Volt drop on outside lighting

[allybally]

I got called to a job this afternoon, it's outside lighting, about a 1000w of lamps on the whole circuit, and when trying to quote for it done a few calcs and the volts drop is massive, it's 24 volts, the circuit is run in 3 core 4mm swa, and is 300m in length, now I know that it's well over the allowed 3% allowed, but after reading into the regs a bit I've found a reg- 525.100 would this apply, the max measured Zs is 2.43 and is on a 10 amp rcbo, so given that disconnection times will be satisfied can I disregard the volt drop as safety is not affected?


many thanks in advance

 

[Silly Sausage]

How did you calculate the volt drop?
With the full 1000W @ 300m (crude), or with the load spread along the cable length (accurate).

 

[allybally]

Like I said quick calcs, even still well over, snd back to my question does that reg apply?

 

[HandySparks]

If this circuit exists, what exactly are you quoting for?

 

[Zebra Stripe]

If its failed on volt drop then its extremely unlikely the circuit will comply with max Zs, however it will depend on how you calculated volt drop…load all lumped at one end will not give the most accurate value as already stated. I believe if the manufacturers minimum voltages are met at most onerous position and Zs is ok, then you are good to go

 

[allybally]

I was called to sort an issue with the rcbo keep going down and to replace all lamps and ballasts if they need it, I just don't want to touch it if the volt drop is too high, I'll have to have another measure and go back to the drawing board, many thanks for now

 

[HandySparks]

I don't think that high VD would put me off the work particularly if, as Zebra says, its within the lighting manufacturer's spec.

As this circuit is on an RCBO, have you determined whether it's tripping on overcurrent or earth leakage?

 

[allybally]

There's a dead short on live-earth, the company normally have a maintenance team to do this but haven't for some reason, thanks

 

[davesparks]

You aren't clear about whether this is an existing circuit or a circuit you are quoting to install.

If existing then measure the voltage at the intake and the voltage at the end of the circuit when fully loaded and find out exactly what the volt drop is rather than doing rough calculations.

If this is a circuit you are quoting to install from scratch then do an accurate calculation and adjust your cable size to achieve the correct volt drop, not forgetting that you can start with a bigger cable and reduce the CSA of the more distant legs of the circuit if the distribution of the load allows.

 

[allybally]

It's existing, and I didn't think you could actually measure the volt drop with a tester for some reason

 

[HandySparks]

There's a dead short on live-earth,

So it's just a matter of locating the fault, which should be easy if, as you say, it's a dead short, and then doing the repair. Don't think I'd worry about the volt drop.

Yes, volt drop can be measured, although I guess it would best be done by measuring at the supply end and the load end at the same time, to avoid errors due to variations in supply voltage.

 

[allybally]

Finding the fault isn't an issue, just pretty interested in the design side as well that's all

 

[HandySparks]

Finding the fault isn't an issue, just pretty interested in the design side as well that's all

Fair enough. What design current(s) were you using for your VD calc? Is the power factor poor, as I was getting a lower VD (based of a PF of 1), than your figure?

 

[davesparks]

It's existing, and I didn't think you could actually measure the volt drop with a tester for some reason

Why not, it's just testing the voltage at the end of the circuit and subtracting it from the intake voltage.

 

[allybally]

Might have made a mistake!!! It's been a while, the ballasts on 21 lights have a PF of 0.36

 

[HandySparks]

Might have made a mistake!!! It's been a while, the ballasts on 21 lights have a PF of 0.36

Ah! If that was the PF for the whole circuit, you'd have some chance of it tripping the RCBO on overcurrent.

 

[allybally]

Right this is what I have, 3 x twin lamp posts, and 21 single lamp posts, all sodium, thanks

 

[HandySparks]

Right this is what I have, 3 x twin lamp posts, and 21 single lamp posts, all sodium, thanks

Ok, so what's the spec for each lamp (Watts, VA, PF)? Are they all the same or are there two different types?
Is the circuit in one run, or is it branched?
What are the distances from the origin to the lamps?

If it's complicated, you might need to do a diagram to convey the information.

 

[allybally]

HQL | Mercury vapor lamps for open and enclosed... | OSRAM

These are the lamps I'm intending to use for the lights, they are pretty much spread out along the 300 meters equally, roughly every 10-15 meters and it's one complete run apart from a 15 meter run from the 70 meter mark

thanks

 

[wozz]

If you are looking at the design side of things look at the thermal constraints at the far end.