Sizing for Duty/Standby pumps

[usher00]

Hi,

i need some input on this please;

i have a duty/standby pump configuration.(one of them running at a particular time).

right now the wiring of the to the pump includes a single point power with a main disconnect with 2 secondary disconnects for each pump. the main disconnect/wire are sized on the assumption that both pumps are running.

will it be possible to size down the main disconnect and wiring to as if only one pump is running as it is an duty/standby configuration?

 

[ScottyUK]

Does something, e.g. a mechanical interlock, prevent both pumps operating simultaneously?

 

[usher00]

there is no mechnaical interlock, but there is a software interlock as the pumps are connected to the vfds which are connected to the plc.

 

[MacGyverS2000]

I would not rely on software to prevent things from turning on/off... electrical glitches have a funny way of rewriting software on the fly

Dan - Owner

http://www.Hi-TecDesigns.com

 

[usher00]

how about using a hardwired interlock between the VFD's(n/o contact to Run permissive of the other?) does this follow NEC code?

 

[ScottyUK]

Can't comment on the NEC, not my patch at all.

Is there any possibility this will operate as a duty-assist mode rather than duty-standby, i.e. if the duty pump can handle the flow then the second pump stands idle, but if the duty pump can't cope alone the assist pump also cuts in?

How big is the cost difference between cable sized for one pump and cable sized for both? Are you the client, or the contractor? Is the small cable already installed, or is this question still on paper?