Main Vs. MCB

[mstrlucky74]

I've never seen this before where it lists the main as 600A but the MAC 1200A.

 

[petersonra]

maybe it is a typo. or a copy and paste error.

 

[Jraef]

maybe it is a typo. or a copy and paste error.

Yeah, otherwise it would make no sense and nobody could build it. You can't put an MCB in a panel with bus bars rated for 1/2 of the MCB rating.

 

[jim dungar]

Yeah, otherwise it would make no sense and nobody could build it. You can't put an MCB in a panel with bus bars rated for 1/2 of the MCB rating.

It is possible to build a panel with a 1200A frame main circuit breaker dialed down to a LTPU = 50%, with wire bending space for 600A conductors and put a 600A nameplate on it.
Depending on the breaker, it is probable that the protective functions are based on the Sensor and not the LTPU.

 

[Jraef]

It is possible to build a panel with a 1200A frame main circuit breaker dialed down to a LTPU = 50%, with wire bending space for 600A conductors and put a 600A nameplate on it.
Depending on the breaker, it is probable that the protective functions are based on the Sensor and not the LTPU.

Sure. But as an MCB for a panel, what's the point behind using a 1200AF breaker with a 600A rating plug, when you cannot increase the size of the rating plug because your panel bus is only rated 600A? It's an automatic limitation, so why not just use a 600A breaker? The difference in the breaker frames is huge!

 

[jim dungar]

Sure. But as an MCB for a panel, what's the point behind using a 1200AF breaker with a 600A rating plug, when you cannot increase the size of the rating plug because your panel bus is only rated 600A? It's an automatic limitation, so why not just use a 600A breaker? The difference in the breaker frames is huge!

To achieve coordination between similar sized devices, like a 600A Main with a 500A Feeder. Instantaneous settings are usually based on the sensor size, so a 1200AF might start at 6000A, while a 600AF probably stops there.

 

[mstrlucky74]

Thanks

 

[Tom Zacharias]

use of 1200 MCB and 600 amp mains

use of 1200 MCB and 600 amp mains

I've never seen this before where it lists the main as 600A but the MAC 1200A.

I have seen this happen before. Assume that the panel is a long distance from the source. Due to significant voltage drop, the feeder cables need to be significantly oversized. Even thou you only need 600 amps of service. Typically the oversized cables will not fit in the standard 600 amp frame. The 1200 amp main circuit breaker will have large enough lugs to handle the over sized feeder cables on the incoming side. I have used this trick myself. Just set the trip on the 1200 frame to 600 to handle the 600 amp bus downstream. Hope this is helpful

 

[kwired]

I have seen this happen before. Assume that the panel is a long distance from the source. Due to significant voltage drop, the feeder cables need to be significantly oversized. Even thou you only need 600 amps of service. Typically the oversized cables will not fit in the standard 600 amp frame. The 1200 amp main circuit breaker will have large enough lugs to handle the over sized feeder cables on the incoming side. I have used this trick myself. Just set the trip on the 1200 frame to 600 to handle the 600 amp bus downstream. Hope this is helpful

Can't say I have seen it but was going to suggest that kind of scenario as one possibility.

 

[Sahib]

Instantaneous settings are usually based on the sensor size, so a 1200AF might start at 6000A, while a 600AF probably stops there.

If 600A bus bars are not suitably brazed to withstand the higher fault current of 1200AF, then such MCB not to be used, IMO.

 

[Smart $]

Perhaps switchgear/MCC with 1200A horizontal bus and 600A vertical, i.e. each section is 600A max while feedthrough bus is 1200A?

 

[jim dungar]

If 600A bus bars are not suitably brazed to withstand the higher fault current of 1200AF, then such MCB not to be used, IMO.

Actually, the 600A bussing only needs to be capable of handling the maximum available short circuit current, whether the actual or design. The nameplate AIC rating of the 1200AF breaker possibly being larger than a 600AF is immaterial.

 

[Jraef]

Actually, the 600A bussing only needs to be capable of handling the maximum available short circuit current, whether the actual or design. The nameplate AIC rating of the 1200AF breaker possibly being larger than a 600AF is immaterial.

In fact the MCCB AIC rating is often higher than the MCC bus bracing when you get into 1000A and above... i.e. the MCCB will have an 85kAIC rating, but the MCC might only require 65kA or even 42kA bracing.

Back to the other issue, it seems to me that jumping from a 600AF breaker to a 1200AF breaker just to get bigger lugs is an expensive way to solve a minor problem. There are lots of less expensive options to get there and in most MCCs and some switchboard designs, you are looking at a half section of space for a 600AF breaker (maybe even less), compared to a full height section for a 1200AF main breaker. But, to each his own I suppose...

The coordination issue though, that makes more sense to me. 600AF breaker will have I trips at 400-1000%, so 2400 to 6000A, vs 1200AF will have 4800 to 12,000A I trip settings.

 

[jim dungar]

The coordination issue though, that makes more sense to me. 600AF breaker will have I trips at 400-1000%, so 2400 to 6000A, vs 1200AF will have 4800 to 12,000A I trip settings.

I first ran into this situation when manufacturers created a 600AF breaker, prior to that the sensors were commonly 800 or 1200A. It was easy to have an INST setting of 7000-8000A especially for fire pumps and other larger inrush loads.

 

[jim dungar]

600AF breaker will have I trips at 400-1000%, so 2400 to 6000A, vs 1200AF will have 4800 to 12,000A I trip settings.

I first ran into this situation, when 600AF breaker became an industry standard, prior to that the 'sensors' were commonly 800 or 1200A so it was easy to get an INST setting of 7000-8000A for fire pumps and other large inrush loads.

 

[ron]

maybe it is a typo. or a copy and paste error.

This is likely the case. This happened to one of my junior EEs recently