US Electrics

[ECW]

Chaps,

I am trying to get the max heating power from an electrical boiling setup in the US. The voltage is 120V and the max household current is 20A so the power available is 2.4kW. I've only found elements up to 1.5kW for 120V. With said 1.5kW element the boil is slow to arrive at and weak when it happens. How can I use the 2.4kW available? Can I use the 240V 4.5kW elements at 120V to give approx. 2.25kW? All ideas gratefully received.

Cheers,

Ed

PS: Those in the UK with 240V 13A and cheap kettles don't know how lucky you are! Plus, your fuse boxes aren't on the outside of your houses - in a country where winters are long and harsh that must count as one of the most stupid ideas I have yet to see.

[OldSpeckledBadger]

What you would need is an element which has a resistance of 6 ohms. This means it would draw 40 amps at 240 volts, which is 9.6KW.

[Dr. Dextrin]

Why not use two 1.2 kW elements designed for 120V?

[sparky Paul]

What you would need is an element which has a resistance of 6 ohms. This means it would draw 40 amps at 240 volts, which is 9.6KW.

It's not that simple with an ac supply, resistance will not give you an accurate current calculation. Other factors apply with an ac supply, combining to give an impedance at a particular frequency - this is used in calculations. Basically, impedance is the ac equivalent of resistance, but is difficult to determine without the correct equipment.

Better to work with given kW ratings at the correct voltage.

[OldSpeckledBadger]

The inductance of the element at 50HZ is going to be very small. The increased in resistance due to heating might be more significant depending on how hot the element gets internally. Either way it's clear that there's no domestic 240V element which is going to do what the OP wants.

[sparky Paul]

The inductance of the element at 50HZ is going to be very small. The increased in resistance due to heating might be more significant depending on how hot the element gets internally. Either way it's clear that there's no domestic 240V element which is going to do what the OP wants.

Agreed. Halfing the supply voltage will not half the output of the element - it doesn't work like that.

EDIT: I've just re read my earlier post, and it does sound like I am disagreeing with you. What you said is quite correct, and definitely along the right lines WRT wattage required at 240V. What I was actually trying to convey is that measuring the resistance of a heater element to determine it's wattage is not always accurate.

My apologies.

[Graham]

Was talking rubbish here, so deleted it.

[sparky Paul]

a 240V element run at 120V will give you half the rated power. Yes, in theory, your proposed 240V 4.5kW element will give you 2.25kW @120V.

It doesn't work like that. For a resistive load, if voltage is doubled, current also doubles, and power is a product of the square of the current (P=I²R), so the power output is multiplied by four. Similarly, if the voltage is halved, the power output is reduced to one fourth, or 25%.

So a 4.5kW 240V element will give an output of approx. 1125W at 120V.

[edit1now]

Your US power supply should have a 220V line coming into the house, which is split to give two feeds of 110V. In your utility room (if you have one) there should be a peculiar socket for running the washer/drier at 220V - this is where you plug-in the brewing boiler. If you're in a flat then ask the building superintendent/janitor?

[Graham]

a 240V element run at 120V will give you half the rated power. Yes, in theory, your proposed 240V 4.5kW element will give you 2.25kW @120V.

It doesn't work like that. For a resistive load, if voltage is doubled, current also doubles, and power is a product of the square of the current (P=I²R), so the power output is multiplied by four. Similarly, if the voltage is halved, the power output is reduced to one fourth, or 25%.

So a 4.5kW 240V element will give an output of approx. 1125W at 120V.

That is true, wasn't thinking straight. Halve the voltage, quarter of the power.

[ECW]

And yes, you are all right. I did the calculations and (disappointedly, but I used to teach physics and really ought to have known better ) I have just bought another element to give 2x 1.5kW = 3kW (tvm Dr Dextrin). I needed an extension cable to use the sockets on the other side of the kitchen as there's only 20A available each side and each element takes 15A. However, US kitchens do have what they call GFCIs fitted as standard for safety. GFCI = Ground Fault Circuit Interrupter - a safety device similar to a UK RCD (Residual Current Device) although not as good, as an RCD doesn't require an earth connexion to work.

Point taken about the 220V socket in the utility room and yes it is available but a pig to actually get to as it is low down behind the dryer.

As an aside, it isn't surprising that most US brewers use gas (UK:propane) to heat their boilers given that 120V 15/20A gives pretty useless power, it's just that my particular rental agreement means I don't pay for the electricity, so I'm keen to use as much of it as possible! It's not unlike the large number of vehicles over here that contain "V8 Power" but can't actually pull the skin off a rice pudding...

Thank you all for the advice.