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I took a look at some of the other threads, but none seemed to give me a clear answer:

My house currently has a mess of a load center setup that I need to correct. There are two Square-D 22? space panels wired to each other, with one being a subpanel with a 60 AMP link from the primary one (there is a 60 AMP breaker in each panel that are linked together with some super-thick aluminum braided wire). The primary panel has a 100 AMP main breaker, connected to the meter outside.

Aside from this thing being a monstrosity, the "subpanel" has corrosion in spots on some of the breaker contact points, reducing voltage to 108 or so on some circuits (diagnosing why some things worked and others didn't when plugged into certain outlets was a multi-day adventure.) The thing that just pushed me into action is that the screw-downs for the feed from the meter on the 100 AMP breaker are corroded, and one is actually loose and occasionally arcs.

I don't have a lot of vertical space to go with a 200 AMP panel with a lot of spaces. I can get the same footprint Square-D panel with either a 100 AMP or 125 AMP main breaker. I can also reduce the number of breaker spaces I need by consolidating circuits (LOTS of the interior outlets are direct runs to the panel, there's no reason the LR, for example, can't all be on one breaker instead of 3), and by using a few tandem breakers.

My questions:

  • Does going from a 100 AMP to a 125 AMP panel require any change in the gauge of the wire coming from the meter?
  • Is there any limit to the number of tandem switches that can be used in one of these panels (e.g., Square-D HOM2448M125PC)?

Thank you!

John

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    "feed from the meter on the 100 AMP breaker are corroded, and one is actually loose and occasionally arcs" - that is a problem you need to have fixed immediately. Like today. Before your house burns down. Because it will. Soon. – Grant Apr 7 '16 at 23:27
  • Just a note that if you're a long distance from the road, voltage drop may also be an issue. – Someone Somewhere Apr 8 '16 at 6:43
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    I agree with Grant. The very first house I wired in the 70's Had a smoking main less than 9 months after being built. Aluminum wire needs a anti oxidation compound and the mains should be tightened every few years. the loose connection is probably the reason for your voltage being low but in some areas 112V is a normal voltage with 117vac being the high value. unlike some answered below it is totally acceptable to use "split" or tandem breakers as long as the panel is rated for it. many older panels only had split locations in the bottom few spaces, some newer allow for them all to be tandem. – Ed Beal Apr 8 '16 at 16:16
  • @EdBeal, new aluminum terminations DO NOT need to "be tightened every few years". This is a myth perpetuated by olde tyme electricians and handymen. – Speedy Petey Apr 10 '16 at 14:26
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    As a licensed electrician that has been called to repair fire damage on aluminum services I disagree. Most all industrial sites have IR scans of all load centers and most times ~5% need tightening some are copper but the majority that need tightened are aluminum – Ed Beal Apr 10 '16 at 21:14
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My questions:

Does going from a 100 AMP to a 125 AMP panel require any change in the gauge of the wire coming from the meter?

If you have a 125 amp breaker you need 125 amp wire. See the table here. You need to use the 75° column since no one makes a 90° breaker just yet. Since this is a single phase dwelling service you can use #2 AWG copper or 1/0 aluminum for a 125 amp service or main feeder.

Is there any limit to the number of tandem switches that can be used in one of these panels (e.g., Square-D HOM2448M125PC)?

The limit on the number of circuits in a panel was removed from the National Electrical Code in 2011. So, your only limit is the manufacturer's equipment.

Good luck!

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This one popped up for some reason, so I'll edit in some new thoughts.

Corroded or arcing power feeds are "burn your house down" serious business. Fix immediately! Aluminum feeder wire is fine stuff, but you do need to use the anti-ox goop, and torque it correctly. Many electricians in the 80s and 90s did not get the memo. Nip it in the bud fast.

Likewise corrosion (read: arcing) on breaker contact points is absolutely unacceptable. Catch it early and you may be able to save the breaker space, otherwise that's a dead space in your panel. The #1 cause is using the wrong brand of breaker. Brand X breaker won't engage Brand Y buses with the correct contact shape or clamping force, causing the arcing.

If the panel is a goner, swap it. Let me come back to that.

The cable between main and sub doesn't need 2 breakers. It needs a breaker upstream (in the main panel) to protect the cable. A 60A breaker is correct to protect #6Cu or #4Al. At the subpanel, the cable/wire can land on main lugs (provided the subpanel is rated for 60A+) or on the subpanel's main breaker (of any rating - a smaller main protects the panel, a larger main is just a shutoff switch, which you do not need since it's in the same building.) So if the subpanel has main lugs, use them instead of backfeeding a breaker.

One more thing. If the main panel is breakered at 100A, and the cable supports 100A, and the subpanel supports 100A, then the main main breaker protects them all. You don't need any breakers for the sub. You can use thru-lugs or snap-in lugs in the main panel.

With that in mind, back to replacing the panel(s). Talk to the power company and see if your service drop/meter can support 125A. Even if not, you can order a 125A main panel and swap the main breaker to 100A.

See if you can upgrade the main-sub cable to 125A. If so, order a main panel with thru lugs, which will save 2 breaker spaces, and a 125A sub. Voila, no breakers needed, space maximized!

You can also look at the CH or QO panels, which use 3/4" high breakers, so more spaces in the same panel footprint. GFCI and AFCI breakers are available for them.

What doesn't work is "double-stuff" breakers (including GE's 1/2" Q-line breakers). Those are useless as almost all new work today requires GFCI, AFCI or both - and you cannot obtain those in double-stuff. So a "24/48" panel is actually 24, period.


Original:

Wire gauge is decided by what NEC requires (and those are in stair-steps) and what your electrical distributor has in stock.

So with numbers as close as 100A vs 125A, you might get lucky: imagine the original installer found #2 wire was good for 95A, #1 for 110A and #0 aka 1/0 was good for 125A. He can't use #2, the distributor does not stock #1, so he uses 1/0. You get a happy surprise when you aim to upgrade. Hey, it could happen - check. If not, you need to pull bigger wire.

Check the markings on the aluminum conductors to make sure they are stranded and the modern AA-8000 series alloy, which are legal and safe. (NEC 310.106b). The problem alloys from the 1950s are now outlawed, and your distributor won't even sell them.

Your reduced voltage to 108 is more likely to be a problem with your neutral. Poor connections can't drop 12 volts without destroying themselves from heat (volts x amps = watts of heat), more likely your other leg went to 132 volts because of a lost neutral. This is an extremely dangerous condition that needs to be taken out of service immediately because the imbalance can be much worse than 12V, and that will destroy appliances and start fires. If you know where the problem is, but can't de-energize your house now, a short-term work-around is disable one "hot" - doesn't matter which one. Look at how the hot busbars are arranged inside the panel, pick one leg, and remove every breaker which uses that leg. Rearrange as necessary to keep the lights on. At that point, if the neutral fails, the circuits will shut off instead of giving dangerous voltages.

Look for other options (i.e. other brands or suppliers) that will give you more spaces in the panel. You don't want to be forced into using duplex breakers, because - especially when you remodel - you'll be forced to follow current code, and will need more kitchen breakers and GFCI and AFCI breakers in many slots. Those are a lot more expensive as duplex. You can completely fill the panel with double-stuff breakers (in fact, doing so is the basis of their "48 circuit" marketing claim).

Don't buy this at the big-box home improvement stores, go to a real electrical distributor such as Greybar - they tend to be locally owned. They will have better options (eg more breaker spaces in smaller boxes, Homelites are huge), better quality and sometimes better prices. For instance, Homelite is Square D's bottom-tier brand. There's better out there, and it's worth an extra $100 to not have problems like the ones you are having.

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The panel model number will usually tell you quickly if twins/tandems can be used. The example you gave; HOM2448M125PC. This is a 24 space, 48 circuit, 125 amp panel. So 24 one inch spaces, but you can use all 24 twins giving you 48 potential circuits.

  • You won't get anywhere near 48 if you're in NEC 2014 territory, with so many circuits requiring AFCI... – Harper Jan 6 at 18:05

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