Rating Question

[MKELLY516]

Morning All,

1st post so if I'm in the wrong area I apologize in advance.

So I'm a little confused maybe I never noticed it before but I am seeing more and more panel schedules come out with things like 400A buss with a 100A MCB, or 250A buss 100a MCB or MCS. My question is most panel building programs don't work that way, it's either heat rated copper buss, 750 sq inch or 1000A sq inch. I lean to 1000A sq inch since in NYC that's good for up to 1200A but it does drive up the cost. Is this correct, is there anything else I can do? I'm curious.

Thanks in advance

Mike

 

[Dennis Alwon]

It sounds like the engineer is planning for the future. Is this a main distribution panel with no main? I was assuming he is just planning for future additions but IDK

 

[MKELLY516]

No they are branch panels, I agree about the future but the problem is building it configuring the bus.

 

[kwired]

If these are feeder supplied I think I would just put in the 400 amp main and put a 100 amp breaker at the feeder supply.

If you later wanted to purchase a 400 amp main to install in that panel, it probably cost more then purchasing another panel with a 400 amp main - that is just how they like to sell them:blink:

Do they also specify large enough raceway to get 400 amp conductor to such a panel should that ever be desired?

 

[MKELLY516]

Kwired my problem is a 400A MCB will only support a 200A trip. So I would have to downsize the frame to 250A to get a 100A trip unit which is not really that big an issue untill they go to put a 400A frame in the space.I think later they may run into trouble trying to fit a 400A frame breaker in a 250A space.

That's fine I can still over size the buss to a 400A capacity by way oversizing up to 1200A by specing 1000A sq in buss. My question is why are the engineers doing this?

If it's MLO then I oversize the bus to 1000A sq in but the cost goes way up and I'm knocking myself out of the bid game if I run into someone who is not doing it.

 

[electrofelon]

I have no idea why they would do this. Probably a case of "its easy to spend someone else's money".

do the panels need a main breaker?

 

[MKELLY516]

I have no idea why they would do this. Probably a case of "its easy to spend someone else's money".

do the panels need a main breaker?

Some are MCB some are MLO it's always varied.

 

[electrofelon]

Some are MCB some are MLO it's always varied.

When they are MCB, are they nec required to be MCB, i.e. in a detached structure or fed from a transformer?

 

[Jraef]

I think you are misinterpreting something here. When an Engineer is specifying bus rated 1000A per square inch, that is just the standard rating for copper bus bars. Aluminum bus is typically going to be rated 700A/sq. in. All that is referring to is basically the resistivity of the alloy used in making the bus bars. Lower resistivity of the alloy would mean the mfr could get away with smaller bus, but would result in softer bars so more susceptible to damage in a fault. Higher strength then ends up with less bracing, but higher resistance, so it would run hotter and have more voltage drop. 1000A/sq. in. has been the industry standard for as long as I have been in the industry. Basically whenever you see a spec calling for 1000A/sq. in., that's just a "boilerplate" spec for copper bus.

The actual Ampere Rating of the bus then relates to the CROSS SECTIONAL AREA of the bars themselves. So if I use bus rated 1000A/sq. in., but it has a minimum cross sectional area of 0.40 sq. in., it is 400A bus.