CABLE CALCS

[soulman]

Hi a bit of help please guys, currently doing cable calcs and a bit confused

lighting circuit (40 metres) 1000w/230 =4.34a

5a 3036 fuse

5/0.725 =6.89a

cable size 1.0mm or 1.5mm

volt drop 1.0 cable = 44x6.89x40m /1000 = 12.12v

volt drop 1.5 cable = 29x6.89x40m /1000 = 7.99v

both are above the 6.9v 3%

when looking at table 7.1 in the onsite guide

max cable length for 3036 5 amp fuse 1.0 cable is 71m and 108m for 1.5

Can you please tell me where i'm going wrong! this has been driving me crazy today headbang

cheers

 

[SPECIAL LOCATION]

I am guessing the whole of your 1000w load is NOT all at the far end (40m away) along the cable?

e.g.

Try considering if 500w was 20m down the cable

and another 500w at the far end 40m down the cable...

the first 20m with carry full load..

but the last 20m will ONLY carry 500w NOT 1000w!

On this example you get something like 3.8v drop @ 20m then 1.9v drop at last 20m total 5.7V which is ok within the 6.9tolerance..

expand the illustration a bit further an assume you got 100w every 1/10th down your total length...

Volt drop becomes even less..

And if you also then allow for diversity e.g. only 66% of lights on at one time...

you find that the cable can get much longer.

Somewhere in the bowels of the forum there are some more in depth calcs I did to illustrate this problem!

:Salute

 

[soulman]

well explained thank you!!! your a credit to the forum.

 

[meerkat]

Hi Soulman, cable calcs sequence (1) Ib = W/V = 1000/230 = 4.34 Amps

(2) In = BS 3036 5 Amps

(3) Ref Method = clipped direct etc, has a bearing on the c/c/c of the cable later on

(4) Correction Factors = BS 3036 = 0.725

(5) It = In/CF's = 5/0.725 = 6.89 Amps

(6) Iz = Min of 6.89 Amps C/C/C

(7) Volt Drop = mV/A/M x Ib x L (All Divided by 1000) = 44 x 4.34 x 40 = 7.63 Volts

(8) 3 % 0f 230 Volts = 6.9 Volts, so our 7.63 Volts does not comply with the regs

increase the cable size to 1.5 mm and recalculate step (7)

(7) mV/A/M x Ib x L = 29 x 4.34 x 40 (All divided by 1000) = 5.03 Volts

(8) 3 % of 230 Volts = 6.9 Volts, now our circuit of 5.03 Volts does comply with the regs

so the answer to the question is, we would need a 1.5mm cable in this instance, I hope this helps soulman, PM me I have some good info on Cable calcs I can send you, (MEERKAT)

 

[Hefferl]

My understanding is Volt Drop = (mV x Ib X L) / 1000

So in your case you should be using 4.34A as your design current (Ib) not the tabulated current carrying capacity of the cable.

You would then get (44x4.34x40)/1000 = 7.64V For 1.0mm

and (29x4.34x40)/1000 = 5.03V for 1.5mm

 

[PC Electrics]

Heff and Meer have spotted one of your mistakes, your design current is 4.4A. The 0.725 only applies to the cable carrying capacity when hitched to a 3036.

Also you haven't applied any diversity. For a domestic lighting circuit you can apply 66%. This would bring your calc below the 3% I expect.

If it's commercial and all the lights will be on, then consider how the load is spread along the cable (as per Specs suggestion) and calculate each section, then add up the sections. Do it in 3 sections first as a rough calc to see if it works for you.

Is this an exercise or a real job? if it's real, use 1.5mm for the loop through and 1.0mm for the switch drops. On a 3036 1.0mm is on the edge of it's ccc on a 3036 if it runs through any significant insulation. And you don't know if someone is gonna hang a load of GU10 downlighters on the end of your 40m.

 

[meerkat]

Hi PC Electrics, I think this is a single circuit cable calculation only, no diversity involved.

In a consumer unit where there are several circuits, the main supply cable and its protective device are calculated using Maximum demand and diversity, using the tables found in Appendix 1 of the OSG, cheers (MEERKAT)

 

[SPECIAL LOCATION]

Hi Soulman, cable calcs sequence (1) Ib = W/V = 1000/230 = 4.34 Amps(2) In = BS 3036 5 Amps

(3) Ref Method = clipped direct etc, has a bearing on the c/c/c of the cable later on

(4) Correction Factors = BS 3036 = 0.725

(5) It = In/CF's = 5/0.725 = 6.89 Amps

(6) Iz = Min of 6.89 Amps C/C/C

(7) Volt Drop = mV/A/M x Ib x L (All Divided by 1000) = 44 x 4.34 x 40 = 7.63 Volts

(8) 3 % 0f 230 Volts = 6.9 Volts, so our 7.63 Volts does not comply with the regs

increase the cable size to 1.5 mm and recalculate step (7)

(7) mV/A/M x Ib x L = 29 x 4.34 x 40 (All divided by 1000) = 5.03 Volts

(8) 3 % of 230 Volts = 6.9 Volts, now our circuit of 5.03 Volts does comply with the regs so the answer to the question is, we would need a 1.5mm cable in this instance, I hope this helps soulman, PM me I have some good info on Cable calcs I can send you, (MEERKAT)

Your points (7) & (8) are ONLY true if the 1000watts is at the end of the cable 40m..

IF 4.34A is NOT being drawn at the 40m length then those calcs are irrelevant!

Hi PC Electrics, I think this is a single circuit cable calculation only, no diversity involved. ?In a consumer unit where there are several circuits, the main supply cable and its protective device are calculated using Maximum demand and diversity, using the tables found in Appendix 1 of the OSG, cheers (MEERKAT)

Diversity can be applied to single circuits... See Paragraph 3 page 95 One site Guide!

And most certainly lighting circuits, (of all the various type of circuit) are very much a circuit where diversity is a VERY relevant factor.

Which is why as Soulman states..

Table 7.1 page 44 On site guide

suggests a 5A 3036 on 1.0mm cable can be up to 70+ meters long..

(5A @ 230v is >1000w!!!!)

 

[Chr!s]

And most certainly lighting circuits, (of all the various type of circuit) are very much a circuit where diversity is a VERY relevant factor.

For lighting circuit the onsite guide states " Current equivalent to the connected load, with a minimum of 100 W per lampholder"

The 66% may be used for assessment of Maximum Demand.

Which is why as Soulman states..Table 7.1 page 44 On site guide

suggests a 5A 3036 on 1.0mm cable can be up to 70+ meters long..

(5A @ 230v is >1000w!!!!)

Diversity plays no part, the Onsite Guide assumes a load equivalent to that of the protective device.

 

[meerkat]

Hi SL, I was told at college that the only single circuit where diversity played a part was the cooker circuit, I thank you for your input and would like to understand more about this subject.

Using the lighting circuit in soulman's post, is it feasable to reduce the C/C/C of the cable by 33% by utilising diversity, or have I understood this wrong.

I have been told many things at college which didnt weigh up in the real world, one that springs to mind is that maximum demand is the rating of the service cut out fuse, I thought that was why the tables mentioned were used, cheers (MEERKAT)

 

[SPECIAL LOCATION]

For lighting circuit the onsite guide states " Current equivalent to the connected load, with a minimum of 100 W per lampholder"The 66% may be used for assessment of Maximum Demand.

Diversity plays no part, the Onsite Guide assumes a load equivalent to that of the protective device.

I was referring to the fact that Meerkat implied Diversity could NOT be used on a single circuit...

Where'as in the real world it most certainly can.

And that way he was doing his calculation putting the whole of the load at the end of the lighting circuit..

Which is the exception to the rule with a majority of real world lighting circuits.

As always the OSG is just "general guidance" and does not preclude the competent person at the site using any applicable calculations and allowances.

(Paragraph 2 pg 96)

:C :coffee

---------- Post Auto-Merged at 13:50 ---------- Previous post was made at 13:43 ----------

Hi SL, I was told at college that the only single circuit where diversity played a part was the cooker circuit, I thank you for your input and would like to understand more about this subject.Using the lighting circuit in soulman's post, is it feasable to reduce the C/C/C of the cable by 33% by utilising diversity, or have I understood this wrong.

I have been told many things at college which didnt weigh up in the real world, one that springs to mind is that maximum demand is the rating of the service cut out fuse, I thought that was why the tables mentioned were used, cheers (MEERKAT)

Been I while since I done my college exams an all...

I was unclear if this was a Real work question or a college question...

You may be right that college want you only apply things in a certain way..

BUT

However you want to say it..

The real world is The Real World!!!!

e.g.

Two identical halves of a semidetached property with identical lighting circuits arrangements

could have very different actual current max loadings and thus volt drop...

cuz..

one half is the elderly widow..

who never puts the lights on..

and the other half has been converted to a Dentist with all light on ALL day.

even in daylight!

I shall now go and finish me lunch!

then go and investigate a dodgy alarm system!!

opcorn :coffee

 

[Chr!s]

I was referring to the fact that Meerkat implied Diversity could NOT be used on a single circuit...Where'as in the real world it most certainly can.

And most certainly lighting circuits, (of all the various type of circuit) are very much a circuit where diversity is a VERY relevant factor.

Table 1A is for current demand of a final circuit. In a lighting circuit you would not normally use diversity for current demand, unless commercial or industrial where timers or management systems may be employed.

And that way he was doing his calculation putting the whole of the load at the end of the lighting circuit..Which is the exception to the rule with a majority of real world lighting circuits.

Agreed, but not only lighting circuits.

As always the OSG is just "general guidance" and does not preclude the competent person at the site using any applicable calculations and allowances.

Agreed, but if your using there standard circuits, your using there design criteria.

(Paragraph 2 pg 96)

This is in reference to maximum demand, not design current of the final circuit.

 

[meerkat]

Hi Plumber, well not sure I understand any of this anymore, using the circuit in soulman's post is special saying that we could reduce the C/C/C by a third using diversity, or can diversity only be used in a commercial/industrial environment? cheers for your help (MEERKAT)

 

[steptoe]

You dont alter ccc using diversity, you alter the demand, ie, load.

Therefore a 6A load would be 4A after applying diversity.

 

[Chr!s]

Hi Plumber, well not sure I understand any of this anymore, using the circuit in soulman's post is special saying that we could reduce the C/C/C by a third using diversity, or can diversity only be used in a commercial/industrial environment? cheers for your help (MEERKAT)

Soulmans design current is 1000w, so this is the figure he would use, the 66% is applied to Maximum demand not the final circuit current demand

 

[a1]

well explained thank you!!! your a credit to the forum.

He is an asset to the forum. :x

 

[meerkat]

Hi Plumber, this is what I was trying to say in my original answer to soulman's post, so diversity only affects the load, diversity is only used when considering a group of circuits as found in a CU, where not all of the capacity of every circuit is being used, therefore we wouldnt reduce the circuit protection on a single circuit as it is there to protect the whole circuit load, should it be used.

Diversity is used to calculate the cable size and protective device rating of the cable which feeds all of all of the circuits of a CU, as most of the time only a fraction of the total capacity of each circuit is in use, does this make sense or am I not getting this at all, cheers (MEERKAT)

 

[Chr!s]

Hi Plumber, well not sure I understand any of this anymore, using the circuit in soulman's post is special saying that we could reduce the C/C/C by a third using diversity, or can diversity only be used in a commercial/industrial environment? cheers for your help (MEERKAT)

Soulman is questioning his design to that of a standard circuit within the onsite guide, now if you know how to design the standard circuits you will know no diversity is applied to the lighting circuit. People are complicating things by going on about applying diversity to a final circuit.

Now the guidance for final circuit current demand is table 1A onsite guide, table h.1 guidance note 1. along with chapter 13 Bs 7671.

Both state demand is calculated by adding current demand of each point of utilization, the table state minimum of 100w per point, the only reference to diversity is as you said cooking appliances.

In a domestic or even commercial applying diversity to a final circuit may lead to a small overload, now unless you have specific criteria which allows you to apply diversity i would advise using the tables provided in the onsite guide and guidance note 1.

---------- Post Auto-Merged at 15:41 ---------- Previous post was made at 15:35 ----------

Hi Plumber, this is what I was trying to say in my original answer to soulman's post, so diversity only affects the load, diversity is only used when considering a group of circuits as found in a CU, where not all of the capacity of every circuit is being used, therefore we wouldnt reduce the circuit protection on a single circuit as it is there to protect the whole circuit load, should it be used.Diversity is used to calculate the cable size and protective device rating of the cable which feeds all of all of the circuits of a CU, as most of the time only a fraction of the total capacity of each circuit is in use, does this make sense or am I not getting this at all, cheers (MEERKAT)

Diversity may be used for a final circuit, though you would need specific criteria to utilize this, in general i would advise using the table 1A within the onsite guide.

Yes you may use table 1B of the onsite guide to equate the maximum demand for a group of circuits, say for a distribution circuit.

 

[meerkat]

Hi Chris, thanks for your help I do now understand how this works and I am keen to learn how all the smaller pieces fit into the bigger picture, thanks to you, steptoe, Special location and PC electrics for your input, cheers (MEERKAT)