Retrofitting an existing service panel

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Old 06-04-15, 10:50 PM
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Retrofitting an existing service panel

I posted several months back with some question regarding load calculations to add a circuit for a portable spa. The unfortunate result of that exercise was that I realized I had a Zinsco panel. I’ve been looking at my upgrade options since then and was hoping some of the knowledgable folks here could provide some thoughts. I’m an electrical engineer, though not an electrician.

Apologies in advance for the length of this post.

As is common in California, I have an outdoor all-in-one service entrance that includes the meter, main breaker, and branch circuit breakers. The house was built in 1972, the electric service is 100 Amps, original, with the exception of an AC compressor circuit added ten years ago. The house has all gas appliances, the only heavy load is the AC which we only use ~10 times a year since we’re near the coast. Here’s a picture of the panel and one of the top portion (breaker cavity) with the dead panel removed.





The service entrance is fed from underground, the transformer is approximately 30 feet from my panel in the front corner of my lot. The service feeder is directly buried (confirmed via POCO documentation and when a lineman opened the transformer box to check for me). This transitions to a conduit presumably at/near the base of my foundation. The service feeders are #2 aluminum with a #4 aluminum neutral. The feeders appear to be what’s known as “Clemson” in the trade, based on conductor size, strand size, number of strands, and insulation thickness. My reading of the NEC limits this to 100 Amps for service entrances, though Clemson is rated for 155 Amps direct burial, 115 Amps in conduit for non-NEC applications per various manufacturers. Here's a picture of the service entrance (bottom portion of panel).



I’ve got to get that old Zinsco out of there before I’m sorry. No signs of arcing or overheating, but it dawned on me that in the twelve years we’ve lived here, we’ve never popped a breaker. It makes me wonder if they are doing anything at all, regardless they are over forty years old and need to be replaced. I’d also like to upgrade the service but at this point I am not willing to tear up my yard to replace the feeders.

I’ve was looking at replacing the panel with a new all-in-one, when I started to consider a retrofit of the Zinsco can. The idea would that I leave the service entrance and meter portion intact, but gut the portion of the can that includes the main/branch breakers. I would then retrofit the resulting cavity of the Zinsco can with a new main breaker/small busbar and use that to feed a new subpanel indoors. (Our POCO and AHJ require a main service disconnect outdoors for emergency responders so the main has to stay outside.) All branch circuits would move to the new indoor panel with the exception of the A/C breaker that I would leave outside with the main since the compressor is located right next to it. I would also leave a small number of spare spots at the outdoor panel to accommodate things that might be needed in the future.

Eaton makes retrofit kits specifically for this type of thing but it doesn’t look like they have one that exactly meets my needs. Square-D QO makes a small panel that includes a main and small number of branch circuits. The guts of this look like they would fit perfectly into my existing can as the dimensions of the Square D can are almost identical to the cavity size I would have available.

CH Retrofit Interior Kit for Replacement of Vintage Brands

Square D QO112M100C QO 100 Amp 12Space 12Circuit Indoor Main Breaker Load Center with Cover - Fuses - Amazon.com

The outdoor panel would now include a main breaker, the AC breaker, a few spare spaces, and a breaker that feeds the subpanel (could be the same Amps as the main but probably smaller). The new main breaker would be a good quality 100 Amp 22K AIC, unless I can get the POCO and AHJ to sign off on using a 110 or 125 Amp with the existing service entrance feeders. The Zinsco box is rated at 125 amps although all I’m keeping from it is the service feeder connections and meter contacts/housing.

I’d use 1.25” EMT from the existing panel to the new indoors subpanel, a distance of about 10-15 feet depending upon the final location. The subpanel feeds would be #2 THHN/THWN. The new subpanel would be main lugs only. I should be able to reroute all of the branch circuits to the new subpanel without splicing if I locate in a bedroom (not preferred). I may need to splice some of the branch circuits if I put the subpanel in an adjacent hallway (preferred). Both locations are good regarding “dedicated space” and access.

Having thought through all of this, I’m curious what people think. It would probably be easier to just replace the panel with a new all-in-one, but I like the idea of moving my branch circuit breakers indoors for convenience, protection from the elements, and more easily supporting future remodels. I also really don’t want to tear up my stucco. Retofitting the existing can seems like a reasonable, if complicated, solution.

Here are some of the questions I have

Has anyone had experience with retrofitting an existing can? The Eaton kits have UL certification but I’m sure taking the guts out of one can (such as the Square D product linked above) and putting in another can raises all sorts of certification questions. Does anyone have experience with this?

Is feeding the subpanel with THHN though EMT the best choice? My main concerns are protecting these high amperage lines from future errant nails. I have good access for routing.

What about locating a subpanel in a bedroom? The code seems to allow it, though I ‘ve seen some NEC interpretations that all a bedroom “circuits” must be protected by arc fault breakers means you can’t put a panel in a bedroom (since the feeder or some circuits in the panel are technically "in the bedroom" and aren’t arc fault protected). It’s currently an unoccupied spare room with no plans to change that.

If I do use the hallway location for the subpanel, I would need to splice probably six branch circuits. Is having the JB in a closet allowable? It’s along the path and accessible but out of sight.

Any thoughts about the max ampacity of the existing underground feeders? I realize POCOs have their own rules but how firm is the NEC regarding service feeder ampacities?

Thanks for reading and any feedback is welcome and appreciated.

Jeff
 

Last edited by Jeffc3497; 06-04-15 at 11:00 PM. Reason: typo
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Old 06-05-15, 06:21 AM
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I have never used any of these retrofit kits,but if they are UL listed for that purpose and the AHJ is good with it then I don't see a problem,hallway location would be best for the panel,splice box could be in a closet as long as it is accessible.
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Old 06-05-15, 07:32 AM
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Hi Jeff, as long as the local inspector is willing to approve the retrofit kit I think you're good to go. I have never used one for a Zinsco panel, but it does seem they are designed specifically for your purpose. I have swapped out panel guts of similar makes and sizes without a problem.

The indoor plan seems good to me. I would prefer the hallway panel, because it would be difficult to maintain 36" clear space in front of the panel in the bedroom with the various possible configurations of bedroom furniture. It's also easy to hide a panel in the hallway by hanging a painting over it. The AFCI thing sounds like a misinterpretation to me as the panel is not an "outlet" by any reasonable definition. Circuits do not require AFCI protection, outlets do.

A splice box in the closet is ok as long as it is accessible without removing building finish. Essentially don't cover it with a built in shelving unit.

The underground feeder is entirely at the discretion of the power company -- somewhere around 100-125A seems reasonable for that size and type of wire. The POCO usually will also bump up the max ampacity rating quite a bit on residential because they know it is not continuous use. The average load over a couple hours is much lower than the peak.
 
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Old 06-05-15, 06:02 PM
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Thanks for the responses. I've discussed this with the local inspector anonymously over the phone but wanted to give the plan a sanity check before I met with them and filed. I appreciate the help.

Jeff
 
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Old 06-09-15, 11:29 AM
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I think you're good with either your retrofit/subpanel idea or a complete replacement.

If it were me, I'd probably go with a complete replacement, leaving the panel outside, sized with future expansion in mind and not worry about junction boxes and taking up space inside with an 'ugly' panel.

It would probably be simpler and cheaper, even if some stucco work is required.


One other note - your feed from the main outside panel to your new subpanel (presuming you go with that option) can be in conduit or can be 4-conductor w/ground cable. Depending on how it's run, it's likely easier with a single cable - but it's up to you.
 
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Old 06-09-15, 11:47 AM
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I'm just always surprised to see pictures from climates of 40+ year old steel outdoors and it looks nearly new. :-) We really don't do refits here because at that age an outdoor box is in very rough shape and not really reusable at all. You usually have to be careful to make sure the hinges haven't rusted so much that either the panel cover falls on you or you need a hammer to break it open.
 
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