Cable Calculation Help - Adiabatic Equation

[iDragonBox]

Need help check that my cable calculations are correct as still new to doing them.

6A Lighting Circuit 60898 MCB Type B wiring in 1MM²/1MM² T+E supplying a single 10W outdoor light

Length = 3.3M

Zs@db = 0.22

R1 + R2 mΩ/M = 36.20 mΩ/M - Table lt P218 OSG

Correction Factor - 1.2 - Table L3 P220 OSG

R1 + R2 = 3.3 x 36.20 x 1.2 / 1000 = 0.14Ω

Zs = 0.22 + 014 = 0.36Ω

Fault Current = 230 / 0.36 = 638A

Disconnection Time = 0.1s

K = 115

√ I²t / k = √ (638² x 0.1) / 115 = 1.75 Minimum CPC size.

Seems like I will have to make the circuit longer to increase the Zs value which seems ridiculous or change my cable for a larger CPC which also seem ridiculous for a single 10W light.

Can someone check I am doing my calcs right before I go down the rabbit hole of adapting!

Many thanks

 

[Pretty Mouth]

Need help check that my cable calculations are correct as still new to doing them.

6A Lighting Circuit 60898 MCB Type B wiring in 1MM²/1MM² T+E supplying a single 10W outdoor light

Length = 3.3M

Zs@db = 0.22

R1 + R2 mΩ/M = 36.20 mΩ/M - Table lt P218 OSG

Correction Factor - 1.2 - Table L3 P220 OSG

R1 + R2 = 3.3 x 36.20 x 1.2 / 1000 = 0.14Ω

Zs = 0.22 + 014 = 0.36Ω

Fault Current = 230 / 0.36 = 638A

Disconnection Time = 0.1s

K = 115

√ I²t / k = √ (638² x 0.1) / 115 = 1.75 Minimum CPC size.

Seems like I will have to make the circuit longer to increase the Zs value which seems ridiculous or change my cable for a larger CPC which also seem ridiculous for a single 10W light.

Can someone check I am doing my calcs right before I go down the rabbit hole of adapting!

Many thanks

Adiabatic's quite tricky to get the head around. You're on the right lines, but as you can see, you've come up with a larger CPC than expected. What is the circuit protective device?

 

[iDragonBox]

Adiabatic's quite tricky to get the head around. You're on the right lines, but as you can see, you've come up with a larger CPC than expected. What is the circuit protective device?

6A 60898 MCB Type B

 

[Pretty Mouth]

6A 60898 MCB Type B

P147 of the regs, fig 3A4. I guess you've already seen this to come up with what you have so far, but read the note at the bottom of the chart.

What brand circuit breaker?

 

[timhoward]

P147 of the regs, fig 3A4

Try P417 !

 

[iDragonBox]

P147 of the regs, fig 3A4. I guess you've already seen this to come up with what you have so far, but read the note at the bottom of the chart.

What brand circuit breaker?

Its a theoretical circuit design so haven't chosen a brand yet.

 

[Pretty Mouth]

Its a theoretical circuit design so haven't chosen a brand yet.

Apologies, as Tim says, p417!

 

[timhoward]

Your calculations are correct and as accurate as you can achieve right now (see the note that @Pretty Mouth refers to in order to understand why)

A general comment, remember 543.1.1 - you can either calculate OR select.
In general for smaller size line conductors and shorter lengths it's better to select from table 54.7 which would give you a much more welcome answer!
In general for larger line conductors or longer cable lengths, it's better to calculate.

 

[Pretty Mouth]

It's hard to get a realistic CSA using the data from BS7671. As you've seen the time only goes down to 0.1s, but real world a fault current like that would disconnect much quicker, giving much lower let through energy (I²t), therefore a smaller CSA.

We're concerned with the worst-case situation: for a B or C type breaker, that will be right at the start of the circuit, where the fault current is greatest. If the conductor can withstand a fault here, it can withstand it anywhere downstream.

 

[timhoward]

Let's assume you went for a Wylex B6 .
Here's some data from Wylex:

You had 638A prospective fault current and a 6 amp breaker, so the multiple is x 106
That graph shows you that the disconnection time will be less than 0.01 seconds.

If you run you calculation again with 0.01 as the disconnection time you end up with a CPC of 0.55 sq mm!
You obviously need to round this up to something that actually exists, and then be mindful that it it's separate to the cable or buried there are minimum sizes that apply.
You are using T&E so that last bit isn't relevant.