Tandem Circuit Breakers

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dime

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The NEC code 408.35 defines the number of spaces on one panelboard. To allow for the addition of new circuits I have found panelboards that had single pole breakers replaced with tandem breakers. Doing this increases the number of circuits the panel was designed for. Does this not violate the 408.35 code?
 
dime said:
Doing this increases the number of circuits the panel was designed for. Does this not violate the 408.35 code?
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No not necessarily.

The panel is likely designed to accept tandem breakers.
 
There is listing and labeling for panels. If you read the label (usually on the inside of the enclosure), you will be able to determine if the number of circuitbreakers installed is too much or fine. Tandem breakers count as 2 breakers, even if the tandem breaker only occupies one standard breaker location.
 
Often, tandem breakers will fit in a panel for which they are not designed. In this case it is a violation as the panels are not designed for their use. Violation of 110.3B.

Information inside the cover of the panel will generally tell you whether the panel can accept tandem breakeres.
 
I do believe the panels and tandem breakers are designed so that they will or won't work together therefore the panel can't accept more than is allowed by code ... 42 rite ? ... m
 
Yes, 42 is the limit, although I invariably see 40-circuit panels for single-phase use. (42 is not divisible by 3, while 42 is.) Unless not allowed, panels of fewer spaces will accept the number of tandems that would allow the circuit count to hit the max.

Most panels are labeled for both the maximum number of spaces and the maximum number of circuits, such as 30-spaces/40-circuits or 40-spaces/40-circuits. Obviously, the former will accept 10 tandems, while the latter will accept none.
 
I agree with Larry. I would add that a power panel does not have a 42 circuit limitation.
 
The problem I see in most new homes when a HO hires me to add a few circuits is that the original EC put in a 30/30 200 amp panel and then filled it to the max. I understand that a 30/30 is a little cheaper that a 30/40 but not by much and then I have to tell the HO that it is gonna cost them for me to put in a new panel too. I like the extra earnings don't get me wrong but I don't understand the first EC designing a system this way. I have seen this in homes that have unfinished basements and they don't have space for a breaker for a sub panel. I have had to find home runs in the basement and take them off the main panel and route them into my sub just to get space for the sub panel breaker.
 
Mario,
I do believe the panels and tandem breakers are designed so that they will or won't work together therefore the panel can't accept more than is allowed by code ... 42 rite ?
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While that would be true if you are using CTL tandem breakers, that have a physical design that does not permit the installation of too many breakers in a panel, they also sell non-CTL breakers that do not have the rejection feature. In this area none of the supply houses stock the CTL breaker, so it is physically possible to fill the panel with tandem breakers.
Don
 
dime said:
The NEC code 408.35 defines the number of spaces on one panelboard. To allow for the addition of new circuits I have found panelboards that had single pole breakers replaced with tandem breakers. Doing this increases the number of circuits the panel was designed for. Does this not violate the 408.35 code?
Click to expand...

30 slot panels have room for 12 tandems, plus one additional tandem for each 2 pole used, before they violate 408.35
40 slot panels have room for 2 tandems, plus one additional tandem for each 2 pole used, before they violate 408.35

But all bets are off if the listing is exceeded 110.3(B)

don_resqcapt19 said:
Mario,
While that would be true if you are using tandem breakers, that have a physical design that does not permit the installation of too many breakers in a panel, they also sell non-CTL breakers that do not have the rejection feature. In this area none of the supply houses stock the CTL breaker, so it is physically possible to fill the panel with tandem breakers.
Don
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Most breakers sold by supply houses around here are non-CTL "cheater" breakers. Some people think they can fill the whole panel with them and then land the blacks and reds from multis on the same tandem.

The "perfect storm" brewing inside the panel.

David
 
dnem said:
30 slot panels have room for 12 tandems, plus one additional tandem for each 2 pole used, before they violate 408.35
40 slot panels have room for 2 tandems, plus one additional tandem for each 2 pole used, before they violate 408.35
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Not correct: 2-pole breakers count as 2 breakers for this purpose.
 
I would like to thank everyone for there heplful answers to my question. One last note, the panels in question are Frank Adam. These panels have no modern labeling except 120/208, 100 amp, 3 phase, 4 wire. They have 20 spaces and are used commercially as lighting panels. It appears they were built in 1950's and 1960's.
 
Physical differences in panels?

Physical differences in panels?

I have educated myself on tandem breaker "do's and don't" by reading this forum. However, I have a question. What is the physical difference between a panel that has 30 single pole spaces and is only rated for 30 circuits versus a panel that has 30 single pole spaces but in rated for up to 40 circuits (allows additional circuits using some tandem breakers).

I know the manufacturers provide this rating, but what are the physical differences in the panels?

Thanks
 
It is worth noting that the 42 circuit limitation will (probably) be eliminated from the 2008 NEC, as is the distinction between power panelborads and lighting and appliance panelborads. :)
 
wjones885 said:
I have educated myself on tandem breaker "do's and don't" by reading this forum. However, I have a question. What is the physical difference between a panel that has 30 single pole spaces and is only rated for 30 circuits versus a panel that has 30 single pole spaces but in rated for up to 40 circuits (allows additional circuits using some tandem breakers).

I know the manufacturers provide this rating, but what are the physical differences in the panels?

Thanks
Click to expand...

The physical difference is the shape of the bus that the breakers snap into, whether it is solid across or if it has a small slot in it.
 
ryan_618 said:
The physical difference is the shape of the bus that the breakers snap into, whether it is solid across or if it has a small slot in it.
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It depends on the manufacturer and the model of panel. For Square D QO loadcenters the busbars are the same but there are slots in the mounting rail that holds the breakers.
 
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