Conductor Size for Control/Reference Tap on 3000A Bus

[hcubed]

Hi Guys,
I'm trying to find some guidance in the NEC on a voltage reference tap and having trouble.

So, I'm working with a developer that has a load monitoring device (like an Emon Dmon) that they want to install in a customer's gear. The gear is a 3000A bus that goes has only a 3000A bus duct tapped off of it - there are no breakers for the circuit except for several floors up. Now, if I want to connect the voltage reference for the monitoring unit, I keep ending up at the 10ft tap rule. I would tap off the bus with 350kcmil (10% of 3000A is 300A...) and go to a 30A fused disconnect with 15A fuses. Then I'd connect the voltage reference for the monitor there...seems overkill for something that might pull 100mA at most.

However, we all know when you look inside gear we see customer monitoring units just tapped directly to the bus using #12 or #14 and either inline 600V fuses or some other UL508 mini circuit breakers. Anyway, I'm wondering in the code where it says you can use a greatly reduced wire size if it's for control or reference voltage or monitoring, or something along those lines?

Thanks.

 

[iwire]

Take a look at 240.21.

 

[hcubed]

Ha, thanks. I'm trying to understand if there's other code that allows for control or reference wiring to be of a reduced size in a tap situation. 240.21(B)(1) gets me to 300A conductor on a 3000A bus, but that seem ridiculous for a load monitor.

 

[iwire]

Ha, thanks. I'm trying to understand if there's other code that allows for control or reference wiring to be of a reduced size in a tap situation. 240.21(B)(1) gets me to 300A conductor on a 3000A bus, but that seem ridiculous for a load monitor.

Are you really going to leave the enclosure before getting to the OCPD?

240.21(B)(4) For field installations, if the tap conductors leave the
enclosure or vault in which the tap is made, the ampacity
of the tap conductors is not less than one-tenth of
the rating of the overcurrent device protecting the
feeder conductors.

 

[infinity]

Are you really going to leave the enclosure before getting to the OCPD?

That's the key having the OCPD within the enclosure so the tap conductors do not leave. That's how all of our panels with sub-metering are set up.

 

[iwire]

That's the key having the OCPD within the enclosure so the tap conductors do not leave. That's how all of our panels with sub-metering are set up.

Same here, any needed OCPD we add right inside the enclosure containing the tap.


We used to use inline fuse holders but now more often three pole versions of these

 

[hcubed]

I see. Thank you.

Two follow ups:
1) So, if you have your OCPD inside the gear then you can basically use whatever conductors you want as long as you comply with 241.21(B)(1)(1)-(3). I assume then you can run your conductors from the OCPD inside the gear to your monitor or whatever outside. The monitoring equipment doesn't also have to be inside the gear, does it?

2) The plan checkers at the City/County of Honolulu have specifically forbidden us to tap anything inside the switchgear because even tapping conductors (and, in this case, I'm talking about tapping conductors with insulation piercing connectors, not tapping the bus) inside the gear could be considered a "modification" to the switchgear and would need to be included as a "listed modification" in the switchgear manufacturer's documentation. Any ideas here?

Thanks.

 

[steve66]

I see. Thank you.

Two follow ups:
1) So, if you have your OCPD inside the gear then you can basically use whatever conductors you want as long as you comply with 241.21(B)(1)(1)-(3). I assume then you can run your conductors from the OCPD inside the gear to your monitor or whatever outside. The monitoring equipment doesn't also have to be inside the gear, does it?

2) The plan checkers at the City/County of Honolulu have specifically forbidden us to tap anything inside the switchgear because even tapping conductors (and, in this case, I'm talking about tapping conductors with insulation piercing connectors, not tapping the bus) inside the gear could be considered a "modification" to the switchgear and would need to be included as a "listed modification" in the switchgear manufacturer's documentation. Any ideas here?

Thanks.

1). Yes. Once the conductors hit the fuse (say we are using #12 wires and 15 amp fuses), they are protected at their ampacity, and no longer considered tap conductors. So they may leave the enclosure and run to an external meter with a chapter 3 wiring method (like conduit).

2. That could be a tough one, unless you can get the manufacturer to provide instructions that allow tapping the bus. Or maybe if there are factory holes already in the buss, maybe they would allow a tap to be made there. There may also be an option to have UL do a field listing where they come out and field inspect the modification. Expensive and time consuming, but others may know more about this.

Any chance on adding a bus plug to the bus duct, and supplying the meter from that? Or adding a distribution section to the switchboard if you have space? I know that seems extreme to supply a power meter.

 

[hcubed]

OK. Got it. Thanks.

Luckily the current project is not in HI and I don't expect to run into the listed modification issue, but just curious if anyone has had the same experience. If we do run into that issue with permitting then we'll go the bus plug route.

 

[infinity]

2) The plan checkers at the City/County of Honolulu have specifically forbidden us to tap anything inside the switchgear because even tapping conductors (and, in this case, I'm talking about tapping conductors with insulation piercing connectors, not tapping the bus) inside the gear could be considered a "modification" to the switchgear and would need to be included as a "listed modification" in the switchgear manufacturer's documentation. Any ideas here?

IMO using an IPC on a conductor is not modifying the equipment. Most switchgear has some extra holes in the bus somewhere that can be used to attach you tap conductors or your may find a long bolts on the bus where you can attach a ring connector with an extra nut. IMO neither of those methods would be considered a modification.

 

[hcubed]

I completely agree. Thank you!