Volt drop in the real world

[TJC1]

Has anyone ever had a volt drop issue?

Just for fun.......... yes i said fun, i cant believe i would be saying that in my mid 20s

But i did a calc for a radial circuit in in swa

B32 mcb
4.0mm 3core swa
35m run

(mV/A/m)

So...... 11X32X30/1000 = 10.56

Way over the 5% in table 4Ab

Am i missing something? Would the design current me lower as there is no way the circuit would pull 32a

 

[Deleted member 105166]

as there is no way the circuit would pull 32a

In that case try the calcs with a smaller CPD

 

[Andy78]

230 x 0.05 = 11.5V
so 10.56V would be under that and permissible for a 5% tolerance.

 

[EricMark]

Yes four marked faults caused by volt drop.

1) Algeria around 1982 in Algiers, we had a three phase 110 volt to neutral supply, and an air conditioning unit normally used in the 220 volt temporary camps, so 190 volt phase to phase, and the overload built into the compressor would regularly burn out as it stalled so often, even today most fridge/freezers say on the instructions not to use an extension lead as unless inverter control still same problem today.
2) A radio was transmitting mains hum as the voltage dropped to low for the power supply to smooth the output, this was in a caravan site.
3) A shrink wrap machine would not work correctly again because of volt drop.
4) A string of fluorescent lights and last 5 lights would not strike.

In the main the switch mode power supply has removed the volt drop problem, however we don't now when doing an installation what equipment will be used.

So...... 11X32X30/1000 = 10.56

Not quite if you correct the mV/A/m then 11.5935070873342 I put in the whole figure because it's so daft, when we can only measure the loop impedance to 2 decimal places, it is so easy to make mistakes with calculations I use a Java program, and I get the volt drop to 13 volts, but to measure we would measure loop impedance at incomer say 0.35 ohm then at end of run say 0.756 ohm now both can be out by 0.01 ohm so keeping rest of figures static, so 0.34 and 0.77 = shows 37 meters used and 0.36 and 0.74 shows 34 meters used. With 13.76 and 12.16 volt drop so in real terms we must allow for both errors on our measurement and the person passing the installation in the first place so it would need to be out by 4 volts before we could say an installation error has been made.

I have used installation method 100 never bothered changing it, but again if a 70 deg C or 90 deg C cable it will also change the results.

And a B32 MCB does not mean the design current Ib is 32 amp, it is normally considered with a ring final with a B32 that Ib is 26 amp.

It was my worry that if I did a PIR (now EICR) and I failed to spot excessive volt drop then some one could claim from me, however once one has done the calculations it becomes apparent you can't be 100% sure anyway, so unless wildly out, may be best to say nothing!

 

[davesparks]

I'm sure we've all seen volt drop on sites with long 110V leads everywhere, everybody's drill slowing down and the lights dimming.

 

[TJC1]

230 x 0.05 = 11.5V
so 10.56V would be under that and permissible for a 5% tolerance.

Why the 230/0.05?

 

[Deleted member 105166]

Why the 230/0.05?

It's 230 x 0.05, not 230/0.05
to get 5% of nominal line voltage 230V

 

[davesparks]

Why the 230/0.05?

It's 230x0.05 which gives you 5% of 230

 

[TJC1]

If im using a 32a mcb is that the desing current? Or should i use the realistic load of the circuit?

 

[Andy78]

Why the 230/0.05?

As above
Or you could say that 10.56/230 = 0.04591 = 4.6%

If im using a 32a mcb is that the desing current? Or should i use the realistic load of the circuit?

Yes you should use the design current in the volt drop calc which is Ib. The OCPD rating is In which should be equal or greater than Ib.

 

[Deleted member 105166]

Iz ≥ In ≥ Ib

 

[Dustydazzler]

Run a 50M extension lead to the bottom of the garden and plug in a kettle full of water and see how long it takes to boil...vs plugging directly into the kitchen socket

This was one way I was actually physically shown as apprentice how volt drop affects a circuit

 

[haptism]

Just read this in "The Electricians Guide, 17th Edition" by John Whitfield, which may be of interest (page 79).

"If the actual current carried by the cable (the design current) is less than the rated value, the cable will not become as warm as the calculations used to produce the volt drop tables have assumed. The regulations include (in [appendix 4]) a very complicated formula to be applied to cables of cross sectional area 16mm^2 and less which may show that the actual volt drop is less than that obtained from the tables. This possibility is again seldom of interest to the electrician, and is not considered here."

Im not sure what this formula is (im not looking tonight) but it could be useful for peace of mind if for example a 10mm was just out of limits with the standard tables !

 

[D Skelton]

In my industry (railway) voltage drop is one of the largest problems we have to contend with.

Always remember that as a designer, the appendices (apart from 1) are informative, not normative. You are not obliged to follow the advice within. You'd be rather silly not to for every day work, but there are many circumstances where it's advantageous not to because it can often lead to significant inaccuracies. It's also worth remembering that the most important factor when determining suitable voltage drop isn't some arbitrary value in a book but the actual voltage tolerance of the equipment you're supplying. A closer look at the requirements set out in 525 will show you that the table in appendix 4 is ultimately there to aid calculation for worst case scenario.

One of the largest factors that affects voltage drop in my industry for example is cable operating temperatures. We spend a lot of time debating and correcting based on average weather predictions. Imagine for a moment a mile long length of black cable and how its resistance fluctuates based on whether it's been baking in the sun for a week or whether it's exposed to regular frosty mornings. Add to that the wildly fluctuating loads depending on time of year and the drop will fluctuate between the odd volt here and there to the odd hundred volts here and there.

We deal with this in a number of ways, chief among which is the use of step up transformers and automatically reconfigurable feeder switching arrangements. We also use equipment with very high voltage tolerance bands.

Essentially I can summarise my whole reply by saying that yes, voltage drop has real world consequences.

 

[Deleted member 105166]

Essentially I can summarise my whole reply by saying that yes, voltage drop has real world consequences.

There is an upside to this. Within temporary power scenarios, particularly smaller, lower budget events (and some not so low budget marquee events) it can be a fight to convince a client that it is simply NOT acceptable to let them 'daisy chain' diy shed extension reels over unlimited distances to export DNO power from a building, instead of hiring a generator and distribution.

Voltage drop is the one aspect that will noticably impact straight away, whereas high Zs (and associated inability of CPDs to provide ADS within the required time) and the issue of exporting a TN supply outside of its equipotential zone are, to a layman, not considered or understood.