in this q a video we'll be answering the question when calculating grouping for a cable tray
do i need to count every cable on there
now this question relates to numerous videos that gordon and gary have made on cable tray and basket
from Atkore so check those videos out to see more details of those items but in this video we're
going to look at a technical question relating to grouping factor on cable tray and basket now
we all know why grouping factors are important ultimately when current passes through a conductor
it generates heat if excessive this heat can cause the insulation on the conductor to break down
and in some cases cause damage to the fabric of the building and may even start fire so if
a cable needs to carry more current we install a larger cable it's not just that the copper
is physically capable of carrying more current a cable with a larger cross-sectional area can
dissipate its heat more effectively so managing the safe dissipation of this heat is absolutely
critical to making sure that our installations are safe now when we install cables together onto
a tray the fact that the cables are touching or even just in close proximity means that they can't
dissipate their heaters effectively because the heat from each cable will affect its neighbours
this means that the cable cannot be allowed to get as hot as it could do if it was by itself
table 4c1 and vs7671 gives a factor that gets used to effectively increase the maximum current
a cable must be able to carry and in reality this results in needing a cable with a larger
cross-sectional area so let's look at an example if we've got eight cables fixed to a tray feeding
eight lighting circuits we look at item three of table four c1 in the wiring regs and track across
to find the column marked eight looking at where they intersect we've got a value of 0.72 now this
factor goes into a calculation that helps us to determine conductor size and it looks like this
i t must be greater than or equal to i n divided by c g in this formula i t is the tabulated value
of current that the cable can carry tabulated just means in a table by the way i n is the nominal
rating of the circuit protective device and cg is the grouping factor that we just found now there's
a whole load of other rating factors that we can put under here to keep the grouping factor company
but we'll cover those in other videos so if we're considering this lighting circuit protected by a
10 amp mtv then the cable we install must be able to carry more than 10 divided by 0.72 so 13.9 amps
and then we'd select the correct cable accordingly but is that the end of the story in this case do
we really need to use all eight cables to figure out our grouping vector well there is a way that
may allow us to discount some of those conductors note 9 to the table sheds a little bit of light on
this it reads if due to known operating conditions a cable is expected to carry not more than 30
of its grouped rating it may be ignored for the purpose of obtaining the rating factor
for the rest of the group so in other words if the cross-sectional area of the cable is bigger
than it needs to be which sometimes happens to make the circuit have a compliant vault drop or
earth fault loop impedance value then we may not need to include it in our calculation for grouping
this is because the circuit is not drawing enough to make the cable hot enough to cause a problem
but there's two points just to be careful about first of all this is only applicable if there
are known operating conditions in other words you know what the circuit is feeding and that it is
not likely to change so a socket circuit would not be a suitable candidate for this allowance
but a lighting circuit with a fixed and known number of lights on might also notice it read
not more than 30 percent of its grouped rating what does this mean well the note continues for
example a group of n loaded cables would normally require a group rating factor of cg applied to the
tabulated i t however if m cables in the group carry loads which are not greater than 0.3 cg
it ampers the other cables can be sized by using the group rating factor corresponding to n
minus m cables so let's try to unravel that one suddenly this note is telling us that
the grouping factor needs to be applied to the tabulated current carrying capacity of the cable
but earlier we applied the grouping factor to the nominal rating of the protective device
so what's going on there well looking back to our formula from earlier if we transpose it we'll find
that the current carrying capacity multiplied by our grouping factor must be greater than the
nominal rating of the protective device which kind of makes sense as we want the cable to be
able to carry more current in the conditions it's installed in than the protective device will allow
to flow so if as it says in the final sentence of the note the grouping factor times the tabulated
current carrying capacity times 0.3 for the 30 is higher than the current expected to flow then you
don't need to count that cable in your grouping calculation so if your tabulated current carrying
capacity was 17 amps multiplied by the grouping factor of 0.72 and by 0.3 and you get 3.672 if
the circuit should never draw more than that value of current for instance if it's feeding a
fixed amount of light fittings then you don't need to include it in your grouping factor value now
that's all well and good however amazingly it's only scraping the surface of grouping and there's
lots of other things to bear in mind also such as the fact that note 1 states that the values in
table 4 c1 are only applicable to uniform groups of cables equally loaded so the question arises
what about when you have groups of cables that are non-uniform and unequally loaded well that's the
subject of a future video but we've made a good start here as always we want to hear from you do
you use grouping factors in your electrical work or has note 9 to table 4c1 ever made a difference
to an installation that you've been designing or installing whatever your thoughts or questions on
this subject please leave them in the comments below and as always thank you very much for
watching