Feeder cable sizing bs7671

[STONE85]

Hi,
With regards to a feeder cable feeding a small building (bs7671) I have reading conflicting information on whether to use the diversified load or the undiversified load.

Say building load is 150kw then after diversity (0.8)and load utilisation factor( 90%) this drops to 108kw. For the sizing of the feeder cable would the design current be calculated from 150kw or the after diversity rating of 108kw?

Thank you

 

[che12345]

Dear Mr. STONE85
"...With regards to a feeder cable ...(bs7671). I have reading conflicting information on whether to use the diversified load or the undiversified load. Say building load is 150kw then after diversity (0.8)and load utilisation factor( 90%) this drops to 108kw. For the sizing of the feeder cable would the design current be calculated from 150kw or the after diversity rating of 108kw?.."
1. It may be based on 108kW load.
2. Otherwise, why are you calculating "diversity (0.8)and load utilisation factor( 90%)" for ?
Che Kuan Yau (Singapore)

 

[STONE85]

Thanks for this. That’s exactly what I was thinking.

 

[STONE85]

Hello,
I have a further query related to this topic if that’s ok. Say we have have three machines each rated at 100kw at full load. From the information I have there is diversity and load Ultilisation applied. This lowers the three machines and the third machine load figure is extremely small. I believe it would be best to size each of the cables/protection for the full load of 100kw even if I’m reality they won’t run at full load.

Is there something I’m missing or would you size each based on it’s diversified load?

Thank you

 

[che12345]

Dear Mr. STONE85
"...I have a further query ..... Say we have have three machines each rated at 100kw at full load. From the information I have there is diversity and load Ultilisation applied. This lowers the three machines and the third machine load figure is extremely small. I believe it would be best to size each of the cables/protection for the full load of 100kw even if I’m reality they won’t run at full load... or would you size each based on it’s diversified load?..."
I think you are confused. There are two different issues.
1. the conductor size from building A to B may the sized to taking into consideration of " diversity and load Ultilisation" . This is making the assumption that NOT all THREE motors are going to run at (100kW each) FULL LOAD at the SAME TIME , for long hours.
1.1 It does NOT mean that it "lowers the three machines and the third machine load figure is extremely small".
2. In building B, the incoming breaker and busbar (if any) may be sized to, see above 1).
2.1 However, each of the three breakers, conductor size and the motor thermal over-load on the starter; feeding to each motor shall be rated for 100kW, i.e. NO " diversity and load Ultilisation". This is to ensure that all the three motors can be loaded to 100kW full-load any time. Attention: NOT all three at full-load for a long time.
Che Kuan Yau (Singapore)

 

[STONE85]

Thanks for this. Just to be clear.


From my second post. If we have three 100kw machines then the supply to each one of these must be rated for 100kw. The feeder cable supplying the building housing the machines may take into consideration that these machines will not all be running at the same time at full load?

Thanks again.

 

[che12345]

Dear Mr. STONE85
".... Just to be clear....
1) If we have three 100kw machines then the supply to each one of these must be rated for 100kw...". YES.
2) "...The feeder cable supplying the building housing the machines may take into consideration that these machines will not all be running at the same time at full load?...[add for long period of time] ". YES.
3) BTW (SI unit of Power) is [watt], not [Watt] . But its (symbol) is [W] not [w]. Therefore, 1000 W = 1 kW not 1kw. The machine is [100kW ] not 100kw.
Che Kuan Yau (Singapore)