Discuss cable sizing Ib or In in the Electrical Forum area at ElectriciansForums.net

M

mullock

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Help needed please everyone

This week on my course we have been instructed that when designing a circuit and specifically cable sizing that to work out the cable size you devide Ib (design current) by grouping/ temperature factors etc. However everything i read is telling me its In (pretective devise nominal rating) thats devide not Ib.

I have questioned this several times but continue to be told i'm wrong. i was told that it is done this way, when a circuit with a single devise is installed but not for say a multiple devise circuit say a ring circuit. i'm still not convinced.

Hope this makes sense.

please tell me i'm not the one going mad.
 
N

neilw

  • Thread Starter Thread Starter
  • #2
Hi,

You are right about this - the cable must be able to carry the full amount of current that can pass through the protective device before it operates, not just the design current. What often doesn't get taught about these calculations is that protective devices never actually operate at their stated value anyway - when a fault such as a short-circuit occurs, there's a certain amount of current "let-through" before the protective device has time to react to the fault, and this can be as high as 1.5x-2x the device rating (this is essentially because of the mechanical delay in the fuse or CB operating and probably in the order of around 10 milliseconds). So, you should really take this additional factor into account when sizing your cables if you want to be doing everything correctly. If you need very accurate calculations for the "let-through" current you should consult the manufacturer's technical documentation. These figures are usually also available in professional circuit design software.

Hope that helps,

Neil
 
S

Shakey

  • Thread Starter Thread Starter
  • #3
Help needed please everyone

This week on my course we have been instructed that when designing a circuit and specifically cable sizing that to work out the cable size you devide Ib (design current) by grouping/ temperature factors etc. However everything i read is telling me its In (pretective devise nominal rating) thats devide not Ib.

I have questioned this several times but continue to be told i'm wrong. i was told that it is done this way, when a circuit with a single devise is installed but not for say a multiple devise circuit say a ring circuit. i'm still not convinced.

Hope this makes sense.

please tell me i'm not the one going mad.
i agree with Neil

work out your design current, then apply diversity iff appropriate, this becomes Ib

size your In equal to/larger than Ib

Then size you cable Iz equalt to /larger than In

gruoping factors etc then get applied to the Iz, and the final result is It, which is your cable size;)
 
N

neilw

  • Thread Starter Thread Starter
  • #4
Thanks Shakey :)

Just to clarify one of the points I made earlier, the 1.5-2x figure I mentioned is referred to as the "fusing factor" and this figure needs to be kept in mind for all overcurrent situations (overload and short-circuit). According to my books, the standard equation for cable calculations is

Ib "must be less than or equal to" In "must be less than or equal to" Iz

but if you take the fusing factor into account (such as 1.4 for a CB) the equation should then look like

Ib "must be less than or equal to" (In x 1.4) "must be less than or equal to" (Iz x 1.45)

The 1.45 figure for Iz is a requirement of regulation 433-02-01 point iii of the 16th edition. If the fusing factor for In is greater than 1.45 (such as for rewireable fuses) then the situation gets more complicated - I'll post my notes up if anyone wants to read them though :)

Hope that helps,

Neil
 
S

Shakey

  • Thread Starter Thread Starter
  • #5
Thanks Shakey :)

Just to clarify one of the points I made earlier, the 1.5-2x figure I mentioned is referred to as the "fusing factor" and this figure needs to be kept in mind for all overcurrent situations (overload and short-circuit). According to my books, the standard equation for cable calculations is

Ib "must be less than or equal to" In "must be less than or equal to" Iz

but if you take the fusing factor into account (such as 1.4 for a CB) the equation should then look like

Ib "must be less than or equal to" (In x 1.4) "must be less than or equal to" (Iz x 1.45)

The 1.45 figure for Iz is a requirement of regulation 433-02-01 point iii of the 16th edition. If the fusing factor for In is greater than 1.45 (such as for rewireable fuses) then the situation gets more complicated - I'll post my notes up if anyone wants to read them though :)

Hope that helps,

Neil
Neil, aint got the regs in front of me, but read on a few paragraphs (over the page)

says something like if you are using standard cable sizes (such as 2.5/1.5 flat tw&E) then this requirement is automatically satisfied
 
S

snaban

  • Thread Starter Thread Starter
  • #6
For cable sizing you should consider whether the load is a)domestic(lights/small power) or b)motor and type of load(steady,periodical,fluctuating,suddenly applied big load-AC1,AC2,AC3 etc)
 
G

GrannySmiff

  • Thread Starter Thread Starter
  • #7
To work out cable size you need the OSG and BRB

IT= IN/Ca x Cc x Cg x Ci then once you have your IT you go to the page for cable carrying capacities and match it up with you IT for the Size you need according to the fixing method.
 
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