Cable Calculation

rme · Feb 19, 2010

Hi
Sorry if I’ve posted this in the wrong area.

Do the tabulated values for the current carrying capacity of conductors in bs7671 include the 1.45 rule of thumb?

I.e. a 32amp type B should trip within an 1hr at 1.45x it’s In (46.4amps)

So do I look for a cable that can take 46.4amps, even if the Ib is only 32amps?

I’m sure I’ve seen a reg that states the 1.45 is satisfied when using the tabulated values for twin & earth.

This is all hypothetical and I’m trying to keep my question simple (leaving out the other correction factors and install method)

Cheers

pushrod · Feb 21, 2010

rme said:
Hi
Sorry if I’ve posted this in the wrong area.

Do the tabulated values for the current carrying capacity of conductors in bs7671 include the 1.45 rule of thumb?

I.e. a 32amp type B should trip within an 1hr at 1.45x it’s In (46.4amps)

So do I look for a cable that can take 46.4amps, even if the Ib is only 32amps?

Cheers

the breaker is there to protect the installation so, disregarding install method and cable factors, the cable needs to just take a current larger than (or equal to) Ib.

rme · Feb 22, 2010

pushrod said:
the breaker is there to protect the installation so, disregarding install method and cable factors, the cable needs to just take a current larger than (or equal to) Ib.

Thanks for your reply. I always thought the cable needed to take the current larger than (or equal to) I2?

pushrod · Feb 22, 2010

rme said:
Thanks for your reply. I always thought the cable needed to take the current larger than (or equal to) I2?

Pretty sure that is not the case. The procedure as i understand it is
1) work out design current (Ib)
2)choose the rating of the mcb/protective device (In)
3) calculate (It) by dividing In by the relevant cable factors
4) choose a cable with the correct size current carrying capacity (Iz) from the tables
5) calculate if voltage drop (Vc) is within limits

The tabulated values allow for current operating the device (I2) at approx x1.5 (fusing factor)
I2 depends on the protective device as rewireables have a fusing factor of 1.8, cartridge fuses of approx 1.25 to 1.75 and mcbs approx 1.5

edit ; just an extra thought - the cable factor of the rewireable (.725) automatically pushes up It in the calculation so the problem of of a higher fusing factor and consequent bigger I2 is built in.
hope this helps.

Guest123 · Feb 22, 2010

Seems to be a bit of confusion gents (or I may be having a moment).

Step 1. Calculate Ib
Step 2. Select In
Step 3. Select a cable with a tabulated CCC (It) greater than In.

If no factors need to be applied then that is it, however if any of the factors are required (Ca Cg Cf Ci) then you must calculate Iz (CCC of cable under the particular service conditions)

the factors are applied to In... Iz =

______In______
Ca x Cg x Ci x Cf

You then select a cable with it's It greater than Iz.

Hope this helps.

Edit......unless it's a ring circuit, bit different then.