Vd = I * L * Vc / 1000
5033 4.3.6.1 says to use 3008 for current carrying capacity but I'm just wondering if we can use the same for Vd
Basically I just want to know what table to use to obtain Vc values for DC runs
DC Voltage Drop
Re: DC Voltage Drop
3008 currently does not include any d.c. values.
For CCC, the 50 Hz values will be close enough, and the citation in 5033 provides bum-cover to ignore any differences.
There are d.c tables widely available, but rather than using Vc values that allow us to calculate actual VD; they generally tabulate max run vs current to provide the standard size of cable that will not exceed a stated amount of VD.
So a different table for each VD limit, and for each nominal voltage.
Typically they are available for 12 V & 24 V; and the VD limits generally used are 3 % for voltage-sensitive equipment1 (eg winches) and 10 % for general loads.
Also the distance axis of these tables is typically not "route length" but total circuit length, ie there & back again (via any switching detours)
Just to complicate matters further, many use AWG cable sizes (as commonly used for automotive electrical work)
But FirstFlex have one that uses CSA in mm2; they also provide d.c. resistance data (ohms per km)
www.firstflex.co.nz/information/technical-information/
The tabulated format for VD works OK for single circuits at the nominated voltages; but difficult for composite circuits where different sizes of cable are used for various parts and no use of you happen to be using a different nominal voltage.
So if dealing with those, need to back-calculate the Vc values from the data given - and the results will be close to the single-phase 50 Hz values in "3008".
I've only found one source to date that directly gives Vc values for d.c. circuits, in a UK book on wiring for boats; and the values are all a bit below those for single-phase derived from Table 42 in "3008.1.2" .
While single-phase values for mV/A/m derived from 3008 will be close enough (in fact conservative); what's missing is the limit for max VD.
In the absence of any specific rule for d.c. circuits; clause 3.6 applies; so max = 5 %.
As above, auto-sparkies generally work on 3 %. Noting that when you start with only 12 V, 3 % is SFA.
And that makes the variations in resistance (and therefore Vc) for temperature more significant.
If you regard solar power as "free" energy, maybe losses in the d.c. cabling can be disregarded compared to the cost of reducing losses by using bigger cables. But volt drop is essentially using the harvested energy to heat the environment, when it would be better used to do what we want it to do.
Due to d.c being increasingly used in installations, d.c values are being added to "3008" for next edition.
For CCC, the 50 Hz values will be close enough, and the citation in 5033 provides bum-cover to ignore any differences.
There are d.c tables widely available, but rather than using Vc values that allow us to calculate actual VD; they generally tabulate max run vs current to provide the standard size of cable that will not exceed a stated amount of VD.
So a different table for each VD limit, and for each nominal voltage.
Typically they are available for 12 V & 24 V; and the VD limits generally used are 3 % for voltage-sensitive equipment1 (eg winches) and 10 % for general loads.
Also the distance axis of these tables is typically not "route length" but total circuit length, ie there & back again (via any switching detours)
Just to complicate matters further, many use AWG cable sizes (as commonly used for automotive electrical work)
But FirstFlex have one that uses CSA in mm2; they also provide d.c. resistance data (ohms per km)
www.firstflex.co.nz/information/technical-information/
The tabulated format for VD works OK for single circuits at the nominated voltages; but difficult for composite circuits where different sizes of cable are used for various parts and no use of you happen to be using a different nominal voltage.
So if dealing with those, need to back-calculate the Vc values from the data given - and the results will be close to the single-phase 50 Hz values in "3008".
I've only found one source to date that directly gives Vc values for d.c. circuits, in a UK book on wiring for boats; and the values are all a bit below those for single-phase derived from Table 42 in "3008.1.2" .
While single-phase values for mV/A/m derived from 3008 will be close enough (in fact conservative); what's missing is the limit for max VD.
In the absence of any specific rule for d.c. circuits; clause 3.6 applies; so max = 5 %.
As above, auto-sparkies generally work on 3 %. Noting that when you start with only 12 V, 3 % is SFA.
And that makes the variations in resistance (and therefore Vc) for temperature more significant.
If you regard solar power as "free" energy, maybe losses in the d.c. cabling can be disregarded compared to the cost of reducing losses by using bigger cables. But volt drop is essentially using the harvested energy to heat the environment, when it would be better used to do what we want it to do.
Due to d.c being increasingly used in installations, d.c values are being added to "3008" for next edition.
Re: DC Voltage Drop
I had more of a look through 3008 and actually saw that 4.3.1 is probably of better use, replacing Zc with Rc due to DC having no reactance
I'm assuming I'd use 4.3.2 for a two wire supply and probably Vd = I * L * (2Rc) / 1000
And table 37 (with the note stating a 1.01 factor for tinned)
Does this sound correct?
My next question is this gives me Vd in volts but what value of V should I use when I have a DC Array which voltage fluctuates? The highest? The lowest? Average?
I'm assuming I'd use 4.3.2 for a two wire supply and probably Vd = I * L * (2Rc) / 1000
And table 37 (with the note stating a 1.01 factor for tinned)
Does this sound correct?
My next question is this gives me Vd in volts but what value of V should I use when I have a DC Array which voltage fluctuates? The highest? The lowest? Average?
Re: DC Voltage Drop
Always design / plan around worst case.
For VD on a PV array; you don't have a known load current nor a known voltage .
so to work out max VD; I'd use Isc as max current, and Voc as the nominal voltage ( rather than a lower figure based on an assumed load effect of the PCE). Then apply factor for cable temp based on cable temp at max current on hottest day.
Divide VD max I max, then divide result by cable route distance to get max Vc .
Then for cable selection; once I know my max Vc, I'd probably just use Vc values for nearest conductor type; eg table 41 for single-core Cu; adjusted for single phase, and maybe the factor for tinned copper. Noting that the values for the smaller sizes are the same or closely similar across multiple tables.
Much easier to select cable size from the Vc values than to do a bunch of calculations for various sizes
I already have a spreadsheet that converts the 3-phase Vc values to 1-phase; would just need to produce another version to apply the tinned copper factor as well.
For VD on a PV array; you don't have a known load current nor a known voltage .
so to work out max VD; I'd use Isc as max current, and Voc as the nominal voltage ( rather than a lower figure based on an assumed load effect of the PCE). Then apply factor for cable temp based on cable temp at max current on hottest day.
Divide VD max I max, then divide result by cable route distance to get max Vc .
Then for cable selection; once I know my max Vc, I'd probably just use Vc values for nearest conductor type; eg table 41 for single-core Cu; adjusted for single phase, and maybe the factor for tinned copper. Noting that the values for the smaller sizes are the same or closely similar across multiple tables.
Much easier to select cable size from the Vc values than to do a bunch of calculations for various sizes
I already have a spreadsheet that converts the 3-phase Vc values to 1-phase; would just need to produce another version to apply the tinned copper factor as well.
Re: DC Voltage Drop
If I'm using worst case scenario shouldn't the voltage I use to calculate my allowed Vd be the lowest voltage that could be present and not Voc? So that it would give me the smallest value of Vd allowed for such array?
Re: DC Voltage Drop
True basing volt drop on a lower voltage will reduce the max permitted VD; but you won't have the lowest voltage output at same time as max current, and it's current, along with distance, that's the biggest influence producing volt drop.
Having PCE connected will mean actual voltage at Imax will be a bit lower; but not easy to calculate what it will be. A least with using Vmax we have a known figure to work with.
For normal installation VD calculation, we use nominal voltage (as per clause 3.6 of "3000") not actual voltage (as supplied by the network within a permitted range).
The nominal voltage of a PV array is Voc, modified for temp. That's the condition that will generate Imax.
looking again at the clause in "3000"; I'm don't think it sets a requirement for VD in PV array cables.
VD is specified only for parts of the installation between "point of supply" and "any other point in the installation".
Point of supply is a defined term; though the definition [1.4.75] is over-ridden by the definition in Electricity Act.
Yes the array output is a point in the installation; but since current doesn't flow from PoS to the array, and there's PCE between the two;
the array cable can't be considered to be within that description - so there may well be no requirement .
Regardless, in "5033", 4.3.6.1 requires VD to be considered in selecting the cable; and clearly VD in the array cable should be kept as low as practicable.
The committee who wrote "5033" clearly believed VD should be taken into account; yet they appear to not have set a limit, nor specified the nominal voltage to be used in the calculation.
The Table specifies the current to be used, and the deemed operating temp for the conductors.
I think it's reasonable to deduce that
- the nominal voltage is the array max voltage (the only voltage specified in the Standard that is remotely relevant)
- the percentage VD between array and PCE shouldn't be higher than 5 %.
If the array cable is thought of as being equivalent to "mains"; then the 3 % used for automotive work is close to the 2.5 % often used as the 'mains" share of VD within an installation.
Having PCE connected will mean actual voltage at Imax will be a bit lower; but not easy to calculate what it will be. A least with using Vmax we have a known figure to work with.
For normal installation VD calculation, we use nominal voltage (as per clause 3.6 of "3000") not actual voltage (as supplied by the network within a permitted range).
The nominal voltage of a PV array is Voc, modified for temp. That's the condition that will generate Imax.
looking again at the clause in "3000"; I'm don't think it sets a requirement for VD in PV array cables.
VD is specified only for parts of the installation between "point of supply" and "any other point in the installation".
Point of supply is a defined term; though the definition [1.4.75] is over-ridden by the definition in Electricity Act.
Yes the array output is a point in the installation; but since current doesn't flow from PoS to the array, and there's PCE between the two;
the array cable can't be considered to be within that description - so there may well be no requirement .
Regardless, in "5033", 4.3.6.1 requires VD to be considered in selecting the cable; and clearly VD in the array cable should be kept as low as practicable.
The committee who wrote "5033" clearly believed VD should be taken into account; yet they appear to not have set a limit, nor specified the nominal voltage to be used in the calculation.
The Table specifies the current to be used, and the deemed operating temp for the conductors.
I think it's reasonable to deduce that
- the nominal voltage is the array max voltage (the only voltage specified in the Standard that is remotely relevant)
- the percentage VD between array and PCE shouldn't be higher than 5 %.
If the array cable is thought of as being equivalent to "mains"; then the 3 % used for automotive work is close to the 2.5 % often used as the 'mains" share of VD within an installation.
- Rating: 16.67%