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So our building has 3, 3 Phase 200A 120/208 I-T-E Indoor Load Centers, each on their own meter. Originally the building was built for 3 suites, 3 tenants. We have the entire building, so we have all 3 interior panels.

One of those panels (Panel B) has a 50A double-pole circuit leading to a 60A switch which has two 50A fuses. That circuit powers a 19" Rack Mount Leviton DS-8 Lighting dimmer with 2400W per channel and 8 channels running on 208V as required.

The project at hand is to move this setup to one of the other panels (Panel A), relocating the entire lighting dimmer pack altogether, and capping off the previous setup.

PROBLEM: Panel A is full, but I found that one of the 30A double pole breakers is switched off, and in fact disconnected from anything. So, we have open real estate in Panel A.

But, we need more, so I'm thinking sub-panel, approximately 36 feet from main panel A.

Question 1: Is this a viable plan?

  • Remove the 30A breakers and replace them with 100A breakers. If I'm not mistaken, and I haven't tested it yet, we would be sending 208V to the sub-panel, not 240V.
  • Feed two hot leads, one ground, and one neutral through 1 1/4" conduit to a surface mount 125A sub-panel from the 100A breakers in panel A.
  • Ensure ground and neutral are isolated from each other in sub-panel.
  • Install two 50A breakers to feed the 60A fused switch with 50A Fuses.
  • Mount the 60A switch next to the sub-panel and connect it to the lighting dimmer.
  • Install one or more 120V 20A breakers for server / sound equipment needs in sub-panel, etc.

Question2: There will be 4 wires feeding the sub-panel. What are the minimum gauges that would need to be run for a 36' distance at 100A, 208V?

Thanks for your insight.

  • Can the fused switch be replaced with a non-fused switch? It seems that all it's doing is acting as a local disconnecting means for the dimmer rack.... – ThreePhaseEel Nov 20 '19 at 5:27
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Yeah, that plan seems fine. Three #1Al THHN wires should get the job done, and will fit comfortably in 1-1/4" EMT pipe. You don't need a ground wire in EMT, IMC or rigid conduit.

You sometimes hear of neutrals being downsized compared to the hots. You can't generally do that with feeders, but especially, you can't do it when pulling 2 of 3 phases of 3-phase "wye". Here, neutral does not handle only differential current, and it gets a real workout - neutral flow can be as much as the hots. (but not more).

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