Tandem Breakers.

Carultch said:
The issue is slash-rated vs straight rated breakers.

A slash rating, such as 120/240V, means it's rated for 240V between any pair of conductors in the system, and not more than 120V to ground on the particular circuit where it's used. What you need for using on an ungrounded system, are straight-rated breakers, that are rated for the full 240V, regardless of where ground is within that range. An ungrounded system may generally be in a metastable state of being 120V to ground, but there's no guarantee that it remains that way. It could have a single line-to-ground fault, that goes undetected.

The majority of 1-pole breakers in general, are slash-rated. This is likely the case for tandem breakers as well.
Click to expand...
Your reply makes more sense if applied to corner grounded delta or even ungrounded 240 delta systems. not really any good reason to use single pole breakers on those though I suppose it could be tempting to try to use tandems on those if you are short on spaces and want to supply two pole loads through adjacent breakers. But the tandems will not have a straight 240 rating.

Two wire 120 volts is still going to be 120 volts between the two system conductors whether grounded or not.
 
Birken Vogt said:
I just stumbled across an Eaton paper that basically says CTL vs non CTL is obsolete now and use whatever you want.
Click to expand...

Here is a similar document regarding Eaton CHT twin breakers until I saw this was of the opinion that the vast majority of Cutler-Hammer CH were manufactured prior to Eaton introducing the CHT twin breaker & where they clipped on the rail was different compared to CH full size breakers, that they were not OK to use other then panels that had factory positions for them.

 
One caution with using tandem even with a panel listed to have all tandem (40/80 panel) is the total load of all the branch circuits combined with all tandems can overload the main breaker and cause unwanted tripping.
Had a customer argue over the main tripping while the individual breakers didn't and couldn't understand how is that possible. 100A panel and 10 near capacity 20A breakers running between 16 and 18A each.
 
Fred B said:
One caution with using tandem even with a panel listed to have all tandem (40/80 panel) is the total load of all the branch circuits combined with all tandems can overload the main breaker and cause unwanted tripping.
Had a customer argue over the main tripping while the individual breakers didn't and couldn't understand how is that possible. 100A panel and 10 near capacity 20A breakers running between 16 and 18A each.
Click to expand...
If all branch circuits in any relatively full panel are loaded like that, you risk tripping the main. Really nothing to do with tandems, just makes it easier. 😀
 
Fred B said:
Had a customer argue over the main tripping while the individual breakers didn't and couldn't understand how is that possible. 100A panel and 10 near capacity 20A breakers running between 16 and 18A each.
Click to expand...
Was there a lot of other loads besides those 10 circuits? If those 10 circuits are 120 volts and you can put 5 on each leg then the main shouldn't trip.
 
infinity said:
Since there is no longer a 42 circuit limit this entire CTL and non-CTL nonsense should be completely removed from the NEC.
Click to expand...
Isn't the CTL issue the part of the UL Listing? Panelboard bussing ampacity is a function of the panel construction. If the panel was designed and listed with only 30 circuits, putting in wiring for 60 might overheat the panel and the neutral bus may not have enough room.
 
jim dungar said:
Isn't the CTL issue the part of the UL Listing? Panelboard bussing ampacity is a function of the panel construction. If the panel was designed and listed with only 30 circuits, putting in wiring for 60 might overheat the panel and the neutral bus may not have enough room.
Click to expand...
These new panels come as say 30/60 or 40/80 with the larger number being all twin breakers. Even with non-CTL breakers you are still required to follow the listing. As long as the listing number of breakers is followed CLT or non-CTL doesn't matter.
 
infinity said:
Fair point but as Larry stated you still have a main CB to protect the conductors and the panel unless it's a split bus. :oops:
Click to expand...
For overloads, but not internal heat transfer/dissipation if there are twice as many conductors in there.
I wish I knew why manufacturers bother to list and inventory different part numbers for what looks to be the same product.
 
You must log in or register to reply here.
Top