CB Sizing

[Alwayslearningelec]

Looking at this drawing the top number is the CB size and the bottom is what it is fused at. Is that how it works? This is a draw out breaker.

 

[zog]

I don't see any fuses. Typically the top number is the breaker frame size and the bottom number is the CT size (or CT tap or rating plug)

 

[bob]

I could interpret that to mean frame size and breaker size although I am not sure the breaker would fit.
What about it?

 

[Jraef]

800
400

800A frame size, 400A trip unit
This means that the FRAME of the breaker will ACCEPT up to an 800A trip unit, so the POWER COMPONENTS can handle up to 800A, but the unit in question currently has a 400A trip unit (or plug, or CT etc. etc.) so it is intended to be used only on a 400A circuit. This is done so that if someday in the future the circuit size needs to increase, all you have to do is change the trip unit, assuming of course that the cables are properly sized (which is often done with future consideration issues).

If you had a 400/400, then there is no chance of some day increasing the circuit size without changing the entire breaker. If in a switchboard, that may end up entailing a rearrangement of several breakers inside to make physical room for the larger frame. This all increases down time. In draw-out gear that is not usually an issue, so often it is based simply on initial cost. If someday you want to increase the size and you started with too small of a frame, a replacement cubicle is a LOT more expensive than buying an oversized one in the first place and changing the trip unit/rating plug

 

[zog]

Well said Jraef but the OP said drawout breakers, which typically do not have trip units with ratings like a MCCB would, it is the CT's and/or rating plug that determine the max trip. All other point made are still valid.

 

[jim dungar]

We also will also use larger frame sizes for coordination purposes.
The Instantaneous and Short time settings are usually based on the sensor/ct/frame and not the trip/rating plug (long time settings).

 

[Jraef]

Well said Jraef but the OP said drawout breakers, which typically do not have trip units with ratings like a MCCB would, it is the CT's and/or rating plug that determine the max trip. All other point made are still valid.

Thanks for the compliment.

(or plug, or CT etc. etc.)

I was trying to be generic, this issue comes up a lot and I am constantly surprised at the numbers of people who don't know this, INCLUDING a few PEs I have run in to! I think it's something that people who do a lot of work with gear and breakers learn early on and take for granted, but it's not necessarily taught in school or in the trades. I know when I was an electrician, I don't recall anyone explaining it to me. I didn't learn until I got into engineering for an equipment mfr.

 

[Jraef]

We also will also use larger frame sizes for coordination purposes.
The Instantaneous and Short time settings are usually based on the sensor/ct/frame and not the trip/rating plug (long time settings).

Thanks, hadn't thought of that. I'll add that to my spiel. As I said in the other post, I get this question a lot.