Just curious of peoples views around parallel conductors and earths
Seems to very much be directed at CCC and live conductors but the rules don't seem to exempt earths in any way
My question stems from an example of 6mm² 2c+e, would it be compliant to parallel 2 cables, the live conductors meet the requirements but the earths being 2.5mm² I'm wondering if 3.4.3 prevents this arrangement or simply doesn't apply to the earths in this case
Does 3.4.3 apply to earths?
Re: Does 3.4.3 apply to earths?
Hahaha when I saw that post today I knew I'd asked it before, didn't realize no one ever had any input though, still curious about opinions on this
Re: Does 3.4.3 apply to earths?
Hi Jamie & Doug, I believe the 2.5mm earths wont comply as 5.3.3.1.2 says under exceptions :
"where the summation of the csa does not correspond exactly with the nominal size of the active conductor given in table 5.1, the conductor shall be determined in relation to the nearest larger size of active conductor"
Therefore the two 6mm get adjusted to 16.0mm so the earth needs to 6mm
What do you think?
"where the summation of the csa does not correspond exactly with the nominal size of the active conductor given in table 5.1, the conductor shall be determined in relation to the nearest larger size of active conductor"
Therefore the two 6mm get adjusted to 16.0mm so the earth needs to 6mm
What do you think?
- Rating: 16.67%
Re: Does 3.4.3 apply to earths?
Like the rest of 3.4; 3.4.3 is about CCC, and this applies for load-carrying conductors.
So 3.4 refers to CCCs as per 3008.1 series for load-carrying
The CCC ratings are maximum constant currents under a variety of installation conditions
It doesn't apply directly to PECs, because they don't carry load.
PECs are also sized in proportion to the current they have to carry; but it isn't load current.
PECs are about carrying fault current (assuming fault protection by automatic disconnection of supply).
The currents are far higher; and the heating effect in the cable is proportional to the square of the current (W=I2R).
On the other hand, the PEC isn't required to carry this current constantly; only for the specified time
So were talking about a very different scenario.
Which is why "3008.1" doesn't include any relevant ratings, and 5.3.3 doesn't refer to "3008.1".
Instead it gives us a choice of either a simple table; or a calculation
As you've noted, once above the smallest sizes the PEC of a composite cable is often of reduced size.
This avoids cost of excessively large PECs.
If the reduction was always in direct proportion, parallelling would result in same proportion; and therefore we could assume all OK.
But that's not how cables are made, the PEC is generally a standard conductor CSA, not a direct proportion of the active's CSA.
In your example, the effective CSA of parallelled As would be 12 mm2.
Interpolating Table 5.1, this would call for around 4.6 mm2.
The effective CSA for the parallelled PECs would be 5 mm2, so I'd say that's probably adequate for all practical purposes.
But as PeteRig has noted, 5.3.3.1.2 (paras 2 & 3) says if connecting in parallel, move to next standard size up, which would be 16 mm2;
requiring 6 mm2 for the PEC
Not necessarily required for safety, but - as so often when applying simplified requirements like the Table - it's best to err on the side of caution.
And for absolute certainty, all you have to do is run the calculation as per 5.3.3.1.3.
As with connecting conductors in parallel; the result may be a non-standard size.
But as long as the size you've got is at least as great as the size required, all good.
So 3.4 refers to CCCs as per 3008.1 series for load-carrying
The CCC ratings are maximum constant currents under a variety of installation conditions
It doesn't apply directly to PECs, because they don't carry load.
PECs are also sized in proportion to the current they have to carry; but it isn't load current.
PECs are about carrying fault current (assuming fault protection by automatic disconnection of supply).
The currents are far higher; and the heating effect in the cable is proportional to the square of the current (W=I2R).
On the other hand, the PEC isn't required to carry this current constantly; only for the specified time
So were talking about a very different scenario.
Which is why "3008.1" doesn't include any relevant ratings, and 5.3.3 doesn't refer to "3008.1".
Instead it gives us a choice of either a simple table; or a calculation
As you've noted, once above the smallest sizes the PEC of a composite cable is often of reduced size.
This avoids cost of excessively large PECs.
If the reduction was always in direct proportion, parallelling would result in same proportion; and therefore we could assume all OK.
But that's not how cables are made, the PEC is generally a standard conductor CSA, not a direct proportion of the active's CSA.
In your example, the effective CSA of parallelled As would be 12 mm2.
Interpolating Table 5.1, this would call for around 4.6 mm2.
The effective CSA for the parallelled PECs would be 5 mm2, so I'd say that's probably adequate for all practical purposes.
But as PeteRig has noted, 5.3.3.1.2 (paras 2 & 3) says if connecting in parallel, move to next standard size up, which would be 16 mm2;
requiring 6 mm2 for the PEC
Not necessarily required for safety, but - as so often when applying simplified requirements like the Table - it's best to err on the side of caution.
And for absolute certainty, all you have to do is run the calculation as per 5.3.3.1.3.
As with connecting conductors in parallel; the result may be a non-standard size.
But as long as the size you've got is at least as great as the size required, all good.
- Rating: 16.67%