# 17th Edition question

hi
A domestic consumer unit on a TN-C-S system uses a 15A Type 2 mcb, to BS3036 to protect a radial circuit wired in 2.5mm^2 two-core pvc insulated cable with a 1.5mm^2 protective conductor clipped directly to a surface. If this circuit supplies socket outlets for hand held equipment, the maximum length of run is:

a. 35m
b. 43m
c. 54m
d. 58m

My assumptions are:

Best way to tackle this problem is to rearrange the formula for voltage drop i.e.

V_drop = [(mV/A/m)*(I_b)*L]/1000

hence

L = [1000*(V_drop)] /[(mV/A/m)*(I_b)]

Problem is the question doesn't give me a value for the design current: I_b.

So I've used a hypothetical value of 14A, since 14A < 15A satisfying I_b < I_n.

Using table 4D5 from the regs I get a voltage drop of 18V (mV/A/m) for installation ref method C clipped direct for 2.5 twin + earth cable.

Using these values in the above formula gives:

L_max = (1000*18)/(18*14) = 71.42m

Some questions then,

Am I missing something?

Am I ok to use a design current of 14A?

:? :?
livewire
Rank: Apprentice
Progress to next rank:
89.5%
Posts: 22
Joined: Wed Apr 15, 2009 4:06 pm

your circuit is peotected by 15A, as it's a socket circuit and load can only be assumed, I'd calculate Ib at 15A.
KB
kbrownie
Rank: Project Manager
Posts: 1995
Joined: Mon Mar 05, 2007 9:36 pm

[quote="kbrownie"]your circuit is peotected by 15A, as it's a socket circuit and load can only be assumed, I'd calculate Ib at 15A.
KB[/quote]

that's interesting, thanks KB
livewire
Rank: Apprentice
Progress to next rank:
89.5%
Posts: 22
Joined: Wed Apr 15, 2009 4:06 pm

Display posts from previous
Sort by
Order by

• DIY How to Project Guides
• DIY how to tutorial projects and guides - Did you know we have a DIY Projects section? Well, if no, then we certainly do! Within this area of our site have literally hundreds of how-to guides and tutorials that cover a huge range of home improvement tasks. Each page also comes with pictures and a video to make completing those jobs even easier!

• Related Topics