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I recently had an electrician install a new 30 space 60 circuit panel in my house, upgrading from a 20 space since I was running out of space. I bought a small number of tandems to save some space.

The electricians did a great job, but after the install put in single breakers in place of the few tandems I had bought. They said they didn't like to install tandems. I'd also talked to a different electrician who said the same thing.

Now, I don't fault them for doing what they think is safe or best practice. But I can't seem to find any real evidence that tandems are il-advised, or create problems. It seems to be more like unsubstantiated bias. Some people say they don't like it because it's "double the heat". (I can't back this up with any real evidence that double the heat makes any significant difference). Also, a single 30 amp breaker is allowed in the same space as a tandem 15, which should generate equivalent heat. Other electricians seem to think they're perfectly safe, otherwise they wouldn't be UL listed, or be banned in the NEC.

Are there any legitimate, backed up with evidence concerns about tandem breakers?

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    Tandems are usually used only when you need to add breakers to a full box, not in new work, in fact, I'm not sure they are allowed in new work, which is what I think a new panel is what an inspector would consider new work. Not meaning to be critical, just curious, why didn't you go with a 40 space panel? The cost difference is minimal compared to the cost of the entire job. – George Anderson Sep 12 '20 at 13:03
  • @GeorgeAnderson I have 100 amp service. I couldn't find a 40 space, 100 amp panel. – Steve Sether Sep 12 '20 at 15:26
  • Note also that some local amendments to the Code (most notably, Chicago's as I understand it) prohibit tandem/"double stuff" breakers even with panels that can accept them. – ThreePhaseEel Sep 12 '20 at 19:50
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    @SteveSether -- you can use a 40 space 200A panel with a 100A main breaker in it, though – ThreePhaseEel Sep 12 '20 at 19:50
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    @ThreePhaseEel I was going to suggest that, but you beat me to it! This is where the electrician should have piped up and said, "you know, for just a few dollars more we could upsize the panel and you'd be set for a very long time". – George Anderson Sep 12 '20 at 20:05
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Edit: THIS. Over-full panels invite problems!

Go look at this situation and the poster's proposed solution. This person wants to add a subpanel. What they really want is a 125A subpanel, which should be easy. But they're willing to settle for a 50A subpanel since they feel forced into using a double-stuff breaker, and they're very comfortable using a 30/50 quad; they're mentally married to that solution. But look closer. Look what breaker will be opposite the 30/50. Yeah, a 40/40. So you'll have 160A of load sharing a single bus stab.

And nobody thought anything of this. See how easy that mistake is to make? Notice how a career electrician also missed it - heck, everybody missed it!

A problem like this is extremely rare in a normal panel, because 80A+ breakers are "black swans" - they exist, but you sure know when you're working with one, and so you tend to watch out... and you rarely have two. And they don't sneak up on you.

So that's essentially a UX argument, but that's a real thing in the real world. Armwave all you like about how the world would be if everyone was careful... they aren't.

That's a UX case, but UX cases are why we handle-tie MWBCs, pigtail neutral, use white for always-hot on switch loops, etc. etc. A huge portion of total NEC code is dedicated to UX factors like that.

They're awesome in 2003 or in a FEMA trailer

Because then, you do not need AFCI or GFCI on anything.

But if you provisioned your panel counting on 2 circuits per space, and you then are required to AFCI or GFCI some of those circuits, you'll be gobbling up spaces at twice your expected rate. AFCI/GFCI don't come in double-stuff form factors.

Other emerging technologies won't fit either

Nobody knows what's coming, but I can think of two right now.

  • Leviton has a new panel in the marketplace that has integrated power monitoring a-la the Sense home energy monitor. There is a hub built into the panel and smart breakers in each slot. If that takes off, all the panel makers will do it. That kind of thing is usually not offered in double-stuff.

  • As we get more into alternative power that is non-dispatchable, we will want to get more into the "smart grid" tech where the power company can suspend your water heater temporarily instead of spinning up a fuel generator to power it. That will be most easily implemented as breakers, and again, not gonna happen in double-stuff.

That last one is already implemented (the brawn part, anyway) in Eaton BRRP or CLRP remote control breakers. All it needs is the electronics to talk to the grid and command the breaker on/off. But one brain could control any number of BRRP/CLRP breakers.

You have more trouble with stab limits

Most panels have "stab limits" limiting all the breakers sharing a bus stab to X amps. Double-stuff breakers cram 4 poles on a stab instead of 2, so they are much more likely to run into stab limit problems. Recently I saw a 50/30 opposite a 30/30, so 140A on those stabs. Stab limits were 125A.

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  • Thanks. This again seems speculative that tandems will never have these controls. I'm not an EE, and presumably neither are you, so I wouldn't want to guess whether it's possible to put the capabilities you're describing in a tandem breaker. I'm not too concerned about the power company shutting down appliances. they do that already with my AC, and it's handled with their equipment, so speculating on how this works seems premature. I guess what I've learned in this question is that some Electricians just don't like these breakers, but don't have quantifiable evidence to back that up. – Steve Sether Sep 15 '20 at 3:45
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    @SteveSether, EE here, and Harper isn't wrong about advanced breakers not physically fitting in tandem breakers. In low-power digital electronics, we can miniaturize all day, but power electronics are a different story -- you need a certain amount of metal to work with a given current, and that's dictated by physics. It's the same reason we haven't found a way to send 100A down a 12ga wire without it melting in the last 100 years. – Nate S. Sep 16 '20 at 16:50
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    Now, what may happen at some point, which Haprer alludes to, is that the advanced electronics don't necessarily need to live in the breaker slot. Instead, each breaker can just be a remote switch and some sensors, and then all the control logic can live in the panel itself -- this may allow for smaller breakers. But this requires a totally new panel design, so any panel you're buying today (except maybe the Leviton) will never have double-stuff advanced breakers available. – Nate S. Sep 16 '20 at 16:53
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    @Nate Right... and even Eaton's remote-throw/remote-reset breaker is full-width... It appears to be their "classic" BR elephant style duplex design, with the 2nd overcurrent mechanism replaced with a motor drive. They won't even make it for 3/4" CH/CHQ; probably because they can't. It's an industrial product, so walking away from the CH/QO market is a really big deal. That reflects just how hard miniaturization is. – Harper - Reinstate Monica Sep 16 '20 at 17:18
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    @SteveSether -- the issue is that the breaker guts in a tandem/doublestuff are basically already as small as they can get, so there's no further room to shrink them, and the measuring CTs in a GFCI or AFCI don't shrink all that well either. (Note that many early AFCIs especially have longer bodies -- Eaton took several product gens to come out with the "short body" BR AFCI breakers, for instance.) – ThreePhaseEel Sep 16 '20 at 23:29
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They are not unsafe.

BUT

If you pick up the tandem breaker and a full size breaker and look at the area that plugs into the bus of the panel and if you take a good look at the panel itself, you will see that there is a lot less area that makes contact between the tandem and the panel bus. In fact about half the area. If the circuit is a heavily used circuit it could be a place that overheats and eventually destroys the breaker and usually damages the bus.

For this reason most experienced electricians like to use full size breakers instead of tandems when they can since they are simply a better breaker. Personally I don't mind using tandems for 15 and 20 amp circuits, but never like to use 240V 30, 40, 50, and 70 amp tandem breakers. Also I try and set up a panel so the heavier loads are not back to back. Usually I try and install them on either the right side or left side of the panel with less loads such as 15 and 20 amp breakers on the opposing side for better distribution and less heat.

I hope this sheds a little light on you question and good luck.

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    This is the basic story I keep hearing. It seems entirely speculative, and not based on evidence. I'm a person that believes evidence should be provided to back up theory. These things are cheap enough that I might even conduct my own experiment with a new panel on a test bench, under controlled conditions, and measure the heat generated with a thermocouple. – Steve Sether Sep 12 '20 at 15:21
  • The other problem I have with this theory is that high amp tandem breakers are UL listed. Certainly someone has thought of the heat concern (The manufacturer, UL, NEC, etc) and done testing to allay the concern? – Steve Sether Sep 12 '20 at 15:30
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    I understand your inclination to just trust UL, but some people haven't found some things that are listed to still be complete junk and no longer give something a pass just because a bureaucracy has put a sticker on it. We have to sleep at night, both feeling like we delivered a good value, but feeling property and life are safe too. That being said I don't know the details of how you ended up supplying parts that the electrician didn't approve of, and maybe your electrician should have made his standards and acceptable products more clear to you before starting. – NoSparksPlease Sep 12 '20 at 16:37
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    @NoSparksPlease UL doesn't test for durability. They test for safety. Junk can get UL Listed. As far as the electrician's rejection of OP's breakers, we don't know the brands in question, but the electrician might have just flagged alien breakers. – Harper - Reinstate Monica Sep 12 '20 at 17:46
  • @SteveSether - I would debate that your argument about being speculative is based on you personal knowledge of electrical construction. My argument would be that my answer is a reality based on training and personal experience. I have observed and replaced many panels based on the information I described. One indecent in particular, the panel was only 5 years old.Keep in mind that UL and the NEC are minimum requirements and in 1965 they approved aluminum branch wiring in residences. I believe we are here to offer expert advice, but it is up to the individual accept it. Good luck. – Retired Master Electrician Sep 13 '20 at 13:49

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