Line Side Tap for metering, overcurrent protection?

Question

I have a generator behind my house. It has its own internal breaker for overcurrent protection.

The generator has a disconnect next to it. Eaton Heavy Duty 200 amp, DH364URK. It is unfused given that the generator has its own internal circuit breaker.

I'd like to monitor the output of the generator (Voltage and Frequency) with a few analog meters in a NEMA 3R enclosure next to the disconnect, using a line side tap off the disconnect.

The wiring to the meters needs overcurrenent protection. This can be provided by fuses.

Problem is that I cannot find a small fuse holder that will take large gauge wire, nor can I double tap two large gauge wires into the lugs on the disconnect.

Is there a proper way to protect the wires between the disconnect and fuse holder? (Colored yellow in the attached picture)

Answer 1

This sort of intra-box tapping is considered OK when kept within reason

You are by far and wide not alone in the situation you run into, where a small control circuit conductor needs to be tapped off of a fat power conductor. In fact, this happens enough in industry that there are special tap conductor rules that apply to this situation, both within the NEC itself, and within NFPA 79 for industrial machinery. While the latter isn't directly applicable here, it's more relevant to an intra-box situation, and can be considered a subset of the NEC's "10-foot tap rule":

7.2.8 Location of Overcurrent Protective Devices. An overcurrent protective device shall be located at the point where the conductor to be protected is connected to the supply except as follows:

(1) Overcurrent protection at the supply shall not be required if all of the following conditions are met:

(a) The current-carrying capacity of each of the conductors is at least equal to that required for their respective load, in accordance with Section 12.5.

(b) Each connecting conductor to the overcurrent protective devices is no longer than 3 m (10 ft).

(c) The conductor is suitably protected from physical damage.

(d) The conductor does not extend beyond the control panel enclosure.

(e) The conductor terminates in a single branch circuit overcurrent protective device.

As a result, I'd consider a tap conductor situation acceptable. I'd use a set of 1/2" stud ring terminals atop the existing lugs (this sort of stacking of terminals is permitted at a base level by the UL White Book, provided the lug flats are making good contact) with 14AWG THHN, routed up and alongside the line conductors until it clears the inter-pole barriers, to make the tap. This rule also means you need to use branch-circuit rated overcurrent protection, which in your case means Class CC fuses in matching rejection fuse holders instead of "midget" or smaller electronic-style fuses, and mount the overcurrent protection inside the safety switch, which should be feasible given that you are running a 2-pole circuit through a 3-pole switch.

Answer 2

Here are two ways current protection could be achieved for the wires off the generator lines to the meters:

• Swap out the Eaton disconnect for a ranch panel. These panels have a main breaker at service-level current and some few spaces for branch circuits. Some if not all ranch panels are outside-rated, too. You could use the main breaker as the official disconnect, and use one of the branch circuits for the meter wires, which could easily be 14 gauge off a branch circuit breaker. The line to the transfer switch could be another branch circuit breaker in the ranch panel, which would give you the ability to kill the house while keeping power at other breakers in the ranch panel. Or some panels may have "through lugs" where the power can come right out at the other end of the bus and continue on to the house (this setup would require killing power to the whole ranch panel to turn off the house connection). You would also have space in the ranch panel for 120V and 240V outlets and/or lighting at the generator if desired.

• Use 3-port multi-tap connectors compatible with 2/0 gauge to send 2/0 to both the disconnect input lugs and through conduit to a 2/0-compatible fuse box, with the smallest-current fuses possible in the fuse holders. If you can get 15-amp fuses for this 2/0 fuse holder, and the fuse holder allows 14-gauge on the outputs, then you attach the meters and you're good. If 15-amp fuses are not available for the 2/0 fuse holder, or the fuse holder does not allow down to 14-gauge wires, then go by stages using compatible-gauge intermediate pigtails to another set of fuse holders, accounting for minimum fuse sizes in the 2/0 holder vs appropriate gauge pigtails, then get down to 15-amp fuses and 14-gauge wire for the meters. The second stage fuses may be small enough to be inline.