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.
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I think this is the issue. It's not "ungrounded" systems, it is 240/120 3 phase 4 wire DELTA systems, where the B phase to ground is 208V potential. Single pole plug-in breakers, and by virtue of the fact that they use a single pole connection, Tandems, are only rated for maximum 120V reference to ground, so cannot be plugged into B phase on a 4 wire Delta panel.

When you have that kind of system, the breakers that plug into the B phase must be "fully rated" for 240V, not "slash rated" as in 120/240V. For most brands, there are 2 pole fully rated 240V options, but for single pole, no, which will mean having to use a 480V rated breaker like an I-Line Square D if you want a single pole plug-in.
 
Jraef said:
When you have that kind of system, the breakers that plug into the B phase must be "fully rated" for 240V, not "slash rated" as in 120/240V. For most brands, there are 2 pole fully rated 240V options, but for single pole, no, which will mean having to use a 480V rated breaker like an I-Line Square D if you want a single pole plug-in.
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In the Square D lineup there is the NF panels that have 277 volt rated single pole breakers and 277/480 volt rated two and three pole breakers.

If you have any version of 480 volt delta you do have to use the I-Line series to get a straight 480 volt rated breaker NF doesn't have that rating.

I am not all that familiar with other brands, especially for over 240 volts as have not been around all that many others very often. But I do believe there are some others that have a line similar to the Square D NF line that are likely 277 for single poles and 277/480 for two and three pole, but need to go to some other line if you need straight 480 rating.
 
kwired said:
In the Square D lineup there is the NF panels that have 277 volt rated single pole breakers and 277/480 volt rated two and three pole breakers.

If you have any version of 480 volt delta you do have to use the I-Line series to get a straight 480 volt rated breaker NF doesn't have that rating.

I am not all that familiar with other brands, especially for over 240 volts as have not been around all that many others very often. But I do believe there are some others that have a line similar to the Square D NF line that are likely 277 for single poles and 277/480 for two and three pole, but need to go to some other line if you need straight 480 rating.
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Siemens is nice because their standard buss provision accepts both the 240V BL breakers and the 277/480 BQD breakers so you can mix and match. If you need a fully rated 480V or 600V breaker however, you will need to go to their xGB provision which is different.
 
electrofelon said:
Siemens is nice because their standard buss provision accepts both the 240V BL breakers and the 277/480 BQD breakers so you can mix and match. If you need a fully rated 480V or 600V breaker however, you will need to go to their xGB provision which is different.
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How often do you run into people installing 240 volt breaker in 480 panel because the 240 cost less? or is this only something found in bolt on versions and maybe that alone sort of lessens that chance to some degree?

I do like the 3/4" wide profile of QO. Put in a few 60 or even 84 space panels that weren't so tall you had to put the bottom of the cabinet right on the floor to avoid having the highest device handle too high. Could see that happening with 1 inch breakers. In fact have installed several NF (277/480) that were (IIRC) 54 circuits with a 400 amp main breaker and also with feed through lugs on opposite end of bus from main breaker - cabinet for those is nearly 7 feet high.
 
kwired said:
How often do you run into people installing 240 volt breaker in 480 panel because the 240 cost less? or is this only something found in bolt on versions and maybe that alone sort of lessens that chance to some degree?
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I don't think I have ever seen it. Yes this is Siemens bolt on panels. IMO its a non issue having the same mounting provision for both 240 volt and 480 volt breakers. If some unqualified person or brother-in-law wants to put a 240 volt breaker in a 480 volt panel, it's none of my concern.

I do like the 3/4" wide profile of QO. Put in a few 60 or even 84 space panels that weren't so tall you had to put the bottom of the cabinet right on the floor to avoid having the highest device handle too high. Could see that happening with 1 inch breakers. In fact have installed several NF (277/480) that were (IIRC) 54 circuits with a 400 amp main breaker and also with feed through lugs on opposite end of bus from main breaker - cabinet for those is nearly 7 feet high.
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Yeah some of these larger panel boards, especially with larger frame breakers, can get pretty tall. I had one, a 600A Siemens P3, that had 20 branch spaces, 4 200 A breakers, and a 400A subfeed breaker. Had to mount it just a couple inches off the floor to have enough room to 90 out of the top and clear the ceiling. Another one was a 400A Siemens P2 which had I think 30 Branch spaces and then three 200 amp breakers. That one I think I might have had to cheat the height to the highest breaker a few inches because in that one the 200 amp breakers have to mount singly not side by side like they can in the P3. Also, the bottom couldn't be less than two feet above the floor.
 
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