I agree with 20 and 100. M means it has a main breaker. (As in your link
I don't know what BW is either. Need a real old catalog.
I would read "20" as not allowing tandem breakers. "2024" would allow
tandems in 4 positions. But it is an old catalog number.
Seems like UL had maximum number of poles depending on the panel rating.
I think a 100A panel had a max of 20 poles and 200A had a max of 40 or
42 poles. That makes it harder to overload a panel, but doesn't work so
good if you don't have much on each circuit (so you need more circuits).
SquareD has panels with a lot more poles ("circuits") than that now.
SquareD class CTL tandem breakers have the usual horizontal clip at the
busbar end. At the gutter rail end they have what you describe as a
claw, which fits into a slot in the gutter rail. The slot is only
present at the positions where tandem breakers are allowed.
Your farnell link has a crappy picture for a Homeline panel on page 9 of
A better picture is
where the gutter rail claw can be seen.
Regular breakers just have a clip at the gutter rail end, as with your
The difference is only in the gutter rail.
I don't entirely agree with the limit on circuits. If I watch the
connected load I might install tandems where you are not supposed to.
I think one reason for the limit is to limit the heat produced in the
panel. Heat can cause a breaker to trip at a lower current. Would seem
like a 100A panel would only make 100A of heat no matter how many breakers.
There is, in general, a limitation for fuses and circuit breakers of 80%
for "continuous" loads, which are loads that are on for 3 hours or more.
My understanding is this is based on panel heat, and the breaker may
trip at less than 100% if the load is on for more than 3 hours.
An exposed 20A breaker or fuse by itself is supposed to hold at 20A forever.
I've found that publications printed before the internet often never make
their way to the net. I don't see myself spending too much time looking for
the meaning of BW.
Those tandem breakers fit just fine <g>. I looked at the box again and the
rails are continous without interruption or anything that looks like special
slotting meant to accept tandems only in one spot.
It would be nice to upgrade to a newer panel but it wouldn't be cost
effective. This place has to be "shelled" when I am through with it. Boy
will my heirs be disappointed. (-:
No slots in my gutter rail. All the same from top to bottom but at least
now I know why they changed the clamping mechanism. Boy was I certain when
I got them home that they were the wrong style breakers even though I had
religiously copied down the breaker information from the one I had pulled.
I also discovered that the previous owner had replaced 15A breakers with 20A
ones, which I based on the fact that the same gauge wires were feeding both
sizes of breaker. I replaced those 20A units with 15A ones, just to be on
the safe side.
I figured wiring from 1940 was not likely to be 20A capable but the idiots
before me apparently found the kitchen circuits tripped less frequently with
the larger capacity breaker. Sadly, you could see that the 20A breaker feed
wires had evidence of overheating. That's what led me to add the
tandem/dual skinnies and run new wires to the kitchen and workshop areas.
Either kind of breaker fits into any slot on the breaker panel I have. I
saw a printer's mark on one label in the box that said 1961. Of course,
there's no telling if that's related to the date the panel was installed.
But based on the history of this house that I know about, it's entirely
possible the panel is that old. I know at one point it was a screw fuse
panel because I found a bunch of fuses directly below the panel area when I
pulled the old paneling off the basement.
Then I don't have a CTL panel.
I think the offloading of old, cloth-covered wiring to newer 12/2 NM
justified the use of tandems in my case. It has to be safer using new
wiring than muddling through with the old crap.
There are some interesting charts here about temperature, load and other
factors relating to breakers and how they trip and when.
I saw something (not especially credible - one of those "ask me" sorts of
sites) that said because of the heat generated by the electronics in AFCI
breakers, they had to be mounted away from each other. I also read
manufacturer info that said that heat generation was taken into account.
That's where I also saw something about the NEC required that instrument
controls that a user had to operate with fingers not be over 140F. That
seems a little high.
You can't tell that from the rail. The panel could just not allow any
Sounds like from the date you found it is not a CTL panel and it is
kosher to use non-CTL tandem breakers.
I am not entirely convinced that is a problem anyway. If you have a
100A panel seems like you would only get '100A of heat' no matter how
many circuits/poles there are. You would have to watch how much load you
connect so you don't actually overload the panel.
I used what Home Depot had on the shelves that matched the QO designation
and it was approved by the AHJ. Dual skinnies/tandems are a pretty popular
item in this development of houses built to house the army of workers that
descended on DC at the beginning of WWII. Fortunately with CFLs and more
efficient motors the average load for such items has decreased substantially
(although the number of plug in chargers and appliances has increased
geometrically). I assume when I ultimately switch to LEDs the overall load
will be reduced again.
Tell that to the four that are in there now! (-:
That date's not conclusive. They could have been using the same label for
20 years. There must be some other date markings inside the box. Reading
all the details is getting to be like reading "War and Peace" - very
I do. A long time ago I installed two Hall effect sensors on the incoming
feeds. For days I was calibrating them with various space heater loads
(this was the springtime and it drove my wife a little crazy having me run
space heaters with the windows open. She's a frugal New Englander). Those
sensors feed into my HomeVision home automation controller's analog inputs.
From there I can roughly gauge the total load at any one time and it very
rarely exceeds 50A, and only then when it's very cold which self-limits the
overheating panel problem to some extent.
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