9 replies to this topic

[wednesdayite]

Ok I've got the heater and tried wiring it up to no avail. I connected the red to the fuse box and earthed to the car body it worked fine. but when I connected a toggle switch the live wire went up in smoke. ( no damage to the car or heater ). so can someone tell me how to wire it up please. do I need an inline fuse. also can you tell me in lay terms please. many thanks.

[stuart bowes]

just a fuse alone won't stop the problem because it'll either blow, and the heater will do nothing, or it will allow enough current, in which case you're smoking again if the wiring is insufficient

 

what's the wattage of the heater,  what's the current rating of the switch and wiring - this one for example states 50a continuous current which is probably far in excess of most 12v toggle switches you will find  https://www.t7design...35-7cf-593.html

 

600 divided by 12 = 50 .  so take whatever wattage yours is, divide by 12 and you get the answer to what amps you need, get a relay that exceeds that rating https://www.12voltpl...ay-12v-70a.html

 

and then wire the main supply side with wiring of appropriate current capacity via a fuse carrier with an appropriately sized fuse in it

 

the switch (trigger) side can then be through any 12v switch you like of much lower rating and current capacity, as well as the wiring

Edited by stuart bowes, 08 May 2024 - 11:07 AM.

[mini-mad-mark]

A heater that draws 50amps will need fairly heavy cable and as mentioned above you would be best to use a heavy duty relay as many switches are not able to handle that sort of current

 

I don't think any of the standard mini fuses can handle 50A so I'm surprised you didn't blow a fuse when you connected it directly to the fuse box (unless you connected it to the input/unfused side?)

 

If you are not sure how to do this I would suggest you seek professional help, electrical fires can destroy cars

[gav]

Is it an electrical heater or does it have a matrix? The typical race car heaters are very low current as it's basically a PC fan blowing over a matrix, more info needed...

[Shooter63]

The T7 heater as supplied by MS draws about 3 amps and can be controlled by a Mk1 type rheostat heater switch, it sounds like a duff switch to me.or I wired it up:)

Shooter

[stuart bowes]

we have to be careful of stating actual ratings without any info, as stated above fires destroy cars

 

always use the rule of wattage divided by volts to work out amps based on actual product spec

 

that's not to say you're wrong or anything. I still don't know what unit he's using ...  are we talking about a fan in a box that blows through a matrix,  or a unit with a built in heating element

 

need more info

 

 

edit: apologies to Gav who basically just said exactly the same

Edited by stuart bowes, 08 May 2024 - 02:02 PM.

[alpder]

50A load would want a cable of minimum 6mm-sq area. Bigger would be better to give a margin... but the bigger it is the harder it gets to route and to fit the terminals, so I'd probably stick to 6mm. Both the power and the earth cables need to be this size. And the earth cable needs a really good connection to 'earth' (i.e. chassis/body/engine) - not just a self-tapper into the shell.

 

I'd probably take the 12V supply from the starter's terminal (the one connected direct to the battery by a large battery cable). And I'd run the earth to a bolt on the engine, using a beefy eye on the cable.

 

A fuse is essential. The best location would be very close to the start of the 12V supply cable (i.e. very close to the starter's terminal). A 50A Maxi in an inline holder would do the job. Like all fuses, it's only guaranteed to blow at a current much higher than the rated. A 50A Maxi it is guaranteed to let go at no more than 100A. 6mm cable should stand 100A for long enough to pop the fuse. But there is a possible situation where a partial failure of the heater causes it to draw (say) 75A... which is more than the cable is safe for but less than enough to guarantee to blow the fuse... potentially causing overheat and fire. That's why heavier cables than minimum spec are preferred, where practical. You have to decide if such a partial failure is a realistic risk, and choose cable appropriately.

 

A relay (I'd want at least 80A rating, because inrush current at startup will be more than 50A and you really don't want the contacts welding themselves together) will be needed, fitted in the 12V cable at some convenient point.

 

The relay will need one side of its coil earthed, and the other side of the coil connected to a 12V source via a switch. The 12V source would ideally be the aux power from the ignition switch - so it turns the heater off automatically when ignition is off even if the switch is left on. And the 12V source should be a fused one... and ideally not shared with an essential fuse. You wouldn't want (say) the car's ignition or lights to shut down just because the heater circuit has blown the fuse.

 

The hardest job will be to fit terminals to the 6mmsq cable. They'll either need soldering and heat-shrinking, or crimping with the proper tool. A "proper" crimper for 6mmsq terminals isn't a cheap item, so chances are that soldering is your best bet unless you can do 90% of the work and then get an auto electrician to make the final crimped connections for you.

 

vehiclewiringproducts.co.uk is a good place to start looking at cables and fittings... though often the identical stuff can be had from ebay for less.

[sonscar]

I would probably try and send it back and buy a water matrix heater.The figures bandied about sound like major electrical work possibly needing updated alternator and associated wiring.Just me thinking why not able advantage of all the free heat from the motor?Steve..

[stuart bowes]

big lugs are a bit of a pain, to be honest I usually just 'crimp' them with repeated hits with the club hammer  and then a bit of heat shrink to make them look nice

 

I know that's not exactly the professional approach but I haven't come a cropper yet with various heavy duty sound system installs over the years :)  obviously I give them a good tug to check before committing to installation

 

we have the right tool at work but I can never remember to take it home when I need it 

Edited by stuart bowes, 08 May 2024 - 03:54 PM.

[alpder]

The other problem with a 50A electrical heater is the draw it takes from the alternator. Alternators vary but rated output would be between 45A and 70A, typically.

On a wet dull day, with a 50A heater keeping the mist off the screen, I'd bet that the wipers (10A) will go slower and the headlamps (another 10A) will glimmer more dimly. And - maybe - the battery will go flat as the alternator fails to keep up with demand.

 

I've tried a bigger alternator (90A) on a single-V-belt drive (not on an A-series) but at low RPM the belt couldn't cope - slips and squeals at the slightest hint of damp in the air.

 

Also, bear in mind that 600W isn't exactly stacks of heat. A water-matrix heater will give shedloads more.

 

If demisting is the priority then maybe try an "invisible"* wire heated screen. Easy demisting in all weather. And only 17A draw.

 

And, maybe the most persuasive argument against electrical heaters (apart from the fact they don't make much heat, are tricksy to fit, don't give you any emergency option to ditch engine heat into the cabin if overheating in traffic, and might set fire to your car)... is that 600W, after efficiency losses in the belt/alternator, requires roughly 1HP to make it. Even if your car is replete with HP, that's still a significant hit on fuel consumption. So the car won't get as far on a tank. And it'll take longer to get there.

 

*Mine says it is. But it's not. In fact, at night especially, it's pretty damn visible. Even so, I wouldn't be without it.

Edited by alpder, 10 May 2024 - 10:44 AM.