Sparkies: Electrical wiring conundrum

[Andy]

I built a control box to manage the heating of my HLT.

The box uses a STC1000 like device to control a 25A mechanical relay that switches a 3Kw immersion element in the HLT.

I've used 16A wire for the high-power path through the relay.

I opened the control box up tonight and found the live wires in/out of the relay to have burnt insulation and exposed inner copper

Is this because the 16A wire isn't up to carrying the amount of juice for a 3kW element over a prolonged period ?

Should I put 2.5mm wire in there instead ? Or something else ? Are the cable crimps causing a problem (perhaps under spec, getting very hot and causing the cable burn) ?

[Jim]

Were they housed in a confined space Andy? The rating of cable usually assumes that it will be run in a reasonably open space, and not bundled with other wiring either to allow heat to dissipate. (afaik)

Having said that, if the crimp connections aren't good, that kind of current would show it up.

[Jim]

Just another thought; the element might be rated 3kW at 220v. If your supply is closer to 240, the element will be drawing close to 15A.

[themadhippy]

more than likely a loose connection,make sure you use a decent crimper and preferable 6491 spec cable as both flex and solid core cable dont work so well,other option is to solder the wires to the terminals and do away with the crimps

[Andy]

All the connections are really tight and good so don't think it's anything loose.

[Andy]

Were they housed in a confined space Andy? The rating of cable usually assumes that it will be run in a reasonably open space, and not bundled with other wiring either to allow heat to dissipate. (afaik)

Having said that, if the crimp connections aren't good, that kind of current would show it up.

It wasn't a very tight space Jim, sure it was in an enclosure but a fairly spacious one and not much cabling.

[Andy]

You can see black marks on the blue crimp shielding so something has got very hot.

[Jim]

Now that I look closer at the pic, the worst burning has occurred close to the crimps, so TMH has likely got it right - sub-standard crimps.

[jaroporter]

is that 1,5mm wire there then? the location of the burning certainly suggests the crimps. what did you use to crimp it on? possible alternative to loose connection would be overcrimping and damaging the cable (can happen when stripping insulation too)..

[Andy]

Yes, 1.5mm cable. Crimp tool is a cheapo job.

Like this

[Fastline]

I do believe wrong crimps, blue crimps are for 2.5mm, red for 1.5mm

I would up the cable sizes to 2.5mm anyway give yourself a little more leeway

And buy a nice crimping tool give yourself half a chance

[themadhippy]

red 0 .5-1.5,blue 1.5=2.5 ,yellow 4-6mm,all depends on the cable type which fits best,i tend to use reds for flex upto 1.5mm and blues for stranded(6491 stuff) if its solid core then out comes the soldering iron,and yes get a decent crimper,those cheapos tend to twist

[Pinto]

As hippy says, you need quality ratchet crimpers to ensure a decent join when using crimps - those points of compression contact will carry a majority of the current so will be the site of any hotspots caused by poor contact.

http://www.screwfix.com/p/ratchet-crimp ... GwodDfsMQg

If done correctly, the regs consider a crimp joint to be considered a continuous wire. As an aside to this as well, consider buying your crimps from an electrical wholesaler as opposed from an automotive source - from experience, I know the ones sold to the auto industry are inferior in quality - they dont crimp as hard or hold as securely.

I'd also add that there's certainly no foul in over-rating your cable - personally I'd be using 2.5mm to carry anything over 13A draw as a safety exercise - and the cost difference will be negligable. Keep this in mind when you're making your repairs.

[braailus]

Hi Andy

I am not a sparkie but can tell you the following. The 3000 w element will draw in excess of 13 Amps (depending on your mains supply voltage). This will heat up the wire. In addition, if there is a bad connection anywhere, this would increase the heat in excess of what the wire can handle. If the wire is connected to the element, this will also add to the heat.

I would suggest that you change to 2,5 mm wire and use silicone insulated or some other heat resistant type of wire. Also double check and ensure that all connections are perfect. You do not want a bad connection when drawing high amperage. I am not sure what the current ratings are on the crimp connectors but I would lose them if I were you. You want as much copper connected all the way through the circuit as you can get. The more copper, the less the resistance.

I am not sure about the regulations where you are but 2,5mm (or more ) wire is used for heating elements and plug circuits where I am.

Good luck.

[themadhippy]

. The 3000 w element will draw in excess of 13 Amps (depending on your mains supply voltage

in the uk the declared voltage is 230v so 13.05A,realistically the uk voltage sits at 240v or more, so current consumption will be closer to 12.5A,whatever current it is 1.5mm will handle that sort of current day in day out without getting excessively hot,its rated current at 60c is 16A

and use silicone insulated or some other heat resistant type of wire.

why? the temperature inside the enclosure isnt going to get much above ambient

I am not sure what the current ratings are on the crimp connectors but I would lose them if I were you.

more than enough for the job,there used all over the place,and crimps are the most common method of terminating large cables,you wont find many screw terminals that can take a 500mm

I am not sure about the regulations where you are but 2,5mm (or more ) wire is used for heating elements and plug circuits where I am.

All depends on the type of element,no need for 2.5mm if its a 15w element and 2.5mm is a bit small for an 10kw water heater, normal 13A socket are wired in 2.5mm unless it pyro then you can drop down to 1.5.if its a 16A socket 1.5mm is allowed depending on the cable length,and if its a 5A socket then 1mm is good enough