Dealing with voltage drop

[Deleted member 307320]

Just had to run in a long temporary run for some Christmas decorations. According to my calcs, the voltage drop with the required load is going to exceed the permissible 5%, and it's not possible to change the cable.

However, the decorations themselves run from a switched mode PSU which will be happy with anything between 110v and 230v so functionally they'll be fine.

In this instance, can I deviate from the regs, assuming disconnection times can still be met?

Cheers!

 

[bernardgreen]

You also need to ensure the current in the cable does not exceed the maximum current for the size of the cable

 

[Deleted member 307320]

You also need to ensure the current in the cable does not exceed the maximum current for the size of the cable

We're good there, the load is only 8a so even if the voltage dropped by half we're still well within the capability of the cable

 

[ericmark]

As long as it does not cause a danger then no real problem. what we look at is disconnection times, so a B6 MCB needs to have 6 x 5 = 35 amp to trip the magnetic part of the trip, with RCD protection the line to earth is covered, so only line to neutral, however with my own house the UPS system is unlikely to produce enough amp to trip a MCB magnetic release, so really relying on the RCD to disconnect in the event of a fault or the inverter to close down.

 

[bernardgreen]

so even if the voltage dropped by half we're still well within the capability of the cable

If the voltage supplied to a switched mode PSU drops by half then the current taken by the switched mode PSU will double.

 

[Deleted member 307320]

If the voltage supplied to a switched mode PSU drops by half then the current taken by the switched mode PSU will double.

Yeah, I'm aware of that, as I said we're still well within the capability of the cable

 

[Murdochcat]

We're good there, the load is only 8a so even if the voltage dropped by half we're still well within the capability of the cable

8 amps of led lights - are you sure about that?

 

[AdrianUK]

Yeah, I'm aware of that, as I said we're still well within the capability of the cable

Yeah, but then it takes off..... If the low voltage causes the SMPS's to pull more current...... then the bigger current leads to a greater volt drop............

 

[Deleted member 307320]

Yeah, but then it takes off..... If the low voltage causes the SMPS's to pull more current...... then the bigger current leads to a greater volt drop............

Ah, yeah, I see what you mean...

 

[Deleted member 307320]

8 amps of led lights - are you sure about that?

That's the maximum theoretical draw (all on full white), in reality it's going to be far lower.

Should probably add that this is a shopping centre Christmas display!

 

[EFLImpudence]

If the lights will still work, then the so-called 5% VD limit is irrelevant and not mandatory - 3% for lighting anyway.

 

[plugwash]

Something to watch out for with SMPSUs and large volt drops is the possibility of a system collapse. I haven't seen this happen on a mains voltage system, but i've certainly heard of it happening on "ghetto POE" setups.

When the system is turned off the votlage in the primary caps of the SMPSUs will be effectively zero, when you turn it on they will start to charge. At some point the SMPSUs will reach their switch-on threshold and start drawing current.

If the SMPSUs discharge the capacitors faster than the supply can charge them, then you have a problem. This can happen even if the supply would be capable of supporting the loads in normal operation.

 

[Deleted member 307320]

If the lights will still work, then the so-called 5% VD limit is irrelevant and not mandatory - 3% for lighting anyway.

Yeah, I was thinking about the 5%/3% thing...

Why is the VD different for lighting anyway?

I went with the 5% as these are "decorations" which just happen to contain LED, rather than units with the sole purpose to providing illumination!

 

[Deleted member 307320]

If the lights will still work, then the so-called 5% VD limit is irrelevant and not mandatory - 3% for lighting anyway.

In which case, I think I'm all good!

 

[flameport]

Why is the VD different for lighting anyway?

Left over from the era of tungsten filament lamps where a small voltage drop caused a significant drop in brightness.
Mostly irrelevant with modern LED lighting.