Service Conductors for a multi-family building

[augie47]

The following came by IM:
Hello, I am new to the forum. I need help on an electrical design i am doing.

I am providing electrical service to five (5) separate multi-family dwelling buildings with total 52 dwelling units plus a community center. The Power Co. is providing the pad mounted dry type delta-wye transformer inside the apartment complex area. Each Multi-family building will have a meter center with a main service disconnect. The service conductors will connect directly to the secondary of the transformer without OCPD.

Below are the service conductors (using USE-2 or XHHW-2) and main fuse size for each building. The farthest apt. building is at least 160 feet away from the Power Co. transformer

Building #1, 2, 3 (10 apt. units) = 4-#500KCM, 350A main fuse
Builing #4 (8 apt. units) = 4-500KCM, 350A main fuse
Building #5 (14 apt. units) = 2 sets of 250KCM, 450A main fuse
Community Building - 4-500KCM, 250A main fuse

Do I consider the 4th cable of the service conductor as current carrying conductor?

I also calculated demand load for using the 52 dwelling units plus the community center, is this ok?

I appreciate your help in advance. Thank you.

 

[augie47]

Since your loads are basically linear, the 4tyh conductor (neutral) will no be counted as a ccc.

You could consider all 52 units for the overall load but the service for each building would need to be figured for the number of units that building serves.

 

[kwired]

Size of transformer is really up to POCO. They may want to know what you have for load, or at least know if there is electric heat or not and other major loads within the apartments to help them determine what size transformer they want. I'd bet they don't use a dry type transformer either.

 

[bratzpack]

Below are the steps that I did in calculating the demand load for the entire apartment complex as noted on this thread.

1) I used the standard method to calculate the demand load and to size the electrical service for each building.

For the overall demand load I added all the five (5) building's connected load (Gen. Lighting & SAL, Appliance Load & Air Conditioning, electric ranges) and apply the demand factor using the Standard method and added the demand load for the Community Building.

2) Since the electrical service to the the five (5) buildings is 120/208V, 3 ph, 4 wire and the electric ranges are fed from 120/208V, 1 ph panels, I used this formula to calculate the demand load for the total of 52 electrical ranges @ 9.1KW each.


maximum number between any two phase legs = total no. of ranges/3 = 52/3 = 18

2*18 = 36

See Table 220.55 for demand factor for 36 ranges = 15kw + 1kw for each range = 15kw + 1kw (52 ranges) = 67 kw

per phase demand = 67000W/2 = 33,500W

Equivalent three phase load = 33,500*3 = 100,500 W (Electric range demand load)

3) Laundry circuit is not provided for each dwelling unit because the apartment complex will have a common laundry area. Refrigeration & countertop microwave load is considered to be included in the 2 SAL circuits per 210.52 (B1). No water heaters in each dwelling unit.

Can you please take a look at the calculation I did (see attachement), if it is ok or if I have to do it another way (i.e. use optional method). Based on the calculated overall demand load, can i use a 500KVA transformer for the entire apartment complex.

Again, thank you for your help. All comments are appreciated.

 

[George Stolz]

Size of transformer is really up to POCO. They may want to know what you have for load, or at least know if there is electric heat or not and other major loads within the apartments to help them determine what size transformer they want. I'd bet they don't use a dry type transformer either.

Agreed. I have a 500 kVA serving a 79 unit apartment with around 2600A calculated load. It's on the utility.

 

[ActionDave]

Agreed. I have a 500 kVA serving a 79 unit apartment with around 2600A calculated load. It's on the utility.

2600A calculated load......Two hundred bucks, I'll take the under two to one odds.

That's not really fair is it.......okay I'll take over 500kva against the POCO for ten bucks even money.

 

[JoeStillman]

I also calculated demand load for using the 52 dwelling units plus the community center, is this ok?

You only needed the separate calcs for each building. It's up to the utility co. to size the transformer. They don't use the NEC for that. They need to know how many and how big the buildings are, and they need to know what's gas and what's electric. But the NEC demand for the aggregate of all the buildings is not meaningful to anyone. They will use their own demand factors.

 

[kwired]

Agreed. I have a 500 kVA serving a 79 unit apartment with around 2600A calculated load. It's on the utility.

Had a school building(s) a few years back - can't recall the load calculation but we did have 100kVA transformer supplying 1000 amps worth of service disconnecting means (120/240 single phase) - probably did have peak demand over 100 kVA but maybe only for a few hours on hot days at beginning of school year because of air conditioning load we were adding at that time. POCO did not change their transformer - they figured it had time to cool before it was loaded again the next day and it wouldn't necessarily be loaded that hard for that many days of the year either.

Bottom line is it was totally their problem if it did fail, but they were willing to take that gamble vs investing in a larger unit that would not get loaded all that often. They can get away with it easier then us inside electricians though - if their transformer burns up it is outdoors on a pole or in its own padmount enclosure. Us inside electricians have the risk of such a failing unit causing fire damage to the building if we install it indoors.

 

[bratzpack]

Since your loads are basically linear, the 4tyh conductor (neutral) will no be counted as a ccc.

You could consider all 52 units for the overall load but the service for each building would need to be figured for the number of units that building serves.

The service conductors where sized with the 4th conductor (neutral) not considered as current carrying. The apartment building will be fed directly from the delta-wye transformer secondary and terminate at the service equipment at the meter bank. All service conductors will be located outdoors.

Example for apartment building #1 with 10 units and with electric range @ 9.1KW each, I sized the service conductor with 4-500KCM USE-2 cable and provide a service equipment with 350A OCPD. The electrical service is 120/208V, 3Ph, 4 wire. The electrical panel for each apartment as I noted before are 120/208V, 1PH, 3 wire. See attached electrical calculation.

My question is, can I size the service equipment OCPD to 400A, using the next size rule?

If each dwelling unit will use LED lights, do we have to reconsider that the 4th conductor (neutral) of the electrical service conductor will be current carrying?

 

[augie47]

The service conductors where sized with the 4th conductor (neutral) not considered as current carrying. The apartment building will be fed directly from the delta-wye transformer secondary and terminate at the service equipment at the meter bank. All service conductors will be located outdoors.

Example for apartment building #1 with 10 units and with electric range @ 9.1KW each, I sized the service conductor with 4-500KCM USE-2 cable and provide a service equipment with 350A OCPD. The electrical service is 120/208V, 3Ph, 4 wire. The electrical panel for each apartment as I noted before are 120/208V, 1PH, 3 wire. See attached electrical calculation.

My question is, can I size the service equipment OCPD to 400A, using the next size rule?

If each dwelling unit will use LED lights, do we have to reconsider that the 4th conductor (neutral) of the electrical service conductor will be current carrying?

answering in reverse order:::
In order for you to count the 4th wire (neutral) as a CCC on a 4 wire wye service, a major potion of the load would have to be nonlinear. LED lighting would not be a majority of the load so using a 500 kcmil cu with a 75? rating of 380 amps you can indeed go to a 400 amp OCP device as long as your calculated load does not exceed 380 amps..

 

[bratzpack]

answering in reverse order:::
In order for you to count the 4th wire (neutral) as a CCC on a 4 wire wye service, a major potion of the load would have to be nonlinear. LED lighting would not be a majority of the load so using a 500 kcmil cu with a 75? rating of 380 amps you can indeed go to a 400 amp OCP device as long as your calculated load does not exceed 380 amps..

I forgot to mention that the service conductor cable is 90 degree rated but I assume that the ampacity will still be based on the 75 degree table.

 

[augie47]

I forgot to mention that the service conductor cable is 90 degree rated but I assume that the ampacity will still be based on the 75 degree table.

110.14 would limit you to the ampacity of the terminations. The transformer might be 90? but the service equipment end would almost certainly be no greater than 75?

Just a note which will not be applicable unless you have more than one apt feeder in a raceway. When you use a 1 phase feeder to the apt. from a 3 phase wye supply, that neutral (to the apt) will count as a CCC. With one feeder you are dealing with 3 wire so there would be no derating for conduit fill but that's a little note that you might keep in mind for future reference.

 

[Smart $]

@bratzpack

Glancing through your calculation sheet I noticed a couple points are "off".

On Page 1, you multiply demand amps by 1.25 for fuse sizing (and from thread, service conductor sizing). None of your loads are continuous. You do not have to multiply by 125%.

On building level sheets, you note if more than 4 appliances, 75% demand. It is actually 4 or more. And while you may not be able to apply this demand to apartment feeders, you can apply the 75% demand to the service conductors because there are 4 or more supplied by the service conductors.

Also noted a few math errors in the connected loads subtotals area of the building sheets. For example, Building 1 Apartments 1D through 1F show doubled values. And page notes 10 units but only shows 7.

 

[augie47]

I'm a bit concerned about your HVAC load. 10 buildings @ 20,244 to 31,000
. That's around 10 amps per unit. Bit low for AC I would think.

 

[bratzpack]

I'm a bit concerned about your HVAC load. 10 buildings @ 20,244 to 31,000
. That's around 10 amps per unit. Bit low for AC I would think.

The specified AC units are wall mounted units. 7900 BTU (6.4 Amps) and 5700 BTU (4.7 Amps) @ 110V.

 

[bratzpack]

@bratzpack

Glancing through your calculation sheet I noticed a couple points are "off".

On Page 1, you multiply demand amps by 1.25 for fuse sizing (and from thread, service conductor sizing). None of your loads are continuous. You do not have to multiply by 125%.

On building level sheets, you note if more than 4 appliances, 75% demand. It is actually 4 or more. And while you may not be able to apply this demand to apartment feeders, you can apply the 75% demand to the service conductors because there are 4 or more supplied by the service conductors.

Also noted a few math errors in the connected loads subtotals area of the building sheets. For example, Building 1 Apartments 1D through 1F show doubled values. And page notes 10 units but only shows 7.

Thank you very much for your comments.

I intentionally did apply 1.25 * total demand amps of each building to get the size of the main OCPD at the service equipment though to provide some allowance. It just seems that the total demand amps for the whole building is very small considering that each apartment unit has electric cooking, so i decided to add an extra 25% to the demand load. I also assumed that since the apartment panels are all 120/208v, 1 Ph, one phase leg of the electrical service might be loaded more than the other two phases. Is this acceptable?

For the appliance section; I will correct it to read as per your comment.

The values are doubled because Apts. 1D to 1F have two (2) unit each for Building #1.

 

[Smart $]

...

I intentionally did apply 1.25 * total demand amps of each building to get the size of the main OCPD at the service equipment though to provide some allowance. It just seems that the total demand amps for the whole building is very small considering that each apartment unit has electric cooking, so i decided to add an extra 25% to the demand load. I also assumed that since the apartment panels are all 120/208v, 1 Ph, one phase leg of the electrical service might be loaded more than the other two phases. Is this acceptable?

...

Yes, it is acceptable. Code calculation provides the minimums, but dividing total VA by voltage and phase does not account for imbalance. Example D1(B) shows [one method of] imbalance compensation (only 1? though)... but it's not emphasized in any of the pertinent chapters, nor is a prescribed method. Another method would be to pad your building total by an increase to the next multiple of 3. For example, with 10 units which is not equally divisible by 3, one might figure at 12 units (though this is based on equal units). One note on this though is that 220.54 and 220.55 do include compensation for 3? 4W wye systems. So those are something to take into consideration when padding rather than itemizing.

 

[GoldDigger]

Alternate current
Line to ground and ground to line
Current alternates

Shouldn't that be
Alternate currents....
....
Currents alternate

?