Electrical question - what would happen if?

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mriley

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Could anyone advise what would happen if I connected 115 volts to a 230 volt (750w) water heater. Would it be quite safe (but possibly take twice as long to heat up), or would it draw twice the current to heat at the design rate, and therefore possibly be dangerous? Thanks for any advice.
 
We had a voltage drop here t'other night and very little in the house worked except lights and the tv. Even the low energy bulbs struggled to light up, so I'm guessing that the answer to your question is that you would not get much heat, but a dangerous situation would not exist.
CJ
 
[ QUOTE ]
Could anyone advise what would happen if I connected 115 volts to a 230 volt (750w) water heater. Would it be quite safe (but possibly take twice as long to heat up), or would it draw twice the current to heat at the design rate, and therefore possibly be dangerous? Thanks for any advice.

[/ QUOTE ] Not dangerous, but the power would be approx. one quarter.
 
if the 115 v is Alternating Current, agreed you will just take at least four times to generate the same heat in the tank.

If it's Direct Current, you may find that heating is a little more instantaneous /forums/images/graemlins/grin.gif
 
[ QUOTE ]
If it's Direct Current, you may find that heating is a little more instantaneous

[/ QUOTE ] Why? A water heating element is purely resistive. It'd work just as well on DC as on AC. Themostat contacts might arc excessively though.
 
It should be quite safe but would be very slow. According to Ohm's Law (V=IR and W=VI), we can work out that, on 230V, the unit would draw just over 3 amps and therefore have a (nominal) resistance of approx 80 ohms.
If you connect this 80 ohm resistance to 115V, the current would be approx 1.4A and the watts drawn would be approx. 200 i.e. a quarter of the 230V figure.
It would be dangerous to do the reverse i.e. drive a 115V element from 230V.
Hope this makes sense to you!
Roger
 
The Ohm's Law figures above are very correct but when this happens on a domestic feed system.
Your biggest problem would be anything that has a older type relay or contactor it would only be hobbling to close so the armature the bit that flys in to close the contacts would not be fully drawn in and complete the motor effect of the relay or contactor and that would cause it to over heat or burn out maybe making the armature coil winding housing blister and go out of shape causing it to stick.
This used to be a pain in the back side when it happens in industrial 3ph stuff when one ph go's down.
 
To answer your question:

If the original heater is rated at: 750W/240V its resistance will be given by P=V2/R so R=V2/P therefore 240x240/750 = 76.8 Ohms. It will draw 240/76.8 = 3.13A

If you now connect this resistance of heater (which is fixed and assumed to be resistive as others have said) to a 115V supply, the power drawn will significantly reduce, P=V2/R which equates to: 115x115 / 76.8 = 172W hence it will draw just 115/76.8 = 1.5A

You will not have a problem with cables and the like!

However onto time to heat:

Assuming you have say a 10 litre (10kg) tank and you wanted to heat the water from say 10C to 40C, with your 750/240 heater, the energy required is given by Q = M Cp dT, the energy required to do this is:

Q = 10 x 4186 x 40 = 1,674,400 joules

Your existing 750w/240V heater provides 750j/s (note 1w is 1j/s), so:

1,674,400 / 750 = 2,232.53 seconds or 37.2 minutes

At 115V your power is reduced to just 172W, hence 172j/s, so:

1,674,400/172 = 9734.88s or 162 minutes or 2 hours and 42 minutes!

Otherwise no other harmful effects! /forums/images/graemlins/cool.gif

All clear as mud!
 
All of this assumes that the temperature and therefore resistance of the heating element is the same for the two voltages, In practice this will not be so but the difference will depend upon the degree of heat transmission between element and water. Since we have no info. on this probably best to ignore the effect but not to quote too many decimal points for the predictions.
 
If you really want the decimal places and ms expanded there is also the heat loss from the tank surface to be considered which in turn depends on the ambient temperature and the insulation efficiency and integrity. Still think "about 4 times as long" is good enough.
Why not boil a kettle?
 
All OK above except if there are any electronics controlling the beast. I don't include a simple thermostat in this - it's mechanical.

If there are any electronics, there will be an ac-dc converter somewhere. At it's very simplest, it's a transformer + rectifier & smoothing caps. Put 115V into one of these & you won't get the voltage required to power up the controller. You should't do damage to the controller. But the whole thing may not work. Or, it may just switch on the heater permanently.

If it's a switch mode PSU, then it may be perfectly capable of taking the 115. Or, it may not. It may just not switch on. Or it may be damaged.
 
Mmmmm but we do have empirical info to work with.

The OP stated that the original heater consumes 750W when 240V is applied! It is logical then to use this data for the comparison sought.

As it is a resistive element, it is further reasonable to assume that all electrical energy consumed is transfered to heat the medium (water).

The resistance of the heating element is mainly influenced by the materials used in manufacture and thats used to assess the power at the stated voltage!

Yes, the element resistance will of course change with temperature, but then so will the power consumed and current drawn as a result. We have to assume steady state conditions for this analysis!

Assuming a steady state, again reasonable for this comparison, this will not significantly alter the end result. That is of course unless you want to use Newtons laws of cooling, thermal modelling and calculus!

After all, in reality you will not always get 240V, so the calculations provided are correct for this type of assessment.

There are lots of other variables that could be considered, but its pointless and somewhat misleading to do so for the purposes of this comparison: surface area of tank (yes even that changes depending on shape containing a fixed volume), amount of scale in cylinder/on element, water characteristics, tank materials, insulation thickness/characteristics, ambient temp surrounding tank, etc, etc, etc.

But if the heater is consuming 750W of energy at 240V and assuming the medium is water, it will heat in the time stated. It was also assumed the 115V is AC provided via a transformer (not that thats too relevant for a purley resistive load - please dont come back with reactive element)! Once again control losses are not relevant given the original assumption - 750W/240V!

Great fun debating this but I think the original Q's are answered correctly /forums/images/graemlins/crazy.gif!
 
To summarise then

It'll draw half the current, it'll dissipate only a quarter of the power (ie about 187watts). If the thermal insulation were perfect it will take 4 times as long to heat a tank of water, but in practice it'll take a bit longer.

If it is a simple heating element with a mechanical thermostat it'll work, subject to the above limits and it will be safe.

If you were to decide to fit a 750watt 115 volt heater you may have to upgrade the wiring because it will take twice the current, namely just over 6½ amps.
 
I almost hate to point out (but not quite enough to desist) that while your methodology is correct your answer is wrong 'cos you appear to have failed to read the question properly. The OP is quite clear about 230v yet you persist in using 240v. I would lop off 5/10 for this error.
As the only one to mention transformers, I certainly did not assume step down - instead stepping up from 115v would give almost the same heating time as before and would allow retention of the original heating element and wiring for when the boat returns to civilisation.
 
Many thanks to all those who took the time to reply to this question, and in particular for the calculations showing it would take 4x as long to heat the water - this is counter-intuitive as you might expect at half the voltage it would take just twice as long. I think the water would heat up quicker if I sat in it.
 
On the water heater on my new boat there are links at the connection point that can be set to run then heater either on 110/115 or 220/320 Volts.

The unit in effect has 2 internal elements that are either connected in series for 230 v or parallel for 230 v thus giving the same input power for either setup
 
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