Running a subpanel off another subpanel

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Old 05-04-16, 01:14 PM
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Running a subpanel off another subpanel

Hi, I am new to the forum and have been looking around, and have already learned plenty! I am a mechanical engineer located in Jacksonville, FL. I recently built a detached garage about 80 from the house and I am considering how to run electrical service to it. The 200A main breaker panel is on the opposite end of the house from the garage; however, there is a 125A subpanel installed in an addition which is on the side of the house closest to the garage. After doing some research, I believe I can run a 100A subpanel in the garage off the existing subpanel.

The existing subpanel is wired with 2/0 AL SEU cable run through the attic from the main panel. Circuits on this subpanel included a kiln which is no longer there, a water heater which has been relocated (now runs off the main panel), and the well water pump. When everything is finished, the only circuits that will remain on the existing subpanel (other than the garage subpanel) are a 2-ton heat pump, along with the 110V lighting and outlets for the master suite, family room, and 1-car attached garage. The water pump is currently located in a utility room adjacent to the master suite and, due to the undesired noise, I plan to move it to a pump house out by the well and run it off the garage subpanel.

The new garage subpanel will power a 220V air compressor, TIG welder, a bunch of 110V outlets, the relocated water pump, some large (Big Ass-style) ceiling fans, and possibly in the future a heat pump and electric car charging station when I eventually get one of those. I calculate 100A should be sufficient for my planned usage.

I have done a lot of research but still have a few questions:

1) Are there any problems or caveats with this plan to run a subpanel off another subpanel that I may have overlooked?

2) What should I use for the feeder to the new subpanel? Total feeder length will be around 125 with about 85 being underground. From my research I am looking at 1/0-1/0-1/0-2 AL URD in 1.5 conduit. Do I need to go that large? Do I really need a ground conductor since I will have separate grounds at the garage?

3) There is a ufer ground (piece of rebar stick up out the side of the slab) but it is on the opposite side of the garage from where I plan to put the panel. Can I run my ground wire over there (~50 of wire would be required) and avoid having to install ground electrodes?

4) Can I run a pressurized PVC water line from the relocated pump in the same trench as the feeder? Is there a separation requirement?

5) For the circuit wiring in the garage, since it is a metal building and wiring will be secured to the faces of the framing members, I assume I will need to run all the wiring in conduit. Is this correct?

6) I know I need to separate the neutral and ground and install a ground bus in the subpanel. Any other wiring pointers?

One other thing of note the existing subpanel is a FPE Stab-Lok box. I know they are dangerous and I will be replacing it as part of this effort. I plan to pull the necessary permits, but just want to make sure I have my ducks in a row before going downtown. Thanks in advance for the help!

- Matt
 
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Old 05-04-16, 01:24 PM
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You need to estimate the total load that may be on the new 100A subpanel plus the estimated load on the existing 125A subpanel that the sum of those loads don't exceed the capacity of the 125A panel.

You mentioned URD for the feeder between the subpanels. URD is not allowed inside the structure because the insulation is not fire resistant rated. URD must be used and terminated on the outside of the structure. You need to use THHN/THWN or RHH/RHW-2/USE or XHHW-2 type wire in conduit.
 

Last edited by pattenp; 05-04-16 at 01:44 PM.
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Old 05-04-16, 01:29 PM
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1) No problems.

2) Mobile Home Feeder (MHF) cable is good product for this application. It can be direct buried at 24" depth, buried in conduit at 18" or used in conduit above ground and indoors. The size you state is reasonable, although I'd use 2" pipe. Yes a four wire feeder is mandatory. The biggest problem with URD is that due to fire rating it is an outdoors-only cable. You must have a junction box on the outside of the building to convert to an indoor wiring method which adds significantly to the cost and complexity of the install. MHF or THWN/XHHW conductors in conduit avoid this complication.

3) Yes on the Ufer. Bare #6 copper is appropriate for this task.

4) The dual use trench is really up to your local inspector. Electrical code doesn't specify yea or nay. I do recommend dropping in a second conduit for telecom, internet, catv, etc. Bury the electric at 24", backfill 12", drop the telecom pipe, backfill to grade.

5) Yes, all wiring methods in the shop must be protected. Usually this is interpreted to mean that wiring below about 8' in height must be sleeved in conduit or protected by wallboard, framing or similar materials.

6) Correct on the Neutral/Ground separation. All 120V receptacles in the shop need to have GFCI protection. I'm sure you plan to exceed this, but code minimum is that you must have at least one exterior and interior switched light at the human door entrance to the shop. If the breaker panel in the shop has more than 6 handles, a main disconnect or main breaker style panel is required. The metal framing of the building should be bonded to the panel ground (easy to do this as you connect to the Ufer).

Good call on replacing the FPE.
 
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Old 05-04-16, 01:48 PM
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Just an FYI, MHF only comes in #2 or #2/0 or #4/0. To use #1/0 you'll need to go with one of the types I listed in my previous post.
 
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Old 05-04-16, 07:21 PM
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The existing subpanel is wired with 2/0 AL SEU cable run through the attic from the main panel.
I think I'd start by replacing the SEU cable feeder to the existing subpanel with a 4 wire cable such as SER cable.
 
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Old 05-04-16, 07:33 PM
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I was just about to ask that question. The neutral and ground isn't separated in the existing subpanel. As I mentioned, it is a FPE box that I'm going to replace so I know I will need four conductors. Could I just run another separate conductor for the neutral?
 
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Old 05-04-16, 07:54 PM
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Could I just run another separate conductor for the neutral?
Yes, but you will be adding a ground, not a neutral.
 
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Old 05-04-16, 08:28 PM
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Ah, ok. The SEU cable has two insulated conductors and one bare. So if I run another conductor, I'll use that for the ground and just leave the bare one as the neutral? And am I correct in assuming I'll need to install a ground electrode outside for this subpanel since I'll be separating neutral and ground? Thanks again for all the help!
 
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Old 05-04-16, 09:06 PM
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Pros, would a bare neutral be permitted since it is a feeder not service cable?
 

Last edited by ray2047; 05-05-16 at 08:43 AM.
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Old 05-05-16, 07:59 AM
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Pros, would a bare neutral be permitted since it is a feeder not service cable.
Being a 4 wire feeder the neutral needs to isolated from the ground so the neutral needs to be insulated. Plus code requires it to be insulated.
 
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Old 05-05-16, 08:43 AM
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So to sum it up. Matt wrote:
The SEU cable has two insulated conductors and one bare. So if I run another conductor, I'll use that for the ground and just leave the bare one as the neutral?
I asked:
would a bare neutral be permitted since it is a feeder not service cable
Pattenp replied:
Being a 4 wire feeder the neutral needs to isolated from the ground so the neutral needs to be insulated. Plus code requires it to be insulated.
 
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Old 05-05-16, 08:54 AM
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If you're just doing the panel change on the FPE stab-lok, the existing three conductor SEU feeder will probably be grandfathered. This is however up to the local inspector's discretion.

If it is reasonable accessible to do so, I would strongly consider replacing the feeder with a modern 4 wire option even if the inspector does not strictly require it. Four wire feeders improve panel safety quite a bit and brings your entire supply up to modern code. #2/0 aluminum SER cable would be a good choice for this part.
 
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Old 05-05-16, 07:45 PM
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The neutral can only be bare when pare of an assembly of a cable. The bare neutral is covered within the SEU cable so it should be OK. The ground does not need to be run with the other conductors and is OK to be bare or insulated.
 
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Old 05-06-16, 08:22 AM
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If adding an EGC and using the bare in the SEU as a true isolated neutral the SEU outer jacket will be striped away where entering the panel exposing the bare neutral in the panel. The neutral needs to be wrapped with white tape the entire bare exposed length to prevent any contact with case or ground conductors, plus to identify it as a neutral. My opinion is this is a hack job to do it this way. The SEU should be replaced with SER.
 
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