Help! Calculation maximum demand and diversity for 2396 project.

[Groot2024]

Hi All,

This is my first time posting although I have been watching from afar for years. I am a fully qualified electrician working in the industrial sector in the UK.

I completed my City and Guilds 2396 exam with a pass approximately 2 years ago but have been putting off the project since. I could make excuses but the fact of the matter is, I have never found the motivation to start, until recently!!

I really wish I competed the project sooner as all the information I gained on the course has left me.

I’m confused and probably over thinking a lot but I hope someone on here can help me please.

When calculating a 3 phase sub main cable suppling a DB containing both power and lighting single phase circuits, where do you start??

I have calculated max demand for each individual circuit but how do I relate this to the sub main supply.
Would I calculate the load for each phase?

I would also group multiple socket circuits and apply diversity to them but not sure how this works when calculating the load on each phase?

I know how to calculate the max demand and size the cable appropriately for a single 3 phase circuit and I am also aware of multiplying VD from sub main and final circuit but can’t seem to get my head around the max demand of an entire DB with single phase circuits when applying diversity.

I’m sure it’s obvious

Many thanks in advance,

Stuck!!!

 

[Tom256]

Going into the kingdom of the dart arts here hence the need to show an understanding of the loads, usage and special consideration such as future demand for the installation. The designer will need to show an understanding of the demand for the installlation and even then it can never be said to be an exact science.

As such set out the loads, time demands each day and usage, an installation outline will help.
With no specifics in mind by way of a starting point to jog memory the demand on a Sub-Distribution board can be assessed by total installed load (Pmax) (Pdb) x demand factor summed. Tables for demand factor are widely available both for complete installations amd individual sub-boards with mixed loads.
The base calculatiuonis PDB = P1g1 + P2g2 ….. Pngn

P - being loads under continuous duty cycle
g - being demand factor for the load for the type of installation

Pngn = sum of individual types of loads eg, lighting , can also be used

With loads assessed how they are to be allocated to each phase can be set out using a simple spreadsheet

 

[Groot2024]

Going into the kingdom of the dart arts here hence the need to show an understanding of the loads, usage and special consideration such as future demand for the installation. The designer will need to show an understanding of the demand for the installlation and even then it can never be said to be an exact science.

As such set out the loads, time demands each day and usage, an installation outline will help.
With no specifics in mind by way of a starting point to jog memory the demand on a Sub-Distribution board can be assessed by total installed load (Pmax) (Pdb) x demand factor summed. Tables for demand factor are widely available both for complete installations amd individual sub-boards with mixed loads.
The base calculatiuonis PDB = P1g1 + P2g2 ….. Pngn

P - being loads under continuous duty cycle
g - being demand factor for the load for the type of installation

Pngn = sum of individual types of loads eg, lighting , can also be used

With loads assessed how they are to be allocated to each phase can be set out using a simple spreadsheet

Thank you for your reply Tom256,

I’m not sure that has cleared things up for me. Dark arts it definitely is.

The installation is for a new library, the loading on the sockets will be minimal, maybe charging laptops and phones etc.

DB3 (3phase) on my design has a total loading of 103A.
That includes:
1x16A Lift circuit
4x 20A Socket circuits
3x Lighting circuits. (800 - 200w)

Do I design the sub-main supply cable to 103A or do I apply diversity first?
For example, in the OSG table A2 P138 (Brown book) it states ‘100% of largest circuit and 50% of remaining circuits’ for final circuits and ‘90% of total current demand’ for lighting?

Or do I calculate diversity in my professional opinion and use that figure to calculate sub main supply??
Then it’s per phase but the phases are not complete balanced.

Am I missing something?

 

[Tom256]

Don't overthink it. First step make it clear and set out how you have determined the demand such helps with those using the installation and it can become a part of the hand-over documentation.
It can only ever be a judgment backed by information gathered at this stage. A installation in use can be power monitored and measured over a period but that does not apply.
The more you understand the use of the building the greater the accuracy within this figure.

You should always calculate the demand after diversity first on any project without such how can the DNO know what you require by way of a supply. If you calculated cabling without any diversity applied you would not have a relaistic design.
I mention a load diagram for this reason as different areas within a building may see different periods of demand that impact the Pmax (demand at source)

You may choose to use design current in the demand calculations such as with your lighting loads, where it is known, or use the device rating with such circuits as sockets. Using design current can help to produce a realistic figure that does not require over spec cabling sizes and costs.

The on-site guide provides a couple of methods for demand calculations however there are other methods. I am sure you are aware that the DNO's calculate for a group of complete instalations. On domestic properties they may allow a maximum demand average of ~2kw / property.
No one method, there are demand factors for circuits within specific types of buildings and demand factors for sub-boards.

With regards the phases I use a spreadsheet set out in excel. Devide your loads over the three phases and play around with them. Do so to achieve a degree of balance then work from that point.

Consideration on the lift motor circuit in that it may be subject to frequent stop / starting. Consider the effects of starting currents and the effect on cable temperature if the lift is in use throughout the time the premises is open. A factor of 1.4 x full load current is often advised.