Calculating lb and voltage drop

[Penance]

Pre empting my 2382 and trying to calculate lb so as I can calculate voltage drop.

Circuit I have used for this is an upstairs lighting circuit, 1mm2 t&e 4 light points and length of approx 19m.

Cable installed to ref method C.

Getting myself a bit confused but this is what I think I should be doing:

Using the OSG app 5 table 1a I assume a load of 400 watts, 1.818A.
Is this actually lb or should I apply diversity?

 

[High Tower]

Apply diversity, apply de-rating factors, such as insulation, ambient temp, 3036 rule etc, that will give you your IB then MV/M/A x IB x L / 1000

Sorry don't have osg or brb with me to work out any examples for u

 

[telectrix]

use worst case scenario. design for max. loading. i would round off Ib to 2A, then calculate volt drop from that. just makes it easier all round. if you want to allow for future expansion , though, allow for an extra point or 2. i.e. 600watts, 2.6A, round up to 3A.

 

[Penance]

So do I round up to 2A then apply 66% diversity?
Cable is either direct in plaster (from CU to loft) or clipped direct to joists in loft, at each lighting point the cable passes thru approx 400mm of plastic conduit to keep insulation from direct contact, is ref method C correct for this?
Do I assume ambient temp to be around 20•c for a domestic property, that's what we normally have the stat at on the heating?
Protective device is 6A type B mcb so does 3036 apply?

 

[telectrix]

3036 only applies to bs3036 rewireable fuses.

 

[Penance]

Thought so.

What you say about worse case makes sense to me but does that mean I should not apply diversity as that would lower lb?

 

[High Tower]

Thought so.

What you say about worse case makes sense to me but does that mean I should not apply diversity as that would lower lb?

Yup IB after application of diversity and application of appropriate de-ration factors, sorry for the bum steer regarding 3036's I should have explained further, my apologies it only applies to semi enclosed bs3036 fuses and is a value of .75

 

[ezzzekiel]

unless of course this is an exam question in which case dont go wandering with your figures give them what they are asking for

 

[Penance]

Nope not exam, my own house and using it to try and pick up this stuff.
Will do calcs and put them up in a while.

 

[High Tower]

Cool mate give us the figures and your workings out and one of us will give it the once over

 

[Penance]

One last thing, is my ref method quoted correct for the installation described.

No bum steer Hightower, it keeps me on my toes!

 

[Penance]

Here goes-

4 lighting points on circuit assumed at 100W each gives 400W or 1.818A.

Applied diversity of 66% gives 1.199.

Ambient temp of 20•c correction factor of 1.04
1.04 * 1.199 = 1.246.

Mv/A/m for 1mm2 conductor with ref method C = 44
Lb = 1.246
circuit length = 19m

Voltage drop -
(44 * 1.246 * 19) / 1000 = 1.041

 

[High Tower]

Looks good from here!

 

[Penance]

Thanks mate, is there anything else I'll need to calculate?
I know I have to do final ring cooler etc but is it just v drop and ensuring csa is suitable after corrections?

 

[High Tower]

In domestic, showers are the most important calcs as rfc and lighting are always ok!

 

[Penance]

We're old fashioned and use tap mixers so no issue there, obviously something I'll need to know about.
Could you outline an install for me and I'll see if I can do the calcs?

 

[High Tower]

10.5kw shower, 26.5m cable from board, 30deg ambient in loft, 150mm insulation, run inside stud Walls and clipped direct in the loft

 

[Penance]

Clipped under insulation or just vertically through?

Reckon it'll be tomorrow before I'm sober enough to do calcs.

 

[High Tower]

Under

Look forward to
It

 

[Penance]

Few obstacles there I'm unsure about but I'll have a go.

 

[i=p/u]

how do you know , what percentage to apply for diversity in circuits. where do you's get this info from, sorry for hijackin. i know cooker is first 10A plus 30% thansk

 

[Penance]

From the regs, the OSG has it laid out in an easier to find way.

 

[i=p/u]

think we do diversity tonmorrow at college, but dum dum left his books on the train, goneeeeeeeeeeeeeee and thanks

 

[redface]

10mm tw/e will be ok

 

[i=p/u]

cheers but lost me books or would look,,,

 

[pushrod]

Reckon you could be fitting a mixer tap or changing your installation method

 

[Penance]

10.5Kw = 45.652A

No diversity applied, have taken it to be an instantaneous heater so 100% of load.

Ambient temp of 30c correction factor of 1.08
1.08 * 45.652 = 49.304

No conductor CSA specified but would need to be minimum of 10mm2 but with installation ref method 101 10mm2 is rated at 36A.

Using 16mm2 with ref method 101 allows 46A.

Me thinks you need to reroute cables and not under Insulation.

So go on, where did I go wrong?

 

[High Tower]

You didn't, your calcs are spot on!

Realistically yes you would just change the method.

Let's have the vd calc then

 

[Penance]

I'll have a pop but couldn't see the mv/a/m anywhere for 16mm2 with ref 101.
Off to look in the big book.

 

[pushrod]

what table does 1.08 come from for ambient air temp of 30 celcius?

And the formula for It is

It = Ib/(Ca*Cg* Ci* Cc) so that would be dividing not multiplying

Also table 6B of OSG or 52.2 of BRB is needed for the insulation to give you the factor for Cc

checkout p123 of OSG and tables 6A and 6B on the next 2 pages

edit - suppose could use method 101 for cable

 

[Penance]

1.08 came from table 2e page 104 OSG.
Think I may be using the wrong table.

The formula you showed is for current carrying capacity, I got the max rated current from table 6f page 134 OSG. Is that wrong?

 

[pushrod]

1.08 came from table 2e page 104 OSG.
Think I may be using the wrong table.

Yep you are using the correction factors for Zs

The formula you showed is for current carrying capacity, I got the max rated current from table 6f page 134 OSG. Is that wrong?

That is ok if you do it that way but if there are a variety of simultaneous factors you would have to work out It and then go to a table like 4D2A in the brb to select your cable size.

Table 4D5 of brb or 6F of OSG are fine if you can get a direct match to installation method.

 

[Penance]

So my calcs are ok for a simple installation?

What sort of simultaneous factors might arise?

 

[pushrod]

two or more cables grouped together, higher ambient temps, running straight thru thermal insulation (had not read thru the method of installation, for your last example, properly when i commented initially) also could be on a rewireable to BS3036. You only use the worse case factors that are occuring at the same time. eg if it ran thru a metre of insulation and later the cable went thru a boiler house at higher ambient temp you would only use the insulation factor as that is the worst one and they are not occuring at the same time.

 

[Penance]

Cool, thanks.
Answers my question about multiple methods on same cable run. I assumed you should take worse case.

Will do bd calc in a while.

 

[Penance]

Here goes, assuming cable has now been sorted and is clipped direct for whole run.
Lb = 45.652
30c ambient correction = 1
mv/a/m for 10mm2 using ref method c is 4.4 -
(4.4 * 45.652 * 26.5) / 1000 = 5.323

So if now clipped direct 10mm2 t&e with 50a type b mcb will be ok

I think!

 

[pushrod]

what about 6mm²

 

[Penance]

6mm2 has ccc of 46a with ref method c so thought 10mm2 better, but vd would be 8.831.

Sound about right?

 

[pushrod]

Yeah it is a close run thing - 10mm on a 50A breaker gives lots of leeway and would be the sensible choice if you were installing. If, however, the 6mm cable was already in place and you were being asked to fit the 10.5kW shower you could just comply with a 45A mcb.

At 8.8V on the 6mm² the volt drop is still within 5% .
Hope this has helped a bit


edit : 4D5 of brb (the table for T&E) gives 6mm² at 47A

 

[Penance]

Yes mate, it all helps.

I think I may have been using wrong table in OSG. Saying that I would still be happier installing 10mm on a 50a mcb.