Honest Views On Volt Drop?

[HappyHippyDad]

I have just been asked to provide power to a large garage/workshop for a friend. The workshop is approx 55m away from the main CU and there will be a further run of approx 15m for the lights from sub board to end point of lighting circuit. It will be on a 32A MCB in the main CU. measured voltage at the time was 247V

The transformer supplying the property is approx 100 - 200m away.

I dont have all my calculations to hand but that isn't really the point of the thread. The point is 'Do you stick rigidly to VD calculations and always base your cable size on these results?' (I'm assuming all other requirements are met, i.e Zs, acceptable reference method etc).

Basically lets say the VD works out to about 8V on the lights using 10mm SWA and 1.5mm T&E for the lights, this is above the 6.9V limit. I feel 10mm is acceptable, would you use 16mm? It just seems a bit crazy to oversize this cable in the really, really unlikely event VD will ever be a problem. I'd be greatful for some real case examples from the experienced guys and whether they always stick to the 3%/5%?

 

[Leesparkykent]

I have just been asked to provide power to a large garage/workshop for a friend. The workshop is approx 55m away from the main CU and there will be a further run of approx 15m for the lights from sub board to end point of lighting circuit. It will be on a 32A MCB in the main CU. measured voltage at the time was 247V

The transformer supplying the property is approx 100 - 200m away.

I dont have all my calculations to hand but that isn't really the point of the thread. The point is 'Do you stick rigidly to VD calculations and always base your cable size on these results?' (I'm assuming all other requirements are met, i.e Zs, acceptable reference method etc).

Basically lets say the VD works out to about 8V on the lights using 10mm SWA and 1.5mm T&E for the lights, this is above the 6.9V limit. I feel 10mm is acceptable, would you use 16mm? It just seems a bit crazy to oversize this cable in the really, really unlikely event VD will ever be a problem. I'd be greatful for some real case examples from the experienced guys and whether they always stick to the 3%/5%?

The scenario you've posted I also feel 10mm will be acceptable due to the fact volt drop will normally be calculated using the rated current of the protective device for the distribution circuit. The chances of the installation pulling any where near the rated current of the protective device are going to be slim when diversity and other factors are taken in to account. Also when calculating volt drop the tabulated mV/A/m values are based on a conductor operating temperature of 70~C so if the bigger cable has been selected due to EFLI values and the OCPD for the circuit is smaller than the maximum CCC for the installed cable then the conductor operating temp is going to be no where near 70~C so the actual volt drop is going to be less than what's been calculated.

 

[hightower]

Is VD based on your measured voltage or the nominal voltage? Because let's say you are allowed 6.9V on a lighting circuit, based on nominal voltage of 230V that's 223V you're allowed to measure at the end of the circuit. But if your actually incoming is 247V and you're above the 6.9V, let's say 10V - you're still gonna measure 237V at the end which would be higher than 223?

I'm not offering a solution, just using an example to ask the question if VD is to be measured on the nominal value? (I'm guessing it should be as who's to say the supply won't be altered to 230V in the future)

 

[Lucien Nunes]

Agreed with the above that the average submain load is likely to be much lower than the OCPD rating and VD proportionately less. Under normal running conditions you'll probably be well inside.

There are various reasons behind the low VD allowance for lighting. One is that filament lamps become very inefficient at reduced voltage - the light output falls off much more rapidly than the power consumption. Another is that some discharge lighting does not start reliably at reduced voltage. Lighting circuits often run at or near design load, unlike general power circuits, so are also significant contributors to energy losses.

Then there's the effect of the nature of the other loads, given that most of the lighting circuit's VD will be in the shared submain. Workshop loads can be a bit nasty - welders, induction motors starting - significant surges are common which can cause annoying flicker if allowed to modulate the lighting voltage.

So the situation you are trying to avoid is a bunch of fluorescents that won't start in the winter if you turn the heater on first, or that flash annoyingly when you start the circular saw. Interestingly, electronic LED drivers and PSUs are almost immune to most of the above. They are regulated so they work efficiently over the full voltage range, and prevent the LEDs flickering.

If the other loads in the garage are actually a couple of chargers and the 32A MCB is a matter of convenience for circuit rating, I don't think you'll have any issues with the 10mm²

 

[HappyHippyDad]

Cheers guys and thanks for the information on filament lamps/fluorescents etc Lucien.

I do find it very frustrating not being able to click 'thankyou'!!!

 

[HappyHippyDad]

Is VD based on your measured voltage or the nominal voltage? Because let's say you are allowed 6.9V on a lighting circuit, based on nominal voltage of 230V that's 223V you're allowed to measure at the end of the circuit. But if your actually incoming is 247V and you're above the 6.9V, let's say 10V - you're still gonna measure 237V at the end which would be higher than 223?

I'm not offering a solution, just using an example to ask the question if VD is to be measured on the nominal value? (I'm guessing it should be as who's to say the supply won't be altered to 230V in the future)

VD is based on nominal voltage. I know i've made a point of saying that the transformer is really close so I am actually taking measured voltage in to account but for design purposes you should use 230V.

It would be interesting to know the following though:

Voltage supply could be as low as 6% less than 230V = 217V. If you are able to have a VD of 6.9v (approx 7V) on lights does this mean lights are designed to work on voltages as low as 210v (217v - 7v)?

 

[ruston]

Most of the equipment and lamps you will use will have the voltage rating on them.
IE 220 -240...

 

[Richard Burns]

The volt drop limits do state that if there is a product standard for the equipment then this can be followed and I would say from the voltage inputs for many LED lights being 85-265V this would constitute allowing a voltage drop (from nominal) of over 140V! I also agree with LeeSparkyKent and Lucien who have much better approaches.

 

[dmxtothemax]

you really need to base this decision on what the customer wants,
what he's prepared to pay, and what the loads are going to be
So if the loads are fairly tolerant of voltage variation then go for it.
But if they are not, well then you need to go bigger.
You really need to be able to determine what the customer expects from you.
If your not sure, give him the options and let him decide.
otherwise it's best practice all the way !