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#84564 - 04/11/03 08:10 AM How to monitor utility power ahead of disconnect
arclight Offline
Junior Member
Registered: 04/08/03
Posts: 2
Loc: Manassas, VA USA
All: I have several thousand locations where I have a single-phase service (a few are 3-phase), a transfer switch to allow me to bring in a portable generator to run the facility, and a typical load center. I am being asked to provide status of utility power ahead of the transfer switch. Technically this is a no-brainer (a 240 V relay would work nicely) but I am stumbling over how to make this NEC-compliant. NEC 230-82 tells me I can have either an instrument transformer or a "load-management device" ahead of the service disconnect, but I am foggy on how to size and protect the connection to the service entrance conductors.

I had considered installing a small 2-circuit load center with a relay on the output side of the breakers in question, but I am afraid this is overkill, and for several thousand installations any extra cost is a big negative. Is this the right approach? Assuming it is, how do I make the connection from this new load center to the service entrance conductors (which are screwed down to the appropriate lugs on the transfer switch)? Do I have to size the supply conductors the same as the 200-A service conductors? I presume so, but again I don't want to overkill.

As you can see NEC interpretation is NOT my strong suit, so I figured I'd post here and see what advice I could get.

Comments / suggestions are all welcome.
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#84565 - 04/11/03 10:27 AM Re: How to monitor utility power ahead of disconnect
resqcapt19 Offline
Registered: 11/10/00
Posts: 2148
Loc: IL
If you really need to be on the line side of the existing service disconnect, you will have to install an additional service disconnect. You can use some type of tap device to tap the service entrance wires and run to the line side of the new disconnect. The tap device would not be required if the terminals on the service disconnect are suitable for more than one wire (not likely). These tap wires can be sized to the new smaller service disconnet. The new disconnect must be in the same location as the existing one. I don't think that the minimum size requirement in 230.79 applies to a service disconnect for power monitoring equipment.
Why do you need to be on the line side of the service disconnect?
#84566 - 04/11/03 10:30 AM Re: How to monitor utility power ahead of disconnect
Ron Offline
Registered: 03/13/02
Posts: 582
Loc: White Plains, NY
Many transfer switches have an aux contact for monitoring.
I would suggest that you do not want to monitor the utility side of the main disc., but the load side of the transfer switch. You might not care if the utility power is available if the main is tripped, hense the monitoring on the load side.
#84567 - 04/11/03 12:02 PM Re: How to monitor utility power ahead of disconnect
arclight Offline
Junior Member
Registered: 04/08/03
Posts: 2
Loc: Manassas, VA USA
Thanks for the replies.

As things are today the transfer switch is a manual ON-OFF-ON device. In some jurisdictions this apparently has been allowed to pass as a disconnect device as well. The application requires that we be able to sense the presence of utility power regardless of the position of the ON-OFF-ON switch.
#84568 - 04/11/03 02:22 PM Re: How to monitor utility power ahead of disconnect
Bjarney Offline
Registered: 04/10/02
Posts: 2527
Loc: West-Southern Inner-Northeast ...
It is remotely possible that something like a Buss HEB holder/KTK fuse could be acceptable for the tap conductor. They have accessory rubber insulating boots and some, a provision for two larger conductors on one end—sort of a feed-through feature.

There may be a version that accepts a UL class-CC fuse [200kAIR] making it a true branch-circuit-protective device. The load side of the holder is intended to keep the fuse captive on occasional opening of the holder.

If service is 120/240V, simplification using one fuse and a 120V relay may do. {There are otherwise similar two-pole holders available.}

Littelfuse and Shawmut may have similar… The “breakaway” feature is probably undesirable—it’s to save a roadway light standard from being dragged by a crashed vehicle.

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