2.5mm on 12V running warm - normal?

[sparc]

I installed a replacement 300VA transformer, fitting a 2.5mm from secondary to junction box where it splits out, feeding 5 x 50w halogens.

Cable run between the transformer and junction box is less than 18 inches.

The casing of the transformer indicates 2.5mm - you'd struggle to get anything bigger in the secondary output terminals.

The secondary current is going to be 21A - and I noticed immediately the cable was a warm.

Is that normal? I'll appreciate your thoughts please.

 

[johnboy6083]

Bear in mind that PVC cable is designed to run at 70deg and XLPE at 90.

 

[Dunc]

Not surprised really. Clipped direct and working full tilt (operating temp of 70C) 2.5mm will take 27A so at 21 you'll certainly notice a rise in temperature.

 

[sparc]

thanks guys, that's re-assuring to hear.

I'd done the calc to get the current okay, but having only ever fitted one per lamp 60VA transformers so far the heating of the cable for a 250W load still came as a surprise!

 

[Marvo]

Maybe slap a clamp meter on it and check the lamps aren't drawing more current than expected. If the run is only short it might be worth splashing out a few extra pennies and use a piece of 4mm cable. If the secondary of the TX is fused and rated at 25A then you should get away with the 2.5mm....at least until someone decides to put in ceiling insulation.

 

[sparc]

Thanks Marvo - good points. I was showering the damn thing with sweat - must have been close to 40 up there - and it's funny what you forget to do.

I've heard people say it's better to go with multiple 60VA - one per lamp - than one big TX. I'd been tempted to do that here, but ended up doing straight like for like to save myself some time.

 

[Marvo]

I've heard people say it's better to go with multiple 60VA - one per lamp - than one big TX. I'd been tempted to do that here, but ended up doing straight like for like to save myself some time.

Where I am the regs forbid multiple ELV lamps on asingle transformer for exactly this reason. It's scary how many 400 & 500VA transformers used to get their secondary circuits wired with undersized cabling and mostly because 'electricians' can't get their heads around the fact that cable sizes follow the load current and the voltage is immaterial. Couple that with far higher ambient temperatures here and there was an epidemic of roof fires. The UK regs obviously still allow multiple luminaires on one transformer but I suspect your education standards are somewhat higher than here