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In my house I have a main panel with 3-50 amp breakers feeding a subpanel right next to it. The main panel has an opening for a double breaker.

I plan on adding a 50 amp 2 pole breaker to the main panel that will feed a second subpanel in a building that will have a 30, 20, and 15 amp breaker. It will be 240/120 service. I'm using #4 gauge aluminum service wire direct burial between house and building.

Is there a problem with any of this?

  • How long is your run, is it above-ground or buried, and does your cable have 3 wires or 4? – ThreePhaseEel Oct 17 '17 at 22:40
  • Do you have any other conductors running between the house and the building? These might be electrical service wires or other types of conductors - for example, iron water pipe or telephone wires. – Jean-Paul Calderone Oct 17 '17 at 22:46
  • The run is 120 ft and i am burying.the service wire feeding the house runs underground through the back yard. – L brewer Oct 17 '17 at 23:20
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    I am running 4 wire to bldg. – L brewer Oct 17 '17 at 23:21
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    Three 50A breakers feeding one subpanel is paralleling in a manner that is normally not allowed. You can only parallel from an enclosure specifically made to parallel, and only at wire sizes/amperages well over 50A each. – Harper Oct 18 '17 at 4:46
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Use conduit

Putting in some 2" schedule 80 PVC now is a cheap investment compared to having to dig up a busted cable to replace it.

Torque your lugs to spec!

Get an inch-pound torque wrench and torque your breaker and panelboard lugs to manufacturer's spec. This is a new requirement with the 2017 NEC, spelled out in 110.14(D), and simply good practice anyway, especially with aluminum (which is a bit more sensitive to torques than copper is).

Make sure the outbuilding is grounded

Since you have a feeder to a subpanel at the outbuilding, you'll need to put in a ground electrode at the outbuilding and wire it to the ground bar in the subpanel, which will be separate from the neutral bar, by the way. (Make sure to pull that bonding screw or strap in the new subpanel!)

  • Can direct burial cable be run in conduit? – L brewer Oct 18 '17 at 2:27
  • @Lbrewer -- yeah. it's annoying to pull, that's all (hence the generously sized conduit) – ThreePhaseEel Oct 18 '17 at 2:39
  • Usually, you only put direct burial in short lengths of conduit as it emerges from the ground up to a panel. It is allowed by the Code. – ArchonOSX Oct 18 '17 at 7:37
  • @ArchonOSX -- yeah, stub-ups are the more typical way of doing things, but running the whole way in conduit is what I'd do if I already had the cable at this point. – ThreePhaseEel Oct 18 '17 at 11:37
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I agree with ThreePhaseEel and would add a few things:

Make sure your feeder is 4 wires since it is a new installation. Two hots, a neutral, and a ground. The conductors need to be rated for direct burial.

According to Table 310.15(B)(16) of the National Electrical Code #4 Aluminum at 75°C is rated for 65 amps of current. Section 240.4(B) allows you to use the next higher standard size breaker in this case. Since 65 amps does not correspond to a standard size you could then use a 70 amp breaker. Since a 70 amp breaker and a 50 amp will be the same price you could use the 70 amp as long as the sub-panel is rated for 70 amps or more. Usually this would be a 100 amp sub-panel since these only come in certain standard sizes.

Also, as Harper points out in the comments, your first sub-panel should not be fed by multiple breakers. This is a Code violation and safety issue. Whatever the sub-panel is rated for, you should feed it with larger wire rated for that amperage from a breaker sized for that panel. Your conductors need to be sized for the breaker size. Essentially, you should reduce this feed to a single breaker e.g. a single 100 amp breaker feeding a 100 amp sub-panel with 100 amp wire. This would have an added benefit of opening up 4 more slots in your main panel.

Good luck and stay safe!

  • you make it sound like the wires and subpanel must match the breaker. It's totally ok for the subpanel or wire to be larger capacity than the breaker feeding it, so a nice comfy 225A panel can (and should) be used downline of, say, a 100A breaker. – Harper Oct 18 '17 at 14:53
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Sounds good to me. You'll be within 3% voltage drop which should be fine.

I'd maybe recommend laying this cable in conduit so its better protected from excavation and damage later on and easier to service. 120 ft is a lot of digging to do twice!

But your plan sounds good.

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