DMPPT450 Heating Wire

[Ken R]

Hi Dacian and others

Can you post some pictures of how you are connecting the heating wires to the DMPPT please.  I'm particularly interested in what is being used to protect the small gauge heating wire as it comes out of the slab and what is being used to combine them to a DMPPT output.

Ken

[Dacian Todea]

Ken,

In my case I have 10AWG PV cables from the DMPPT to a metal junction box where the 18AWG silicone heating wire get out of the concrete.  Each of those 18AWG wires are rated for 20 or 22A continues in free air and in my application they run at about 17 to 18A max and two of those are connected to a 10AWG wire so that will transport around 35A again not a problem.

Connections are done temporarily with lug type connectors as I monitor the progress over time but is two years since installed and had not seen any problem like increase in contact resistance due to thermal cycling.  I may actually have those connections soldered in the end will just need to try a connection like that to see how it performs for a few months before doing that. While there where no problem using this screw type connectors for the past two years I do not like this for long therm so will either use a spring type connection that will ensure long therm constant contact resistance or I will solder the connections that will probably be my favorite but needs to be tested for at least few months to see how it performs on thermal cycling and what is the max temperature of the soldered joint.

[Ken R]

A few pictures would be helpful.

I am wondering where you got the max current for the wire.  Most of the charts I find show considerably less.  Eg https://www.ebay.ca/itm/2-30AWG-Black-HIGH-TEMP-Super-Flexible-Silicone-Wire-Electronic-Cable-Tin-Copper/173836491940?hash=item2879765ca4:m:mi4swONDqOyugTIQOWTyNvA

Regards

Ken

[Dacian Todea]

Ken,

They just make that table at random the max current of a wire will depend on where it is installed free air or conduit with many other wires that need to be considered and the max temperature of the wire insulation in this case 200C for the silicone cable I have. Here is a table for Teflon and silicone wire last column for 200C rating in free air with 30C ambient AWG 18 can do 24A http://catalog.connectronicscorp.com/Asset/WIREMAX-conductor-CURRENT--2-.pdf

In my case the wire is embedded in concrete that is significantly better than free air ad wire at 18A is likely no more than 40 to 50C for the copper conductor.

A few not great quality photos that I have

The way I install a wire loop 

The wire loops covered with concrete needed anyway to level the floor

The 10AWG transmission wires they will also get covered by about 4 or 5cm of concrete there are also two CAT6 cables just in case I needed them for something :)

And one of the junction box temporary install. You can see two long about 80m wire loops each will use about 18A in parallel connected to a 10AWG PV type wire so that will carry max 36A  O and notice the diode to handle the inductive spike when current is stopped (I need to add this in the user manual).

Hope photos where of some help tho is just one solution out of almost infinite possibilities

[Ken R]

Once again, extremely helpfull.  The 10awg distribution cables are the same you use for the panels?  (not silicone)

[Dacian Todea]

Ken,

Yes the 10 AWG are the exact same I used for PV panels not silicone I think they are rated at 105C 

[Josh Peterson]

I will be using 2950 watts of solar and I planned on using 3 outputs of the dmppt with 3 1200 watt 36v water heater elements I planned on making splitting the elements to make them 600w each I will have it as 1 600 on one output then 2 600w on the next output and 3 600 w on the 3rd, will I need to add diodes to the elements?

[Dacian Todea]

Josh,

No you do not need diodes for those elements.  You will need to use TLD2 (600W), TLD3 (1200W) and TLD5 (1800W).    

[Josh Peterson]

Is it ok to put the 1800 watts on tld5 I thought the max would be 50 amps on that output

[Dacian Todea]

Josh,

Yes 1800W may be a bit much but you can put just 1200W there as the 1200 + 1200 + 600 is already 3000W the max your PV array can put out so it will make more sense.

[Ken R]

What are those charcoal/grey bars that you are using for spacing?  Does anybody else have other solutions?

Thanks a bunch

Ken

[Dacian Todea]

Ken,

If you are referring at the first photo then those are some plastic cable raceways similar to this https://www.digikey.ca/product-detail/en/phoenix-contact/3240193/277-5384-ND/2525022  not sure what was the exact model I got and inside to make them more stiff is a black painted 2x2 lumber.  Is just what I used as a solution there may be better methods.