Design current?

[Newtothis]

Hey new to the forum

And learning to be an electrician.

I have a few questions about voltage drop.

I understand the formula and all that

for example if I had 25 led lights at 24w each spaced evenly over a distance of 65m fed off a type c 10 amp mcb

would the design current (lb) used in the formula be 10 amps or would it be the 24watts x 12 = 288watts and in amps that is 1.25A

so would my formula us the breaker size of 10 amps or the load of the lighting circuit for voltage drop?

so if it was in 1.5mm it would be
mV/A/m x IB x L /1000
29 X 1.25 x 65 /1000 = 2.3 volts

or would it be
29 x 10 x 65/1000 =18.8 volts

 

[Newtothis]

Yes, these are for general circuits that can be loaded up to the MCB rating.

More importantly is to check the limit on typical Zs for fault clearing. That can be less than the VD limit due to T&E smaller CPC and/or the assumed Ze being significant (generally for high current MCB using C/D curves).
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You can rely on a RCBO tripping at high Zs, but it is not something I ever feel terribly happy with unless there is not a sane alternative.

Okay what if you run 3 core and earth from the switch to light led on the circuit as half of the lights were emergency lights. so you had a SL and a PL at every point? the onsite guide on shows two core and earth?

would it double your cable legnth?

would that change your volt drop or would it be the same?

 

[Lucien Nunes]

If the line current were shared equally between the SL and PL conductors, the VD on the line side would be halved due to the doubling of CSA. The drop in the neutral would be unchanged, therefore the total VD would reduce by 25%. But in most cases nearly all the current will still be in one core, so the reduction might be barely noticeable.

 

[dokkan1080]

If the line current were shared equally between the SL and PL conductors, the VD on the line side would be halved due to the doubling of CSA. The drop in the neutral would be unchanged, therefore the total VD would reduce by 25%. But in most cases nearly all the current will still be in one core, so the reduction might be barely noticeable.

The current is not shared at all.

From what I can work out is the switch live feeds all the lights and the permanent live feeds all the batteries.

So that would make the VD higher?

Or the current taken for the battery packs is tiny, but I dont know ? I'm just trying to understand how everything works.
( thanks so much to everyone here I feel like I am learning loads )

 

[Lucien Nunes]

So, if the lamps take most of the current through the SL and the chargers only a little through the PL, the worst case voltage drop (in the SL) will be only slightly less than if it carried the full current.

 

[Newtothis]

So, if the lamps take most of the current through the SL and the chargers only a little through the PL, the worst case voltage drop (in the SL) will be only slightly less than if it carried the full current.

So for my example circuit I calculated the VD from the origin of the circuit but then after the switch only the current drawn down the switched live...

But what about the current the permanent live would draw at every other light for its emergency fitting

Would that need to be calculated as well?

Is it like have more loads on a different leg of the line conductor

As each light would have a line earth permanent line and neutral.

But only lights with the emergency fitting would use the permanent line but share the neutral

So confused xD


(I used to work selling emergency fitting and now its bugging me)

 

[pc1966]

But what about the current the permanent live would draw at every other light for its emergency fitting

Would that need to be calculated as well?

In theory, yes.

In practice, no.

Normally the permanent live is likely to be a fraction of the normal service current so can be largely ignored. Typically lights take many more hours to charge then to discharge, and even then (emergancy light power) they are typically at much less than full design power.

 

[Lucien Nunes]

Also, as a rule, one does not need to look into every watt of load and metre of cable. Assess the load, assess the midpoint, if it looks like it's well within limits forget about it thereafter. Only if the cable is very long or very expensive, and the load known accurately, and the drop right on the borderline between being OK and needing the next cable size up, would it repay very detailed investigation. Most VD calcs take me about 10 seconds per circuit.

The kind of lighting loads where one really does have to account for every fitting are 400W high-bay SON floods in rows along a 200 metre warehouse. In this situation, VD is likely to be the determining factor. Assorted emergency fittings around an office, not so much.

 

[Newtothis]

Also, as a rule, one does not need to look into every watt of load and metre of cable. Assess the load, assess the midpoint, if it looks like it's well within limits forget about it thereafter. Only if the cable is very long or very expensive, and the load known accurately, and the drop right on the borderline between being OK and needing the next cable size up, would it repay very detailed investigation. Most VD calcs take me about 10 seconds per circuit.

The kind of lighting loads where one really does have to account for every fitting are 400W high-bay SON floods in rows along a 200 metre warehouse. In this situation, VD is likely to be the determining factor. Assorted emergency fittings around an office, not so much.

Thanks for the replies everyone, had covid related issues so i wasn't able to reply 0.o

a little off topic but it goes along with the design off a lighting circuit would this be an acceptable way to run a circuit ? using something like this?

Scolmore 4 POLE CLICK FLOW CONNECTOR CT202C

The Scolmore CT202C is a 4 Pole Click Flow Pull Apart Connector. The CT202C helps to increase the load handling to 20Amp in lighting installations.

www.superlecdirect.com

cable basket with click flow connectors? I have seen it done like that above a load of false ceilings.

sometimes with the click flow connectors and sometimes with the klik plugs