Maximum Demand - Standby and Duty Motors

[DrDrreeeaaa]

Hi Again All,

When doing maximum demand calcualations I assume one does not consider both the duty and standby motors where the duty/standby arrangement is provided. Both motors are the same size etc.

Functionally I am not sure how they operate and if they were to both run at the same time, i.e. when swapping from the standby pump to the duty pump, I would be in trouble as my TF would not handle the load

Any thoughts?

 

[wayne440]

I respectfully submit that it would be prudent to learn more about the switching scheme between duty/standby units, and if in fact both will run simultaneously at any time.

 

[jghrist]

Because of the significant thermal time constant of transformers, there may not be a capacity problem if the motors only run at the same time for a short period. There may be more of a voltage drop or motor starting concern.

 

[DrDrreeeaaa]

Yes I thought about it in a similar way.

There is nothing in the local codes over here (Australia) that says your point of supply has to be able to sustain all the appliances on your network.

 

[rbulsara]

123MB:

If you want to be a good engineer, you will have more success it you design a system that "works". Code are intended for safety and they do not care if a system works, but your client would who is paying the bills.

So the onus is on you to to design a sytstem that meets Owner's requirements. Codes are minimum statutary requirements that needs to be met and not a design criteria.

 

[DrDrreeeaaa]

Fair enough, Thanks.

 

[jghrist]

The transformer current during the time that two pumps are running will be less than the starting current of one pump. I wouldn't worry about transformer capacity.