Main Disconnect sizing

[xJAyx]

I am working on designing a control system for a duplex pump station.
The load consists of 2 5hp motors and a small control load.
Supply is 600V. Both motors have an starting current of 31A, and an FLA of 5A. The transformer supplying the control voltage is 3KVA. All branches (both motors and the transformer) have invidual overcurrent protection.
The control system on the motors is set up so that the motors do not start at the same time, however, if the motors are put in manual operation (only used for testing or to draw down the level of the well) at the same time (Normally this would not happen, but it is a possibility), they could possibly be starting at the same time.
I am sizing a main power connector for the system, is a 60A rated disconnect acceptable? The code is unclear on this issue, as the motors are not required to start simultaneously, it is just a small possibility.

 

[jraef]

If you have 1 main disconnect for the entire panel, it must be rated and fused for the maximum current rating of the single feed cable required to feed it (the fuses don't have to be with the disconnect, just in the circuit somewhere). In your case, you will have 10HP connected load, which could be fed with 14ga wire, so technically a 15A disconnect could be used. However, you must also have a HP rating on the disconnect equal to the connected load so that inrush current is accounted for, so I think you will find that a 10HP 600V rating will only be found on a 16 or 20A disconnect. Either way, 60A is plenty (they are typically rated for 30HP @ 600V).

"Venditori de oleum-vipera non vigere excordis populi"

 

[light1]

The main disconnect is supposed to be rated for the worst case current and HP load.
Two 5HP motors with a small chance being turned on simultaneously will result in 10HP load.
30A disconnect with a fast fuse is often rated 7.5HP and with a dual element fuse 20HP, both at 600V.
60A disconnect with a fast fuse is often rated 15HP and with a dual element fuse 50HP, both at 600V.
Therefore, the selection will depend on the selection of a type of the fuse while the worst case HP load is considered.