Discuss Voltage Drop Calculations, Potentially Stoopid Question? in the Electrical Wiring, Theories and Regulations area at ElectriciansForums.net

Welcome to ElectriciansForums.net - The American Electrical Advice Forum
Head straight to the main forums to chat by click here:   American Electrical Advice Forum

GBDamo

-
Esteemed
Arms
Reaction score
3,010
With reference to Voltage Drop calculations and Table F4(ii) OSG page 151.

When designing an installation the cable is selected, among other factors, so that the maximum operating temperature is not exceeded, in this case 6242Y T&E = 70C.

The table, titled "Voltage Drop (per ampere per meter) at a conductor operating temperature of 70C" then goes on list various installation reference methods.

So, bare in mind the table is has been composed based on the maximum working temperature of the cable.


The potentially Stoopid Question is -

Other than increased impedance due to the elevated temperature what relevance does the Ref. Method have when determining Voltage Drop.
 
only inasmuch as the reference method may affect the operating temp. of the cable.
 
Not a bad question at all :)
It's a copy of Table 4D1B - I read it but I don't get it either. If they really have used 70C as the temp and the cable is the same how can it be different mV/A/m ... Is this some mysterious 3 phase thing I wasn't taught? (or more likely forgot)
 
That's what is confusing.

The table is written based on the maximum operating temperature yet has different values based on Ref Method so does one of these take it over the maximum or using the other mean the cable is working under the maximum??

As I have 100% unshakable faith in the veracity of the contents of BS7671 and OSG I was lead to think I was somehow missing something obvious but preferably obscure.
 

As I have 100% unshakable faith in the veracity of the contents of BS7671 and OSG.


youse in a minority there my friend. not even the IET claim that. :oops:
 
I do not have the yellow OSG to hand but in the red and green versions the voltage drop is the same for all installation reference methods, the only change to the voltage drop is the change from single phase to three phase where the calculation is different.

The installation method should have no bearing on the voltage drop as the intent is always to limit the cable to 70°C for T&E.
Whether the outside of the cable is in insulation, plaster or mud will not affect the resistance (or impedance) of the copper cores so long as the temperature remains at 70°C.
 
I do not have the yellow OSG to hand but in the red and green versions the voltage drop is the same for all installation reference methods, the only change to the voltage drop is the change from single phase to three phase where the calculation is different.

The installation method should have no bearing on the voltage drop as the intent is always to limit the cable to 70°C for T&E.
Whether the outside of the cable is in insulation, plaster or mud will not affect the resistance (or impedance) of the copper cores so long as the temperature remains at 70°C.

As I thought but made me wonder if I was missing something obvious.

Cheers
 
... the only change to the voltage drop is the change from single phase to three phase where the calculation is different...

Can you give a steer? I must've slept through that un :(

Edit : I can see the factor they have used is sqrt(3) / 2 but struggling with why ...
 
Last edited:
Can you give a steer? I must've slept through that un :(

Edit : I can see the factor they have used is sqrt(3) / 2 but struggling with why ...
Had first drink and it's all so clear to me now (maybe). When converting from phase/neutral to 3phase voltage drop, a factor of 0.866 is used. This is sqrt(3) / 2. The factor is needed to convert the voltage drop equation from a leading 2 to a leading sqrt(3) as seen in the eqns in the pic below.
Calculation of voltage drop in steady load conditions - Electrical Installation Guide - http://www.electrical-installation.org/enwiki/Calculation_of_voltage_drop_in_steady_load_conditions

View attachment 35995
 
Had first drink and it's all so clear to me now (maybe). When converting from phase/neutral to 3phase voltage drop, a factor of 0.866 is used. This is sqrt(3) / 2. The factor is needed to convert the voltage drop equation from a leading 2 to a leading sqrt(3) as seen in the eqns in the pic below.
Calculation of voltage drop in steady load conditions - Electrical Installation Guide - http://www.electrical-installation.org/enwiki/Calculation_of_voltage_drop_in_steady_load_conditions

View attachment 35995

Hi, the 1.73 is the constant for line to line voltage at 120 degrees. So if you take the per meter impedance for a single core single phase circuit, you times the impedance by 2. Now for three phase you time by 1.73.

So to convert three phase mV/A/m to a single core per meter impedance we divide back by 1.73. If we want to convert three phase mV/A/m to two core single phase we divide by 0.866.

The 1.73 is derived by the fact the phase voltage leads the line voltage by 30 degrees.
 
In answer to my own question, use the correct table, 4D5 and not 4D1B, when looking at Voltage drop for T&E.
 
In answer to my own question, use the correct table, 4D5 and not 4D1B, when looking at Voltage drop for T&E.
Probably a good choice there:)!

Can you give a steer? I must've slept through that un :(
Edit : I can see the factor they have used is sqrt(3) / 2 but struggling with why ...

Had first drink and it's all so clear to me now (maybe). When converting from phase/neutral to 3phase voltage drop, a factor of 0.866 is used. This is sqrt(3) / 2. The factor is needed to convert the voltage drop equation from a leading 2 to a leading sqrt(3).
Yes I think you have the solution.
In single phase volt drop you are concerned with the resistance of the outgoing and return (line and neutral) cables, as these are the same diameter and carrying the same current you multiply the resistance of the single cable by 2.
If you have a balanced three phase load then the resistance of the outgoing and return paths is a complex equation and not a simple series resistance therefore you get a resistance for the circuit multiplied by √3.
 

Reply to Voltage Drop Calculations, Potentially Stoopid Question? in the Electrical Wiring, Theories and Regulations area at ElectriciansForums.net

OFFICIAL SPONSORS

Electrical Goods - Electrical Tools - Brand Names Electrician Courses Green Electrical Goods PCB Way Electrical Goods - Electrical Tools - Brand Names Pushfit Wire Connectors Electric Underfloor Heating Electrician Courses
These Official Forum Sponsors May Provide Discounts to Regular Forum Members - If you would like to sponsor us then CLICK HERE and post a thread with who you are, and we'll send you some stats etc
This website was designed, optimised and is hosted by Untold Media. Operating under the name Untold Media since 2001.
Back
Top
AdBlock Detected

We get it, advertisements are annoying!

Sure, ad-blocking software does a great job at blocking ads, but it also blocks useful features of our website. For the best site experience please disable your AdBlocker.

I've Disabled AdBlock