Max Demand/Design Current/Full Load Current

[Steve100]

I am getting a little confused over the difference/definition between max demand/design/full load currents, can someone explain this in simple terms please?

 

[m4tty]

Have you got a copy of the on site guide matey it's all in there.

 

[Technician]

Maximum demand; an estimation of maximum current

drawn by an installation considering that not all

loads are used at the same time.

Design current; either a given or calculated based

upon plate data or anticipated value in one circuit.

I.E. the current the circuit is INTENDED to carry.

Full load currents; similar to above but NOT including

starting currents/inrush currents. Should include the

current a motor could draw under full load.

As Matty says, OSG.

 

[Doc Hudson]

Think of max demand relating typically to multiple final circuits fed from one distribution board, such as a domestic installation or a sub main feeding part of a larger industrial installation. Where not all final circuits are drawing full power simultaneously. Although some individual final circuits do have their own diversity due to the load connected, such as a cooker that is not using all rings, grills and ovens simultaneously. Whereas the design current and full load currents relate to your calculations used when selecting cables for a single final circuit with a specific known, fixed load, or assumed load, socket outlets, to be connected.

For example a domestic installation may have a 9kW shower a 3kW immersion heater and cooker circuit that has 12kW worth of elements but typically only 8kW is running at one time. Although the three circuits could potentially be 24kW if all added together. The maximum demand may still only be 9kW as the user is not cooking whilst in the shower and the immersion only comes on a night during economy 7 rates. The individual circuits must have cables suitable for connected loads design currents but the distribution board and incoming tails only need to be large enough to cater for the maximum demand after diversity applied not the sum of all the design currents.

Doc H.

 

[Steve100]

Thanks for all your help, did look in the OSG (appendix H) but couldn

 

[Technician]

Thanks for all your help, did look in the OSG (appendix H) but couldn

 

[Sidewinder]

Steve,

You might want to take a look at the books by Brian Scadden, a forum sponsor, as they are quite good, I have them

Also there is the Part P Doctor, I have heard that this book is also very useful.

Then there is this:

Electrical Installation Design Guide: Calculations for Electricians and Designers - IET Electrical

This is also a good book.

Sometimes it takes reading around a topic for the ideas to formulate and provide the key to understanding it.

HTH

 

[Marvo]

Max Demand may also take power factor into account. Design current and FLA doesn't.

 

[Manator]

Max Demand may also take power factor into account. Design current and FLA doesn't.

Electrical engineers calculate everything, the diversity tables in the OSG are a basic fast track calculation. Remember designers of the installation are required to ensure its safety in use and in maintenance.

Designers calculate all loads and the loads are based not on a day to day basis but on a year or in some instances several. Power factor plays a major role in the design process it is also vital for larger installations to help reduce running costs.

I use data supplied by existing installations (those that have fully monitored systems), this helps to see peaks and overall usage over the last year or so, and based on that data other installations can be designed.

 

[Marvo]

Electrical engineers calculate everything, the diversity tables in the OSG are a basic fast track calculation.

I wasn't trying to complicate or confuse the issue I just wanted to highlight a difference that may be relevant. I have never seen the UK on site guide, nor the more recent UK regs so I don't know how or if a domestic or commercial electrician would be required to consider PF or even harmonics in his loading calcs.

 

[Doc Hudson]

I am getting a little confused over the difference/definition between max demand/design/full load currents, can someone explain this in simple terms please?

Max Demand may also take power factor into account. Design current and FLA doesn't.

Another valid point, I think the OP was just trying to get his head around the concept of Max demand and how it relates to individual circuits design currents. But your additional comment raises the concept that the person responsible for the works must use their expertise to bring into play all relevant factors when considering individual circuits and whole distribution boards loading and rating of cables and equipment. The On-Site-Gide does say that its tables regarding diversity are only basic guidance and that all specific installation characteristic must be taken into account. The bottom line is there are no quick fix easy read table to answer all loading and demand calculations.

Doc H.