Main rated current vs Bus rated current in Meter banks

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shortcircuit1

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Hello,

I was looking at a 2000A Meter bank from Square D
http://www.schneider-electric.us/en...r-pak-meter-centers&node=166378555-main-units

I came across two terms Main rated current and Bus rated current. While i do know that the main rated current is going to be the rating of the breaker that it uses which is 2000A but bus rated current was 1200A. So what does this bus rated current refers to?Is this the current that is being taped to stacks of meter banks on the sides of the meter bank?If so is it 1200A on each side of the bus?

Thanks,
shortcircuit
 
I believe you understand it. Branch units are center fed and load to either side of main cannot exceed 1200A.
 
Smart $ said:
I believe you understand it. Branch units are center fed and load to either side of main cannot exceed 1200A.
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Yes..But if you have a 2000A breaker sitting on 1200A bus,how does the breaker even trip?Its never gonna trip between 1200A-2000A unless your breaker's LTPU is set at like 0.6.Isnt it?
 
shortcircuit1 said:
Yes..But if you have a 2000A breaker sitting on 1200A bus,how does the breaker even trip?Its never gonna trip between 1200A-2000A unless your breaker's LTPU is set at like 0.6.Isnt it?
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Because the bus is center fed. A generalization is if you have 1000A going to the right and 1000A going to the left, the main breaker will see 2000A... but you're not even close to exceeding the bus rating of 1200A.
 
Ingenieur said:
How many meters and what size on each side?
6 x 200
8 x 125
???
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I was just doing an example

I came up with 2000A service with load calcs and i am trying to decide how to balance the loads on a meter bank.These have 65 residential units and the Amps for each unit came out to be 121A.So i picked a 175A breaker cause 121*1.25=151A and the next up size is 175A. Now i have to split 65 units into 2 meter bank and i am trying to see how can i go ahead and do the phase balancing on each of them...Any ideas?
 
Smart $ said:
Because the bus is center fed. A generalization is if you have 1000A going to the right and 1000A going to the left, the main breaker will see 2000A... but you're not even close to exceeding the bus rating of 1200A.
Click to expand...

Exactly my point...The breaker LTPU should be set at 0.6 for 2000A breaker so that it trips if it goes over 1200A which is highly unlikely...
 
Smart $ said:
Because the bus is center fed. A generalization is if you have 1000A going to the right and 1000A going to the left, the main breaker will see 2000A... but you're not even close to exceeding the bus rating of 1200A.
Click to expand...

I remember your point on the meter bank when i asked a question about balancing..Here's the problem i am dealing with:
65 units each unit is going to have a meter with 175A breaker but the question is how do you split into two meter bank and still maintain the phase balancing...
 
shortcircuit1 said:
I remember your point on the meter bank when i asked a question about balancing..Here's the problem i am dealing with:
65 units each unit is going to have a meter with 175A breaker but the question is how do you split into two meter bank and still maintain the phase balancing...
Click to expand...
See my post in your other thread on unbalance.
 
shortcircuit1 said:
Exactly my point...The breaker LTPU should be set at 0.6 for 2000A breaker so that it trips if it goes over 1200A which is highly unlikely...
Click to expand...
I never learned what breaker settings should be for center fed bus but I think you are mistaken. This setup is like tapping a 2000A feeder with 1200A taps... less the 2000A feeder. In other words, like the tap conductors are connected directly to the feeder ocpd. The taps are protected by the downstream ocpd, not the source ocpd. You treat the source ocpd the same as you would if it had a 2000A feeder connected with the taps somewhere downstream.
 
Smart $ said:
I never learned what breaker settings should be for center fed bus but I think you are mistaken. This setup is like tapping a 2000A feeder with 1200A taps... less the 2000A feeder. In other words, like the tap conductors are connected directly to the feeder ocpd. The taps are protected by the downstream ocpd, not the source ocpd. You treat the source ocpd the same as you would if it had a 2000A feeder connected with the taps somewhere downstream.
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Lets say you if the current on the feeder tap or the horizontal cross bar is 1400A and all that current is still going to be seen by Main OCPD which is 2000A. If you set the trip of 2000A breaker at 1 which corresponds to 2000A then its never going to trip for a overload...Look at the line side of the bus tap not the load side..
 
shortcircuit1 said:
Lets say you if the current on the feeder tap or the horizontal cross bar is 1400A and all that current is still going to be seen by Main OCPD which is 2000A. If you set the trip of 2000A breaker at 1 which corresponds to 2000A then its never going to trip for a overload...Look at the line side of the bus tap not the load side..
Click to expand...
I understand that. But it is a tap situation. You have no choice but to protect the 1200A bus with downstream ocpds and calculated load.

Yes, there is a possibility of a resistive fault occurring on the 1200A bus, but the industry has weighed that possibility into the probability of occurrence to be unlikely. A fault on the 1200A bus is more likely to be a bolted fault. With a bolted fault, the normal operation bus rating is irrelevant.

Perhaps someone with more knowledge and experience with setting such a service up can step in and advise.
 
I would just add that there is no specific requirement to protect a meter bus at its rating, like we have to do for panelboards (408.36). IMO it just comes down to 110.3 and load calcs for the serving bussing not exceeding the bus rating.
 
Smart $ said:
I understand that. But it is a tap situation. You have no choice but to protect the 1200A bus with downstream ocpds and calculated load.

A fault on the 1200A bus is more likely to be a bolted fault. With a bolted fault, the normal operation bus rating is irrelevant.
Click to expand...

Can you explain a little bit detailed on that point?I didnt get that...
 
shortcircuit1 said:
Can you explain a little bit detailed on that point?I didnt get that...
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In order for the bus on one side to have an overcurrent condition of 1400A it would most likely be through the meters and subsequent ocpds. This would be considered a resistive fault. The probability of several "branches" overloading at the same time, enough to overload the bus to 1400A because the sum of ocpds exceeds the 1200A rating is possible but quite unlikely without tripping one or more branch ocpds. That's why you protect the bus with the calculated load. At least that's the theory behind it and there has not been enough empirical evidence to change the requirements.

A bolted fault is basically a direct short of near zero impedance. With the bus physically protected by the enclosure(s) and insulators, the probability of a conductive anything bridging the buses or bus to the enclosure(s) is minimal, so the probability of a bolted fault is also quite unlikely. However, should a bolted fault occur, the bus would be seeing the maximum available fault current, which is likely well in excess of 2000A... so the 1200A bus rating is moot... but this would be the condition where your main should trip.
 
Smart $ said:
In order for the bus on one side to have an overcurrent condition of 1400A it would most likely be through the meters and subsequent ocpds. This would be considered a resistive fault. The probability of several "branches" overloading at the same time, enough to overload the bus to 1400A because the sum of ocpds exceeds the 1200A rating is possible but quite unlikely without tripping one or more branch ocpds. That's why you protect the bus with the calculated load. At least that's the theory behind it and there has not been enough empirical evidence to change the requirements.

A bolted fault is basically a direct short of near zero impedance. With the bus physically protected by the enclosure(s) and insulators, the probability of a conductive anything bridging the buses or bus to the enclosure(s) is minimal, so the probability of a bolted fault is also quite unlikely. However, should a bolted fault occur, the bus would be seeing the maximum available fault current, which is likely well in excess of 2000A... so the 1200A bus rating is moot... but this would be the condition where your main should trip.
Click to expand...

Thank you sir for the detailed explanation...:):thumbsup:.Now i can understand clearly..
 
Smart $ said:
I never learned what breaker settings should be for center fed bus but I think you are mistaken. This setup is like tapping a 2000A feeder with 1200A taps... less the 2000A feeder. In other words, like the tap conductors are connected directly to the feeder ocpd. The taps are protected by the downstream ocpd, not the source ocpd. You treat the source ocpd the same as you would if it had a 2000A feeder connected with the taps somewhere downstream.
Click to expand...

I would agree that this is somewhat like a tap. The the smaller bus is protected from overload by the downstream OCPD and short circuit/ground fault is provided by the upstream main OCPD. It's similar to a MCC where the horizonal and vertical buses will be different sizes. I think the OP is not recognizing the distinction between overload and short circuit/ground fault.
 
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