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Will this water heater cause an overload?


Boeriemore
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I am busy installing a 7KW instant water heater. I assume that relates to 32 amps.

The circuit breaker it will be connected to is 30 amps. With other appliances also connected to the circuit will this result in an overload?

Also, the wiring cable  from the heater is thicker than the existing cable to the breaker. How can I confirm that the thin cable is OK or

needs to be replaced?  My electrical knowledge is limited but I must point out that my swimming pool pump is rated at 7.5KW yet never had an overload problem

on that circuit. Some enlightenment would be appreciated.

 

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The wire is the more important part to get right. Breakers can be replaced easily.

Two kinds of breakers. Some uses a thermo-mechanism and others a magnetic mechanism. The thermo one can run over it's limit slight without tripping, but will eventually heat up enough to trip. We have such a circuit at work. Fused at 50A but runs up to 60 for short periods. Trips unexpectedly too... :-)

To me it sounds like the answer to your question is likely that it will overload, and you probably should have it on a separate circuit to the pool pump.

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20 minutes ago, Boeriemore said:

My electrical knowledge is limited but I must point out that my swimming pool pump is rated at 7.5KW yet never had an overload problem

on that circuit. Some enlightenment would be appreciated.

Standard pool pumps are 0.75KW / 750W.

7.5KW is quite big for a pool pump unless you have a olympic sized pool and only one pump.

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25 minutes ago, Boeriemore said:

I am busy installing a 7KW instant water heater. I assume that relates to 32 amps.

The circuit breaker it will be connected to is 30 amps. With other appliances also connected to the circuit will this result in an overload?

Also, the wiring cable  from the heater is thicker than the existing cable to the breaker. How can I confirm that the thin cable is OK or

needs to be replaced?  My electrical knowledge is limited but I must point out that my swimming pool pump is rated at 7.5KW yet never had an overload problem

on that circuit. Some enlightenment would be appreciated.

That is a serious load and will certainly trip the 30A breaker, without any other appliances connected at the same time - it might not trip immediately when the water heater is powered up, but it should trip within a short period of time.

Best would be to install another breaker with a higher over-current rating (35A or 40A) which will be used for the water heater only and also thicker cable all the way to and from the breaker all the way to the water heater - use at least the same size of cable as the cable currently used on the water heater, but preferably a size or two thicker if possible. 

Electrical cable has resistance and the thinner the cable the higher the resistance. The higher the resistance, the higher the voltage drop, losses and resulting heating effect of the cable.  It is always better to have the shortest possible length of cable and the thickest possible cable to have the least amount of losses over the cable.

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That is a serious load and will certainly trip the 30A breaker, without any other appliances connected at the same time - it might not trip immediately when the water heater is powered up, but it should trip within a short period of time.

Best would be to install another breaker with a higher over-current rating (35A or 40A) which will be used for the water heater only and also thicker cable all the way to and from the breaker all the way to the water heater - use at least the same size of cable as the cable currently used on the water heater, but preferably a size or two thicker if possible. 

Electrical cable has resistance and the thinner the cable the higher the resistance. The higher the resistance, the higher the voltage drop, losses and resulting heating effect of the cable.  It is always better to have the shortest possible length of cable and the thickest possible cable to have the least amount of losses over the cable.

I concur...

Sent from my SM-N900 using Tapatalk

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3 hours ago, plonkster said:

The wire is the more important part to get right. Breakers can be replaced easily.

Two kinds of breakers. Some uses a thermo-mechanism and others a magnetic mechanism. The thermo one can run over it's limit slight without tripping, but will eventually heat up enough to trip. We have such a circuit at work. Fused at 50A but runs up to 60 for short periods. Trips unexpectedly too... :-)

To me it sounds like the answer to your question is likely that it will overload, and you probably should have it on a separate circuit to the pool pump.

Sent from my GT-I9195 using Tapatalk

Let me clarify. My pool pump is not on the same circuit. It also shares the circuit with my son's room, TV, PC etc.

The heater would only be used for a short while  to fill a kitchen sink with hot water. I'm tempted to connect the existing cables and see the results.

UPDATE: My apologies. Pool pump 0.75KW NOT 7.5KW.

Edited by Boeriemore
Incorrect information
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You might still have an issue. But test it. The trip will go before a wire catch on fire.

That said. If your trip goes constantly the solution will not only be a larger trip. The wire will have to increase to match more than 30A. The only trip in my house that is more than 20A is the stove.

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13 minutes ago, edmundp said:

The only trip in my house that is more than 20A is the stove.

Same here. There's pretty much two gauges of wire in my DB board, the thinner one for the lights, and the thicker one for the plugs. As far as I know, the thick one is only rated for 25A or so. The electricians will know better. I still think it will be best that it has its own circuit. Of course that is going to cost...

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There are special regulations for geyser, and I asume the same would apply to instant hot water systems. For one, it has to be on its own circuit. Secondly it not only has to have a dedicated circuit braker in the distribution board, but also a two pole isolator switch within 1 meter of the appliance. If I remember correctly it also has to be protected with an earthleakage device ( residual current device) typically in the db. As for the wire size, I'll have to look it up, but it depends on the length of wire. The regulations specify the maximum voltage drop of 5% between the point of supply and the point of consuption. Thinner wire will have a bigger voltage drop per meter than thicker wire. The type of wire used also has an impact on the required thickness.  How long will the wire between your db and the instant water heater be? What type of wire are you using, surfix? Is it in a conduit or loose on the ceiling in the roof? Typically it would require a 6 sq mm conductor or more.

i would not recommend a breaker less than 32A in the db. Also remember that when an element is cold it will draw more current than when its hot. So even if the element is rated at 7kw it may use slightly more until it heats up ( this does not take too long though). 

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18 hours ago, edmundp said:

You might still have an issue. But test it. The trip will go before a wire catch on fire.

That said. If your trip goes constantly the solution will not only be a larger trip. The wire will have to increase to match more than 30A. The only trip in my house that is more than 20A is the stove.

Sent from my SM-N900 using Tapatalk

The cable wires from the heater are flat, 3.5 mm x 2 mm. Is there not a formula to calculate the wire diameter required?

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18 hours ago, Janma said:

There are special regulations for geyser, and I asume the same would apply to instant hot water systems. For one, it has to be on its own circuit. Secondly it not only has to have a dedicated circuit braker in the distribution board, but also a two pole isolator switch within 1 meter of the appliance. If I remember correctly it also has to be protected with an earthleakage device ( residual current device) typically in the db. As for the wire size, I'll have to look it up, but it depends on the length of wire. The regulations specify the maximum voltage drop of 5% between the point of supply and the point of consuption. Thinner wire will have a bigger voltage drop per meter than thicker wire. The type of wire used also has an impact on the required thickness.  How long will the wire between your db and the instant water heater be? What type of wire are you using, surfix? Is it in a conduit or loose on the ceiling in the roof? Typically it would require a 6 sq mm conductor or more.

i would not recommend a breaker less than 32A in the db. Also remember that when an element is cold it will draw more current than when its hot. So even if the element is rated at 7kw it may use slightly more until it heats up ( this does not take too long though). 

For many years I have had three Prestigo 5KW instant water heaters. Two share the same circuit, both for showering. As long as both showers are not used at the same time there are no problems. They are very small units and I managed by connecting them directly to the wall socket in a bedroom. I do not know if one could classify them as being "hot water systems" However, based on everything I have read in these posts maybe I should play safe and call in an electrician.

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There is a LOT of very clever guys and this forum.

If you feel you understand the advice and can act on it with comfort and knowledge, go for it.

If not, I would get a electrician in, exactly as you said Boeriemore. 

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17 hours ago, The Terrible Triplett said:

There is a LOT of very clever guys and this forum.

If you feel you understand the advice and can act on it with comfort and knowledge, go for it.

If not, I would get a electrician in, exactly as you said Boeriemore. 

I will do the dirty work first, drilling larger holes through the walls to fit the larger cable I will have to buy, also a two pole switch.

Basically I will get an electrician to came and inspect and correct any error I have made.

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I must ask if I was to restrict the temperature control on the unit to a low setting would it draw less amps thereby enabling the 30 amp breaker to cope?

I cannot simply install a 40 amp breaker as the  breaker is located in the house and the unit is in an outbuilding so the cable from the house must be buried under

paving, then into the outbuilding to its own  25 amp breaker. 

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Hi Boeriemore,

I would advise getting an electrician to have a look after you install it. (Or maybe chat to one before you start, it may save you time and money). Ask the electrician to give you a COC after the inspection. If somethings goes wrong and the insurance company find out it was due to an electrical fault and you do not have a COC they will not pay out.

About plugging in the 5KW water heater into a normal wall socket, its dangerous! The wall socket is only rated at 16Amps and a 5KW element will pull more than 20Amps.

If you are going to do the work yourself remember the following things (The list is a LOT longer but there will help)

  • A Circuit Breaker is there to protect the wire. In other words if the wire is rated to 15 Amps max, then you need a 15 Amp (or less) circuit breaker. This will prevent the wire from carrying more current than it should and will prevent it heating up/starting a fire.
  • The Earth Leakage  (Residual Current Device) is there to protect people. 
  • Isolators are there to switch circuits/appliances on or off. They do not do the same thing as Circuit Breakers (they will not trip).
  • Typical wall sockets are rated at 16 Amps and some standard plugs are only rated at 15 Amps. If you have an appliance that draws more current than that it would be best not to use a wall socket. Instead of a wall socket you can put in a two pole isolator switch and wire the appliance directly to it.
  • Be careful in a bathroom. There are a lot of rules/regulations around where and what you can and can't install in the bathroom. If unsure speak to an Electrician before you start installing something.
  • Run the wires in a conduit (except if its in the ceiling,but even so, there is no rule against using conduits in a ceiling).
  • If its metal, earth it! This includes metal conduits, junction boxes etc. etc.
  • If an appliance has a earth wire or a place to fix one, do it!

If you want some reference document regarding the size and current capacity of the different types of wire have a look at this link : http://www.aberdare.co.za/sites/default/aberdare_cables/files/brochures/649/brochures_649.pdf

For short lengths of cable you should be able to use the specifications in that document. For larger lengths the voltage drop play a part so go for one size bigger.

 

 

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1 hour ago, Boeriemore said:

I must ask if I was to restrict the temperature control on the unit to a low setting would it draw less amps thereby enabling the 30 amp breaker to cope?

It's unlikely to work, because water heaters are full-on or full-off. A lower heat setting merely shortens the time it's on, it does not reduce the peak current required. It's by far the simpler solution for temperature control, because water has such good thermal mass.

Actually went through a similar thought exercise: What if I wanted to run a geyser at half-power, or quarter power? It's a simple thermal element, so if you run it at 115V instead of 230V (for example), you'll get a quarter* of the usual power. My idea was to use surplus power on heating water instead of feeding it into the grid, but to limit the power to what was available, and do it smoothly without creating RF noise (ie no chopping the wave as a dimmer switch does).

The suggestion I got from electronics.stackexhange.com was to use a 230V->115VAC transformer. That is beautifully efficient and simple of course, but only works where the device is purely resistive, as is the case with a geyser.

And it remains a band-aid solution. I understand the resistance to pulling up the paving and installing thicker wires. I also know that sometimes you don't have to do that, if the line is straight enough, you simply attach the new wire to the old one, and pull it through, essentially using it as a fisher line.

* To the uninitiated, it usually sounds strange that half the voltage gives a quarter of the power, but it is ohms law really: Half the voltage means half the current, and P=VI, so power increases as the square of the voltage. Or, in math, because I=V/R, and P=VI, P=V^2/R.

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Thank you to Janma and plonkster. Most informative. The user manual supplied with the 7KW unit in question has been badly translated from Chinese.

The Technical parameters include "The lowest requirement of power meter - 20(40) amps" whatever that means. "Copper core area - 2.5 mm2

Based on my school days memories that equates to a wire of approx. 2.5 mm diameter. (22/7 x r2 /2 )

I now feel that I should remove this heater and buy another 5KW Prestigo "speed heat" as I have been using them for many years without overload problems.

(This 7KW heater cost me R16000 in a scam that was widely published in the local papers)

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Don't skimp on the wires. And don't skimp on the circuit breaker. Rather go a bit higher than what it needed, for two reasons: 1) Too thin cable can heat, melt and cause fire. 2) Too small CB will constantly trip and could actually cause damage in the long run. 

If you want to be complaint, then the unit should be connected to a dedicated dual pole CB, with an isolator close-by - the same as on stoves, air conditioners and geysers. 

7KW @ 220V = 31.81A. You can use a 35A CB and isolator, but that would be borderline. If that unit draws more current at startup, it might also trip. Lastly, you would need at least 4mm cable for this device. 4mm can handle 36A, with 11mV drop, per meter. i.e. on 30m it would have dropped 0.3V - not much. At 100m it would drop 1.1V. Remember, with lower volts you get higher Ampere. 

You will need to have your COC amended by a qualified electrician, or get a COC if you don't have one. 

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I have some info on Prestigo's Speed heat. The 3.5kw unit can be plugged into a wall socket and 20 degree water can be increased to 49 degrees max.

The 5.0kw unit must be connected to a 25 amp breaker and 20 degree water can be increased to 59 degrees max.

Food for thought. 

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@Boeriemore Have a look at the liter per minute that the unit can give you. The smaller units can heat the water but at a lower flow rate. If you use it for a shower it should be fine but if you use if for a bath it may take a lot longer to fill up.

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21 hours ago, Janma said:

@Boeriemore Have a look at the liter per minute that the unit can give you. The smaller units can heat the water but at a lower flow rate. If you use it for a shower it should be fine but if you use if for a bath it may take a lot longer to fill up.

I have always had one in my shower and one for a kitchen sink. The shower unit is now giving problems. I showered last night and using a thermometer I determined that 30 degrees was the max. temp. slowing down the flow resulted in the element switching off. There is probably a switch / thermostat that is faulty. I doubt that this part is replaceable.

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  • 2 years later...

Great insight guys!

 

I am installing 8Kw instant heater (Stiebel Eltron DHC8EL), which is rated at [email protected] (or [email protected]). If I calculate correctly, this means it will draw approx 26-27Amps.

 

If I installed a 30A CB using 4mm wire inside a conduit across 10m distance, with an isolator switch within 1m of the heater, would this be sufficient?

Your help would be greatly appreciated. 

 

Thanks

Quentin

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