32amp as I understand talking to Sparks at work.

[JamesF]

But as long as your sockets are on the ring then you could run 2x2.4kw elements drawing 20amps together and your MCB is rated at 32amp as I understand talking to Sparks at work.

Yes, it's not really the capacity of the ring that bothers me. It's that standard three pin sockets and plugs are usually only rated for 13A and I'd potentially be drawing over 12A depending on the fluctuations of our mains voltage which limits the margin for error somewhat.

James

[Pinto]

You wont see a stable 220V at the socket unless you have a Vphase fitted at the consumer unit. My power averages out at 238V.

End of the day, running a consistant 2.4kW load on a domestic socket on a regular basis isnt going to help with the longevity of the socket - as a maintenance spark on a social housing contract, one of the most frequent repairs I encountered was repair/replacement of 40A pull cord isolators which had burnt out (and frequently taken 2-3" of the neutral/live tails with them) - all of these were connected to 9.5kW showers, so well within the theoretical capacity of cable, isolator and MCB - but 2 hours of consistant use per day in the household could see them ruined again in as little as 3-4 months.

This was negated by my eventual decision to upgrate to 50A pull cords on 10mm2 cabling wherever I could - none of these failed again.

Moral of the story - if you want to do a hard boil's graft - you need to choose workman like, heavy duty switchgear and supplies. Hence why I tend towards the use of "commando" sockets, outlets and rotary isolators in these circumstances.

How you explain a rotary and commando outlet in the middle of the polished chrome platework on the kitchen wall tho to the missus.... thats down to you

[JamesF]

Moral of the story - if you want to do a hard boil's graft - you need to choose workman like, heavy duty switchgear and supplies. Hence why I tend towards the use of "commando" sockets, outlets and rotary isolators in these circumstances.

Eeek! I've just seen the price of that!

James

[Pinto]

You can make it for a lot less can't make them much more attractive tho...

[JamesF]

Fortunately as this is for my own personal brewing space, "pretty" isn't a requirement

James

[Jocky]

I've had a separate 32 amp circuit installed, which supplies 2 commando sockets on the patio for my brewing.

Have to say that the 2.4kw elements and cables I have received from a reputable supplier have C13/14 connectors on them, rather than the proper hot rated C15/16 connectors that have the notch in them.