Application of Diversity

[Happy Jack]

In fact, in light of a recent thread I do have a question. There seems to be some conflicting information concerning the sequence to derive Ib. From a number of different online sources, actually. It has introduced an element of doubt, so I'd be grateful for confirmation and / or a correction. Keeping it local, from the thread I link to below -

http://www.electriciansforums.net/e...-forum/30644-calculating-lb-voltage-drop.html

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

Now I understand the Ib <= In <= Iz. However, High Tower seems to suggest that you apply both diversity and the various correction factors to derive your design current, Ib. I believe that to be incorrect, as the value for Ib is found after the application of diversity, if needed, then the correction factors are applied to In, the nominal rating of the selected protective device.

Can someone confirm this? I have some other questions on diversity right now, but will hold back for the time being.

 

[Happy Jack]

Just a quick bump on this. Any comments would be welcome.

 

[Happy Jack]

Okay, I'm going to drop my other questions now. Still curious to hear about my first as that impacts what I'm about to ask. Anyway. Assuming that one does not apply the various correction factors as described above...

In specific relation to the standard arrangements of final circuits in accordance with Appendix H, am I correct in interpreting that the application of diversity does not inform the nominal rating of the selected protective device, In. That value, as table A1 and the preamble in Appendix H of the green OSG specifies, is fixed, despite the allowance of diversity as detailed in table A2, part 9?

I understand that what is written in Appendix A is not definitive, no one installation is ever identical and so common sense has to be applied. But just to confirm, absolutely, and with a final ramble to end the post.

If the allowance of diversity as detailed in table A2, part 9, reduced the value of Ib for an 32A A2 radial to 16A - a 50% reduction is permissible - there would never be an occasion where one would select an In of 16A? But the cumulative values of Ib derived in this way for every circuit described in Appendix H would inform the current demand for the entire installation and inform the value of the service head fuse/s?

The second word in Requirement 311 is 'economical', so what I've described does appear to make some sense. Any responses would be welcome.

 

[Happy Jack]

I will drop this here, also. I write this with reference to a previous diversity exercise we completed at college and the design of a circuit supplying a cooker rated @ 13kW. I = W / V = 13000 / 230 = 56.23A. After the application of diversity Ib resolves to 23.87A, so In = 30A.

Pulling a sentence from Appendix A -

'The current demand for any final circuit which is a standard circuit arrangement complying with Appendix H is the rated current of the overcurrent protective device of that circuit'.

So In is fixed as in Table H2.1, either 32A or 30A for an A1 or A2, and 20A for an A3, but the application of diversity informs the demand of the entire installation and the selection of the main service head fuse/s? Is that about the sum of it?

 

[Marvo]

Sorry I can't help you with your diversity question, being a foreigner we work to different regs and tables. Hopefully this will bump your thread again and one of the UK lads will chip in with some info.

 

[Happy Jack]

I appreciate the comment all the same, Marvo. I was actually tempted to post it over on the main forum as I know traffic there is a lot higher.

Having mused over it this evening, I'm still not confident my interpretation is correct.

 

[Marvo]

I think your question has merit so by all means copy/paste your OP and start a new thread in the General Electrical forum.

 

[trev]

In fact, in light of a recent thread I do have a question. There seems to be some conflicting information concerning the sequence to derive Ib. From a number of different online sources, actually. It has introduced an element of doubt, so I'd be grateful for confirmation and / or a correction. Keeping it local, from the thread I link to below -

http://www.electriciansforums.net/e...-forum/30644-calculating-lb-voltage-drop.html


Now I understand the Ib <= In <= Iz. However, High Tower seems to suggest that you apply both diversity and the various correction factors to derive your design current, Ib. I believe that to be incorrect, as the value for Ib is found after the application of diversity, if needed, then the correction factors are applied to In, the nominal rating of the selected protective device.

Can someone confirm this? I have some other questions on diversity right now, but will hold back for the time being.

I'm not really sure I understand your question Jack as you seem to be contradicting yourself a little but as an example suppose you had a 10Kw electric hob to install, you'd look up your allowed diversity on hobs which would lower the demand because it assumes that not all elements are going to be switching themselves on at all times. Suppose for arguments sake that the diversified load was now 8Kw you're then going to look at a cable capable of carrying the load (8000w/230=34.8A)
You'll then apply the correction factors.
That's the simplest way I can put it given my potential misunderstanding of your question mate.

- - - Updated - - -

Hope that sorts it.

 

[Happy Jack]

I'm not really sure I understand your question Jack as you seem to be contradicting yourself a little...

I appreciate it isn't particularly concise. I've tried to trim it back for the main forum, but it doesn't appear to be going too well over there, aha! It's just trying to be precise with my terminology.

What you've said (8000w/230=34.8A) would be Ib, is that right? I believe the step after that is to select your protective device, In. In this example that value would be 40A, and then the correction factors are applied to that. As is - 40 / Ca Ci Cg Cf etc.

 

[Happy Jack]

I think your question has merit so by all means copy/paste your OP and start a new thread in the General Electrical forum.

http://www.electriciansforums.net/e...-diversity-standard-circuit-arrangements.html

Okay. It didn't really work, haha! I'll bump it Sunday or early next week.

 

[trev]

I appreciate it isn't particularly concise. I've tried to trim it back for the main forum, but it doesn't appear to be going too well over there, aha! It's just trying to be precise with my terminology.

What you've said (8000w/230=34.8A) would be Ib, is that right? I believe the step after that is to select your protective device, In. In this example that value would be 40A, and then the correction factors are applied to that. As is - 40 / Ca Ci Cg Cf etc.

The correction factors are for the current carrying capacity of the cable Jack, nothing to do with the OCPD. The object being to ensure that it can dissipate any heat arising from carrying the current and to derate it.
EG a cable capable of carrying 20 amps in free air is buried between layers of insulation so the tables say in that circumstance it can only get rid of half the heat it normally can in free air so it's ccc is halfed

 

[Happy Jack]

The correction factors are for the current carrying capacity of the cable Jack, nothing to do with the OCPD. The object being to ensure that it can dissipate any heat arising from carrying the current and to derate it.

That is where the confusion lies. I understand why the correction factors are applied, and that they relate to CCC. While the article is to the 16th, the procedure as listed is how we've been taught. Is that not the correct approach?

http://www.tlc-direct.co.uk/Book/4.3.9.htm

 

[steve100]

We where also taught that CCC is calculated on the In, which is also how it is described in the OSG (green copy), it also gives the equation Ib <= In <= Iz... I also found a good description of how to caculate CCC from the NICIEC which gives the above way...so which is correct?