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What wire size for my 3.5 kw speed heat?


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The info pamphlet states that this unit draws about 15 amps and requires a minimum of 2.5 mm2 cable.

I assume this refers to the  cross section area of the cable with the three copper wires within.

Surely the thickness of wire casings could vary so the individual wire diameter would be a better

and safer way of establishing what is required?  Help here would be most welcome.

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It will refer to the individual copper cables inside, not the casing. You can just ask for 2.5mm three core cabtyre, it sounds like that is what youre looking for, and then as always when dealing with high currents, try to keep the wire short.

Edit: i just checked what a speed heat is, and i wouldn't use cabtyre, from the diagrams it looks like normal single core 2.5mm. 

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15 hours ago, Weasel said:

It will refer to the individual copper cables inside, not the casing. You can just ask for 2.5mm three core cabtyre, it sounds like that is what youre looking for, and then as always when dealing with high currents, try to keep the wire short.

Edit: i just checked what a speed heat is, and i wouldn't use cabtyre, from the diagrams it looks like normal single core 2.5mm. 

The cable needs to be 8 meters in length and will be plugged into a wall socket. So is 2.5 mm the diameter of each wire?

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yes its for each wire. But ok, if you are plugging it into a wall socket then you could rather use cabtyre. the images i saw looked like it was more of a permanently installed thing into a DB. So as i said before then you can just ask for 2.5mm three core cabtyre. 2.5mm here also refers to each wire inside.

http://www.arb.co.za/2-5mm-x-3-core-cabtyre-black-coil-100m.html

for 8 meters you will be fine for 15A load using 2.5mm

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Prestigo's web site stipulates that their 5.0 kw unit requires to be connected to a 35 amp breaker but the 3.5 kw unit can connect to a wall socket.

Thank you for your input.

Edit: Checking online tables and a wire size calculator I get conflicting answers. The calculator worked out 16 AWG,

one table 10 AWG and another 8 AWG (based on 25-30 feet length)

 

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  • 2 weeks later...

A belated update. I bought two 5kw units and one 3.5 kw unit for a shower. Bought what was called 2.5 mm 3 core cabtyre for this unit..

I installed them all but the problem is that the cold winter water does not heat up enough for a warm shower. I now intend to swap by fitting

this unit by the kitchen sink and the 5kw unit in the shower. The information sheet states that should the distance between the 5kw unit and the power source be more than 4 meters then they recommend a 4mm2 cable. Same confusion. Is the thickness of each of the three core copper wire 4 mm in diameter which would make the 

total cable diameter huge?.  

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13 hours ago, Boeriemore said:

A belated update. I bought two 5kw units and one 3.5 kw unit for a shower. Bought what was called 2.5 mm 3 core cabtyre for this unit..

I installed them all but the problem is that the cold winter water does not heat up enough for a warm shower. I now intend to swap by fitting

this unit by the kitchen sink and the 5kw unit in the shower. The information sheet states that should the distance between the 5kw unit and the power source be more than 4 meters then they recommend a 4mm2 cable. Same confusion. Is the thickness of each of the three core copper wire 4 mm in diameter which would make the 

total cable diameter huge?.  

Yes, that would be 4mm2 per conductor. 

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AWG # Diameter
(mm)
Diameter
(inch)
Area
(mm2)
0000 (4/0) 11.6840 0.4600 107.2193
000 (3/0) 10.4049 0.4096 85.0288
00 (2/0) 9.2658 0.3648 67.4309
0 (1/0) 8.2515 0.3249 53.4751
1 7.3481 0.2893 42.4077
2 6.5437 0.2576 33.6308
3 5.8273 0.2294 26.6705
4 5.1894 0.2043 21.1506
5 4.6213 0.1819 16.7732
6 4.1154 0.1620 13.3018
7 3.6649 0.1443 10.5488
8 3.2636 0.1285 8.3656
9 2.9064 0.1144 6.6342
10 2.5882 0.1019 5.2612
11 2.3048 0.0907 4.1723
12 2.0525 0.0808 3.3088
13 1.8278 0.0720 2.6240
14 1.6277 0.0641 2.0809
15 1.4495 0.0571 1.6502
16 1.2908 0.0508 1.3087
17 1.1495 0.0453 1.0378
18 1.0237 0.0403 0.8230
19 0.9116 0.0359 0.6527
20 0.8118 0.0320 0.5176
21 0.7229 0.0285 0.4105
22 0.6438 0.0253 0.3255
23 0.5733 0.0226 0.2582
24 0.5106 0.0201 0.2047
25 0.4547 0.0179 0.1624
26 0.4049 0.0159 0.1288
27 0.3606 0.0142 0.1021
28 0.3211 0.0126 0.0810
29 0.2859 0.0113 0.0642
30 0.2546 0.0100 0.0509
31 0.2268 0.0089 0.0404
32 0.2019 0.0080 0.0320
33 0.1798 0.0071 0.0254
34 0.1601 0.0063 0.0201
35 0.1426 0.0056 0.0160
36 0.1270 0.0050 0.0127
37 0.1131 0.0045 0.0100
38 0.1007 0.0040 0.0080
39 0.0897 0.0035 0.0063
40 0.0799 0.0031 0.0050

 

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47 minutes ago, Boeriemore said:

home's earth leakage switch trips every time. 

Happens when the neutral wire touches ground. Also irritates the crap out of me but perfectly normal and necessary. Cause breaker does not disconnect the neutral line and touching it to ground does create another path for electrons to bypass the ELB.

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In my cast the neutral wire may have touched the earth wire but nothing else except my fingers. Strange that this only happens on this particular speed heat.

I have two others in the home and have never had this occur while working on the wiring from them.

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15 hours ago, Boeriemore said:

In my cast the neutral wire may have touched the earth wire but nothing else except my fingers. Strange that this only happens on this particular speed heat.

I have two others in the home and have never had this occur while working on the wiring from them.

One of 3 possibilities...

  1. Neutral and Earth never accidentally touched while you worked on it
  2. You have a double-pole isolator in the line to the speed-heat and it was switched off, either close to the speed-heat or in the DB feeding it - I'm not sure about the regulations and requirements for the double-pole isolator, but since you have to have one within (I think) 1m from a geyser, the same regulation maybe applies to a speed-heat.
  3. The earth wire running to the speed-heat is faulty / not connected at the other side (DB).
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Back to the need for 4 mm2 cable. Prestigo state that a 5 kw unit with a cable more than 4 meters "they recommend" 4mm2 cable.

I already bought a length of 2.5mm2 cable now its wasted and I have to buy the thicker cable.

However, I have checked the existing two 5 kw units on my property and they have been working for years with no problems and both have cables over 10 meters in length YET the one has 2.5mm2 cable and the thickness of the other is less than the thickness of the 2.5mm2 cable. 

So is there really a need to upgrade the cable????

 

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21 hours ago, Boeriemore said:

Back to the need for 4 mm2 cable. Prestigo state that a 5 kw unit with a cable more than 4 meters "they recommend" 4mm2 cable.

I already bought a length of 2.5mm2 cable now its wasted and I have to buy the thicker cable.

However, I have checked the existing two 5 kw units on my property and they have been working for years with no problems and both have cables over 10 meters in length YET the one has 2.5mm2 cable and the thickness of the other is less than the thickness of the 2.5mm2 cable. 

So is there really a need to upgrade the cable????

The thinner cable will have higher resistance. With higher resistance you will have a higher voltage drop over the cable. The higher resistance in the thinner cable will cause the cable to get warmer and because of the higher voltage drop on the thinner cable, the speed-heat will be less effective because some of the power goes to waste in the cable supplying the speed-heat. If the cable is too thin it might get so hot that the insulation melts, which will cause a short-circuit, which might cause a fire etc. etc.

By using the 2.5mm2 cable, it might not overheat, but your speed-heat will not be able to heat the water as quick as it would have using thicker cable. The incoming water temperature differs all the time, so you might not even notice the effect of the thinner cable. If you are OK with the power going to waste, it should be fine to use the thinner cable. I won't skimp on that - I prefer to over-compensate and would have used 6mm2 cable for that 5KW speed-heat, even if the distance between the DB and speed-heat was less than 4 meters, but that is just me.

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Thank you Superdiy, I have tested and can confirm that the thin cables do not get hot. The water pressure micro switches activates the elements and it does take a while to get fairly hot but not so long as to be an inconvenience. Your advice means I will have to remove the existing cable, presently secured to the wall and drill a larger hole through the wall as well as connecting it to the DB. 

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Heh. I can confirm that when you push 35amps DC through 4mm solar cable, it gets hot to the touch. I did that for a few days as a stop-gap measure until I could buy the correct cable, between my charge controller and the battery. The spec for the wire says up to 55 amps if the wires are in the open, but I can tell you it is probably not a good idea, even over a short distance. For the 15 amps in question here, 4mm should be more than adequate.

Looking at some wiring charts, we're talking 3.3 ohms per kilometer on 2.5mm wire (diameter). That's about 0.5V drop over ten meters, and 7.5W heat dissipated over the length of the wire. I think I'll agree with @superdiy, go with the thicker wire and sleep better.

Edit: If it's 8 meters away, ie 16 meter of wire (there and back to complete the circuit), then it's like double the figures. 15W heat over the 8m length, should be well dissipated I think, but I'd still sleep easier with the thicker cable.

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Boeriemore, I may be opening a can of worms here but what are the chances that from 18 years ago to today, that the wire quality may have changed?

I ask this because looking at extensions leads from 20 years back to the sh_t we get today in the shops, there is just no comparison, yet both are rated same amps.

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I phoned the manufacturers today. Spoke to a technician. When I told him my old speed heats worked fine with the thin cable he said that their elements have been upgraded to SABS specs and they recommend 4mm2 cable, All he said is that the element requires this. I failed to ask him why, what effect would the thinner cable have, would it result in the element not heating up to its full potential? Anyway, I have just bought 4 mm2 cable so one speed heat will be up to spec. but last week I replaced a faulty 5kw unit with a new one and this one is connected to 2.5mm2 cable and I am not going to replace it as part of the cable crosses the yard and is encased in concrete. 

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Well, you're going to drop about a volt over that distance, which is probably nothing to worry about. Also, you will dissipate 10-15W power over the length of the wire, which is also probably no problem seeing as it's encased in cold concrete, that is to say ample space to dissipate that bit of heat. So I doubt you'll burn anything down... :-)

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  • 4 months later...
On 2016/08/01 at 0:13 PM, Boeriemore said:

Somewhat off the topic but here it is. Connecting or disconnecting the unit's wires I obviously first switch off the circuit breaker but the

home's earth leakage switch trips every time. 

If the Earth Leakage is not the main switch it will have it's own separate neutral bar. Circuits feeding from it must connect to this neutral bar and not the main nb.

 

 

Sorry for the necroing.

 

 

 

 

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