Current ratings of wire

[John CutlerParticipant]

I have become a bit concerned about the robustness of my electrical wiring. Having researched a little, I am now looking to replace any aluminium-cored cables (telephone wire?) with copper-stranded wire. Silicon 7-core 20AWG (0.85mm core, gross dia 1.65mm) looks good for flexible layout power supply, is rated at 1.5A and is reasonably priced. Convention in the modellling world seems to be that single solid-core copper cable should be replaced as well but if this is in a fixed position and unlikely to flex, the risk of breakage seems minimal; domestic mains power cable is still single core.

Where I am confused is the wire to use for connecting to locomotive motors. I want to use the minimum diameter possible.

According to most of the (US) websites consulted, the maximum rating of 24AWG wire is 0.58A. This would cope with most of my motors but not a Mashima 1630 with maximum current load of 0.6A (per the EMGS Manual). Moreover the 24AWG wire is quite thick; a 0.51mm conductor core usually translates into a gross 1.3mm diameter.

Contrary to the above, one US engineering website states that the current rating for 24AWG copper wire with 7 cores (strands) is 1.4A! This suggests that I could use much thinner wire. I also possess some flexible copper-stranded wire surplus from a Viessmann electric coupling (used to connect an autotrailer to a Bachmann 64XX  for added pick-ups; supercaps yet to be fitted in the guard’s compartment!). This has a core of 0.4mm and (amazingly) a gross diameter of only 0.6mm. One suspects Viessmann would only use wire well within capacity requirements.

Please can anyone suggest the minimum AWG wire one can safely use for say up to 0.7A?

My immediate solution is to use the Viessmann wire until it runs out. Or is that a bad idea?

 

 

[Nick RidgwayParticipant]

Copper is not a perfect conductor. So the choice of wire is related to the current flowing and the length.

Cable has to survive undamaged until the circuit protective device operates, undamaged from mechanical issues, and with sufficient volts left at the far end to do something useful.

However, private modellers aren’t constrained by the content of BS7671 when building railways.

As thermal dissipation in the cable is hardly a relevant issue at the track voltages modellers use, then one faces the other two constraints.

So let’s pass 1 amp through a hypothetical cable and see what happens:

• If I were wiring the track on a garden railway then I would use minimum 2.5mm2 omnibus cable to get to the other end with 12VDC on the basis of its internal resistance and the consequent low voltage drop in that size cable at that sort of length. 4mm2 might be a better choice if a feedback controller were not in use. Individual droppers onto the track power omnibus cable might be smaller.
• If I were wiring a 4ft-long alcove layout then I would not go below 0.5mm2 on 12VDC on the basis of its mechanical strength – it might have to survive a cat pawing at it, for example (though not in my case).

 

• This reply was modified 2 years, 5 months ago by Nick Ridgway.

[Nick RidgwayParticipant]

The current rating of wire to the motor isn’t a huge issue, as the lengths are so short. Any voltage drop, and consequent heat dissipation, in the pickup wire is tiny in comparison to that dissipated from the motor itself.

Looking at short-circuit faults, the most likely place for this to take place is on the track where a locomotive inadvertently bridges a gap between two rails at different polarity. This particular short circuit current does not pass through the motor pickup wire. While track shorts can be prevented by appropriate signalling and interlocking, and I’ve done it, it remains the most likely place for it to happen.

Short circuits within the loco are rather less likely, and can be designed and constructed out, which means that, in the limit, the connection between pickup and motor can be the same size as the motor winding wire. Practically, it needs to be a bit larger than that to allow for movement during the  fitting of the chassis to the loco body. Stranded, insulated, wire will give more satisfactory performance than single-strand wire.

• This reply was modified 2 years, 4 months ago by Nick Ridgway.

[John CutlerParticipant]

Thanks for explaining that in layman’s terms, Nick.  I now feel a lot happier using the Viessmann electric coupling (for a 64xx and autotrailer) and wire.

I guess therefore that for loco (and coach!) wiring, 30AWG wrapping wire (0.2mm core diameter) is adequate?

 

 

 

[Nick RidgwayParticipant]

“5 amp fuse wire” is 35AWG.

American wire gauge – Wikipedia

• This reply was modified 2 years, 4 months ago by Nick Ridgway.

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