Why such large ga wires?

[aeviaanah]

I see many builds using 14 to 12ga wire for circuits that are drawing >1amp- these often include a few indicators, alarms, contactor coils etc. I am curious as to why they spec out such large wires on these circuits. Wouldn't 28-20 ga wire be safe for these lesser amperage circuits?

 

[doug293cz]

Yes, the finer gauges can be used for low amperage loads, but then you need to add fuses at the point where the wire size drops. Some folks may like the trade off of the harder to work with heavier wire if it allows them to avoid adding fuses.

Brew on

 

[aeviaanah]

Yes, the finer gauges can be used for low amperage loads, but then you need to add fuses at the point where the wire size drops. Some folks may like the trade off of the harder to work with heavier wire if it allows them to avoid adding fuses.

Brew on

Doug,
Nice to meet you. I’ve currently got two fuses in my panel, the hot bus (7amp) as well as the supply circuit (.5amp) to my units (DSPR-310). I chose 7amp because my total 110v circuit draws around 5amps with everything running. The units draw less than .5 amps in total, I added these in case of a surge. Tho I have a hunch these fuses don’t protect against what your explaining.

Are you stating that anytime a wire is reduced in size a fuse needs to be added? If you don’t mind, I’ll give a few hypothetical examples below to help me understand..

1. I use 14ga wire for my pump switch, I jumper hot line using 20ga to illuminate switch when pump is on, I should add a fuse here? Can you explain why?

2. I supply hot bus with 14ga wire, I run 20ga from bus to alarm circuit which includes alarm, switches etc. I should include a fuse here?

 

[doug293cz]

Doug,
Nice to meet you. I’ve currently got two fuses in my panel, the hot bus (7amp) as well as the supply circuit (.5amp) to my units (DSPR-310). I chose 7amp because my total 110v circuit draws around 5amps with everything running. The units draw less than .5 amps in total, I added these in case of a surge. Tho I have a hunch these fuses don’t protect against what your explaining.

Are you stating that anytime a wire is reduced in size a fuse needs to be added? If you don’t mind, I’ll give a few hypothetical examples below to help me understand..

1. I use 14ga wire for my pump switch, I jumper hot line using 20ga to illuminate switch when pump is on, I should add a fuse here? Can you explain why?

2. I supply hot bus with 14ga wire, I run 20ga from bus to alarm circuit which includes alarm, switches etc. I should include a fuse here?

Sometimes it's a judgement call. In the case of short runs to single devices that are inherently current limiting (like lamps or coils), I think there is a very low risk, so don't usually add fuses in this case. So, for your #1 I wouldn't use a fuse, but since #2 includes multiple devices, I would fuse that. You can minimize the number of fuses by using a secondary bus that is fused.

Brew on

 

[Sparkncode]

I used 0.75mm^2 wire for all the relatively low current lines in my control panel as its a cost effective and flexable wire size that csn be protected by a 6A breaker/fuse. To save on multiple wire purchases I brought a 100m roll of it in both black and red as i will use it in other projects. I did the 20A circuits using 2.5mm^2 as that is suitable and i brought a 30m roll of that.
Interestingly properly voltage rated cable at thinner sizes doesnt get much thinner in diameter at lower copper area due to the insulation requirment. Much of the cheap hookup wire sold at hobby stores isn't actually rated for 240Vac. Worth spend a little bit more for the good stuff. Mine has UL CE and some other ratings so i trust it.
I even used it for the PT100 wires in the box as its what i had and well insulated/damage resistant.

Sorry about meteic but 0.75mm^2 is about 7A eated and 2.5mm^2 is 20A in a cable snd more in air.

I definitly minimise high current capacity connected to thin wires so i used 2.5mm for my elrment voltage indication leds ( too long and moved when door opened to more prone to damage and a pain to fuse so just used the fat wire ) but for one other led it got a 1" link of the thinner wire as it was easier.

If a wire is connected to something that can fail short or can be damaged and short then it better be protected by the right fuse or breaker. I used 20A breakers and 6A breakers plus a 32A breaker for the whole panel. Probably overkill since its in a metal panel that should contain fure but better safe than sorry.

Since i have 230V single phase it makes some things easier protection wise like less breakers in some cases than a split phase system like the us 120/240V

 

[itsnotrequired]

sometimes the larger conductor size is driven by the terminations. for example, one manufacturer of class cc fuse blocks indicates that #18 awg is the smallest size conductor their terminal is rated for. class cc fuses are available in very small sizes, down to 0.2 amps but the terminal itself isn't rated for, say, a #22 awg conductor. that isn't to say a #22 awg conductor wouldn't 'work' with the terminal, it is more that the manufacturer hasn't evaluated their terminal with such a small conductor. there is a practical physical limit, where the conductor becomes so 'wimpy' that the terminal can't even hold it (or the conductor is damaged/broken when tightening the terminal).

as doug stated, it comes down to a judgement call. is it practical to install a bunch of decimal-amp fuses to protect every little light or device? probably not. and if you're running really small conductors, they are likely going to act like fuses if exposed to enough current, like in a short circuit.

 

[aeviaanah]

I just divided my hot bus in two. Supply to these is 12ga and I used a 7amp fuse. At the jumper where I transition from 12ga to 18ga, I used a .5 amp fuse. So now the entire circuit that uses 18ga wire is protected as opposed to just the circuit the units were on.

Thanks everyone for the help!