NEC 230.90 and Switchboards for Apartment Building

[new_ee]

I have an apartment building (80 dwelling units total) with three separate meter banks with the following loads calculated for the number of dwelling units on each one under table 220.32. Service is 208 3-phase to 208 single phase meter banks.

Meter bank 1 = 1,167,516 VA X .33 (table 220.32, 30 units) = 385,280 VA = 1069 A
Meter bank 2 = 1,177,533 VA X .33 (table 220.32, 30 units) = 388,602 VA = 1079 A
Meter bank 3 = 812,284 VA X .38 (table 220.32, 20 units) = 308,668 VA = 857 A

Meter bank 1 needs 1200A breaker in switchboard.
Meter bank 2 needs 1200A breaker in switchboard.
Meter bank 3 needs 1000A breaker in switchboard.

Now, can I take the three total loads (before table 220.32 demand) which would be 3,157,333 VA and apply .23 demand factor from table 220.32 for 67+ units and get a total load for bus of 2016 A? In this case I'll have 3 breakers with a total rating of 3400 A on a 2500 A rate bus? Which I believe is ok in accordance with 230.90 exception no. 3.

Or do I have to add the 1069 A, 1079 A, and 857A = 3005 A for the bus rating of the switchboard?

 

[jtester]

You are correct applying Exception #3, and don't need the 3000+amp bus.

Jim T

 

[charlie b]

First of all, you have one or more typos in your NEC article numbers. But I am guessing that you are working with the 2002 version.

The basic question I think you are asking is whether you size the main switchboard on the basis of the total load that it is calculated to supply. The answer is yes. Add all the loads downstream, without yet applying demand factors, and then apply the overall demand factor for the entire set of units.

However, you need to include house loads (lobby, public corridors, elevators, etc.) separately. This load gets added to the 2016 amps that you calculated, before selecting the size of the main switchboard.

I presume that the main switchboard will have a main breaker. You choose a setpoint for that main breaker to a value that is higher than the calculated load, but no higher than the switchboard rating. So even if you have three feeder breakers on the main switchboard, with a total overcurrent setting of 3400 amps, the main breaker would protect the switchboard if you set it at 2500 amps.

 

[new_ee]

Also, I wanted it to be clear that there is no main disconnect the three breakers will be serving as the 1-6 disconnects.

 

[new_ee]

Charlie,

I think you posted before I posted that last bit of information about no main disconnect. I am using 2002 NEC and I have corrected the article numbers in my original post. So, if there is no main disconnect, will the bus have to be rated to carry the sum of the full breaker ratings? Thats why I quoted 230.90 in which I believe exception no. 3 alleviates that requirement. Also, house loads will be on another service lateral to be on a separate meter.

 

[charlie b]

You are correct applying Exception #3, and don't need the 3000+amp bus.

If you ran service conductors from the secondary of the service transformer to a set of three meters and disconnects, then I would agree that Exception 3 allows the sum of the three disconnects (in this case, they are 1200, 1200, and 1000 amp breakers) to be higher than the rating of the service conductors. But in this discussion, there is a switchboard involved.

Here is where I am confused. In 408.13 (2002 code), there is a requirement that the rating of the panelboard must be at least as high as the calculated load. There is no similar requirement for a switchboard.

So how do we tell what the minimum rating of the switchboard must be? Is it based on the calculated load, or on the combined ratings of the overcurrent devices? I think that is the question that NEW-EE is asking, and I don?t have an answer.

 

[flick]

Aren't switchboards and panelboards basically the same thing, except that switchboards are generally free standing, and panelboards are mounted against the wall?

I learned, maybe incorrectly, that switchboards were freestanding units that could have parts (bus bars, etc.) accessed from front, rear, or sides by removing covers if necessary, and that panelboards provided access from the front only because they were mounted against a wall.

Anyway, I think the calculations are the same for either.

Sorry, I don't have my book in front of me and it's been a while since I've read that section of the code.

John

 

[new_ee]

If you ran service conductors from the secondary of the service transformer to a set of three meters and disconnects, then I would agree that Exception 3 allows the sum of the three disconnects (in this case, they are 1200, 1200, and 1000 amp breakers) to be higher than the rating of the service conductors. But in this discussion, there is a switchboard involved.

Here is where I am confused. In 408.13 (2002 code), there is a requirement that the rating of the panelboard must be at least as high as the calculated load. There is no similar requirement for a switchboard.

So how do we tell what the minimum rating of the switchboard must be? Is it based on the calculated load, or on the combined ratings of the overcurrent devices? I think that is the question that NEW-EE is asking, and I don?t have an answer.

Thats exactly what I'm asking.

 

[iwire]

I am lost here.

Why is the switchboard throwing a wrench in this discussion?

The rating of the switchboard need not be higher than the calculated load.

Article 100

Service Conductors. The conductors from the service point to the service disconnecting means.

In this case the buss bars are also service conductors and 230.90(A)Exception 3 applies.

A pretty common installation actually.

 

[charlie b]

I am lost here.

So am I, Bob.

A switchboard is not a conductor, despite the fact that there are conductive materials within, and the fact that those conductive materials are intended to conduct current. The service conductors run from the service point to a point within the switchboard (actually three points). The ?service disconnecting means? within that switchboard is comprised of three breakers. You can?t say that the bus bars within the switchboard are part of the service conductors, because they are internal to a component that is not under the jurisdiction of the NEC. I would say that the bus bars are the point at which the ?service conductors? end, and the point at which the ?service disconnecting means? begins.

But that still leaves us with the question of whether the bus bars (actually, the switchgear as an entity) must be rated for the calculated load or for the load that might be imposed if each of the ?one to six? breakers is loaded to its trip point. You may state that you only have to buy a switchgear that is rated for the calculated load, but I would like to see a code article that backs that up.

Aren't switchboards and panelboards basically the same thing . . . ?

Close, perhaps, but that is not relevant. Article 408 is entitled, ?Switchboards and Panelboards.? Part III discusses panelboards, and it explicitly requires the panelboard to have a rating not less than the calculated load. Part II discusses switchboards, and it has no similar statement.

 

[iwire]

Charlie I think you missed my point, I am lost as I don't see the issue at all.

The bus bars are the service conductors up until they hit the individual disconnects connect to them. Depending on the service point the busing may be the only service conductors.

This is a very common installation in my area.

One I remember had about 3000 amps of total OCPD with 1200 amp busing in the switchgear.

It would make absolutely no sense to require 3000 amps of busing supplied with 1200 amps of 'wire' for a calculated load of less than 1200 amps.

 

[iwire]

Whoa....I think I see the problem

We are confusing panel boards and switch boards.

Given that this 'board' will apparently only contain breakers supplying feeders it is a switchboard

Switchboards do not require OCP

 

[charlie b]

Switchboards do not require OCP

I am begining to see that. But they do have ratings (specifically a current rating) do they not? I think we would agree that it would be "bad" if we were to load a switchboard beyond its rated current. But what prohibits us from doing so? I should like to hope there would be more than just 110.3(B). Indeed, I think there should be something in Part II of 408, but there is not.

So here again, what allows us to use a swichboard that is rated only for the calculated current, and not also for the total connected load, or not also for the sum of the overcurrent devices for the downstream loads? I think that it should be OK, but what code article says it is OK?

 

[iwire]

Charlie when I first realized this I was very surprised,

It was a shock to me that I did not have to protect a switchboard or the conductors supplying it via the exception 3 to 230.90

In the installation we are discussing the only thing that protects either the conductors or switchboards is future electricians and engineers paying attention before adding load to the service.

The service I mentioned with 3000 amps of OCP on 1200 amps of conductor really rubbed me the wrong way as the 1200 covered just the 'shell' of an otherwise empty building, there is no doubt in my mind that the load on that service today would calculate above 1200.

This is a poplar way of installing a large service because it can avoid GFP protection.

I don't believe we need a section that says it's OK as we don't have a section that requires protecting a switch board.