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As a part of a project, I need to supply 24V DC to the terminals of a PTC type heater. The heater will be outdoors 50 meters far away from the 24V DC power supply as shown below: enter image description here Without a long 50 meter cable, I made some tests with the heater applying 24V DC to its terminals and found out at the beginning the heater draws around 2.8A and then gradually it settles to around 1.8A. This also means the PTC resistance starts at 8.5 Ohm and with temperature increase by the time it settles to around 13.3 Ohm. So the electrical power supplied by the power supply starts from 67 Watt and settles at 44 Watt. And temperature settles around 60°C.

Now I need to supply 24V at the heater terminals. But since the resistance of the PTC is very low even I use a 20 AWG wire I will have significant amount of voltage drop at the heater terminals if I use a 24V DC supply and a 50 meters of cable.

I don't know the reason but I have been told that this PTC must be used with 24V DC supply. So I could come up with three options:

1-) I can bring 230V AC by a 50 meters of cable to the heater and place the DC power supply(in an enclosure) just next to the heater. But this is not safe since it will be outdoors.

2-) I can calculate the voltage drop for a given AWG to obtain 24V DC at the heater side. Then I can obtain 24V DC even though there is voltage drop at the cables. But I might come up with something like 36.7V and it will be cable length dependent.

3-) I also consider a DC/DC converter such as this one. So I can place the DC/DC converter just next to the heater and be sure it is around 24V always.

My questions are:

1-) Which options above would be a better practice for this scenario? Or is there a better workaround?

2-) What could be the reason to supply 24V DC for a PTC? What if it receives 12V DC wouldn't it still comes up to 60°C eventually? Im asking because I dont know why I'm told to use 24V.

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2 Answers 2

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Version 1) I can bring 230V AC by a 50 meters of cable"

Even the distribution cables are outdoors, so it's not so difficult. You can use a armoured cable and use a GFCI safety device mounted in your house. In such way, if the live wire gets damaged it will make a contact with the shield and trip the GFCI.

EDIT:

enter image description here

Youtube link 1

Youtube link 2

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  • \$\begingroup\$ It wont be in house it will be somewhere on a bridge. \$\endgroup\$
    – user1999
    Sep 26, 2018 at 11:37
  • \$\begingroup\$ Can I use something like this uk.rs-online.com/web/p/rcds/8929485 outdoors? As you said an RCD. \$\endgroup\$
    – user1999
    Sep 26, 2018 at 11:40
  • \$\begingroup\$ Yes, but you can use a better RCD 30mA + Overcurrent all in one device = RCBO. You should use a current rating matched to the used cable, for example 10A @ 3 x 1.5mm^2. \$\endgroup\$ Sep 26, 2018 at 11:56
  • \$\begingroup\$ These may cost much more than DC-DC converter option. \$\endgroup\$
    – user1999
    Sep 26, 2018 at 11:58
  • \$\begingroup\$ @newage2000 It depends on how the cable is mounted/protected. You don't necessarly need an armoured cable. In both situiations you would need a RCD (if your installation doesn't already have it) and 50m of cable. \$\endgroup\$ Sep 26, 2018 at 12:08
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Just use heavier wire to carry the 24V power. AWG14 (8.1 mΩ/m) will give you about a 6% drop in voltage (at the steady-state current of 1.8A), and AWG12 (5.1 mΩ/m) will reduce that to less than 4% drop.


To answer your second question, supplying 12V to a 24V heater will result in only 1/4 the amount of heating power (P = V2/R), so it is very unlikely that it will reach anything like the same temperature. It would depend on how well insulated the thing you're heating is.

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