Power Distribution and Utility Service

[Samardas]

Hi,

As I said earlier, I am new in the design field trying to make my living. Thank you All so much for helping me in the past.

I have recently designed and asked for a 4,000A service from the Utility Co. in Los Angeles area. The Utility Co. will bring their service disconnect to the Main Switchboard "MS" (A conceptual SLD and MS elevation attached for your reference). The MS has four sections with 1600A, 2x1200A and 800A CB's. The owner already has 4 x 27KVA, 480-120/208V transformers and 4 x 100A sub-panels each of which they want to connect to each section of the MS. Or, can it be a direct cable connection ((Main Lug Only (MLO) on the bus)) of 50A capacity for each transformer? I am not sure where to look for in the NEC code section which allows (or disallows) this arrangement. Can you please suggest if this can be done.

Also if you could shed some light on my perpetual grounding requirement (and problems) for this setup that will be really helpful. Thanks a LOT for your help now and in the past. Samar Das, P.E.

 

[don_resqcapt19]

The transformers need primary protection based on 450.3, so you need a breaker in main switch gear. The secondary conductors need overcurrent protection per 240.21(C). The easiest way to do this on your design is to have main breakers in the sub-panels.

 

[iceworm]

Not sure what you mean by "can it be main lug only". Do you want to connect the 27kva xfm primary direct to the 1600A CB? That really doesn't sound like a very good idea.

Looking at your one-line:
Curiousity Q: It appears to show 4 separate services. Each section has a meter and a main CB. Are there 4 separate customers? If it is just one customer, why not have the metering on the 4000A disconnect? Just curiousity - not important.

Looking at each section as a separate service:
I am a bit baffled as to why one would put in a 1600A service to feed a 27kva xfm, but you certainly can if that is what your customer wants. However, consider the conductors from a 1600A main CB will be on the order of 5 parallel - 500kcmil CU.

A normal line up is to run a full sized 1600A feeder to a multi-section MCC or a 1600A panelboard. Can't say I have ever seen a 1600A panelboard, but I'm sure they exist. One of the buckets (in the MCC) or feeder CB (in the panel board) is a 50A to feed the 27kva xfm. Other spaces are available to feed other future loads.

Perhaps a bit more information on conceptual design. Where is this going?

Grounding:
What you show meets code. Check 250.66 and be sure to read the table notes.

Other things to consider:
You show a 4000A feeder from Edison. That is going to take a 3MVA xfm. They likely won't put that big of one in - after all, you only have 100 kva of load. Set that aside for a few minutes. Supose they do put in a 3MVA, 8%. That gives 50KA short circuit current available . 3MVA at 5% gives 80KVA. You may wish to consider bus differential protective relays. 80kA from a solidly grounded wye make a real mess into a ground fault.

The 4000A disconnect switch: Is that just a manual switch - like a bolted pressure switch, a fused switch, or a CB?

Just a few thoughts. Can't really say more with the information given

the worm

 

[Samardas]

Power Distribution and Utility Service

Not sure what you mean by "can it be main lug only". Do you want to connect the 27kva xfm primary direct to the 1600A CB? That really doesn't sound like a very good idea.

Looking at your one-line:
Curiousity Q: It appears to show 4 separate services. Each section has a meter and a main CB. Are there 4 separate customers? If it is just one customer, why not have the metering on the 4000A disconnect? Just curiousity - not important.

Looking at each section as a separate service:
I am a bit baffled as to why one would put in a 1600A service to feed a 27kva xfm, but you certainly can if that is what your customer wants. However, consider the conductors from a 1600A main CB will be on the order of 5 parallel - 500kcmil CU.

A normal line up is to run a full sized 1600A feeder to a multi-section MCC or a 1600A panelboard. Can't say I have ever seen a 1600A panelboard, but I'm sure they exist. One of the buckets (in the MCC) or feeder CB (in the panel board) is a 50A to feed the 27kva xfm. Other spaces are available to feed other future loads.

Perhaps a bit more information on conceptual design. Where is this going?

Grounding:
What you show meets code. Check 250.66 and be sure to read the table notes.

Other things to consider:
You show a 4000A feeder from Edison. That is going to take a 3MVA xfm. They likely won't put that big of one in - after all, you only have 100 kva of load. Set that aside for a few minutes. Supose they do put in a 3MVA, 8%. That gives 50KA short circuit current available . 3MVA at 5% gives 80KVA. You may wish to consider bus differential protective relays. 80kA from a solidly grounded wye make a real mess into a ground fault.

The 4000A disconnect switch: Is that just a manual switch - like a bolted pressure switch, a fused switch, or a CB?

Just a few thoughts. Can't really say more with the information given

the worm

Thanks LOT to both Don and Iceworm. Yes, according to the customer, he is getting one 4000A service from Edison and then dividing it to four tenants metering through the Main Switchboard (MS).

Main Switchboard (MS) has a 4000A Fused Switch with a Ground Sensor.

As you suggested and I did show on the SLD, a 50A primary protection for the transformer in each of the switch sections. I was also wondering if this can be done physically and also by the NEC, as I tried to show on the elevation. In other words, I am also using the MS as distribution switchboard with one 50A CB, instead of using a separate distribution switchboard. I am not going to show the transformer primary connected to 1600A (or 1200A or 800A) CB.

Yes, originally I did show a distribution switchboard from each section and connected one CB to the transformer and other CB's for future use to meet the capacity of 1600A, 1200A etc. However, the customer does not want to spend money to add distribution switchboard and only wants to have a minimum load hence, for the tenants to decide their needs as they rent the place. The items shown on the drawing are the ones he already has.

I got the Available Short Circuit Current at the service from Edison planner which I will be using for my other AIC calculations. I do not know what their transformer size is. The MS has an AIC rating of 65KA.

Looking at the drawing, please let me know if you need any other information?

Thanks again for all your help.

 

[kwired]

Am I missing something or do you like to just design in a lot of extra capacity - If so how do I get to work on some of your designed projects?

I am seeing a supply gear that possibly could handle up to 3325.44 kVA but only has ~80kVA connected to it. Seems a little like selling a 55 passenger bus to a customer that only needs a 4 passenger sedan.

 

[wbdvt]

Another item to consider is arc flash hazard. Depending on the design of the MS switchboard, the 4000A disconnect may or may not be taken into consideration for the incident energy value on the switchboard. You refer to this as a disconnect switch and symbol shows a circuit breaker but elevation drawing leads me to believe it might be more like a bolted pressure switch with fuses?

Now is the time consider any arc flash hazard issues. Is this all brand new construction? Is the utility pad mount transformer existing? A preliminary arc flash analysis can be done to determine incident energy and it may be possible to reduce any high incident energy levels now.

 

[Samardas]

Power Distribution and Utility Service

Now is the time consider any arc flash hazard issues. Is this all brand new construction? Is the utility pad mount transformer existing? A preliminary arc flash analysis can be done to determine incident energy and it may be possible to reduce any high incident energy levels now.

Yes, it is a BP Fused Switch and the diagram has been revised. The whole setup is New. The transformer from the Utility Co. is New. The Main Switchboard (MS) shown on the drawing is also New. However, the distribution transformers and sub-panels salvaged from the site will be reused.

Again, my question is whether I can use the MS as a Distribution Switch as well, to connect the distribution transformers and the sub-panels from each of the Switch sections as I have shown on the SLD and the Elevation drawings. Thanks.

 

[iceworm]

... Again, my question is whether I can use the MS as a Distribution Switch as well, to connect the distribution transformers and the sub-panels from each of the Switch sections as I have shown on the SLD and the Elevation drawings. ...

Sam -
I've never seen a 1600A service cubicle that also had a 50A CB in it. If you can order one, that's great.

Just curious: This 50A CB, does it connect to the line side of the 1600A Main CB or to the Load side? Is there one Main CB (the 1600A) or two (the 1600A and the 50A)? It's okay either way. I'm just curious how the mfg is internally connecting the CBs.

ice

 

[Samardas]

Sam -
I've never seen a 1600A service cubicle that also had a 50A CB in it. If you can order one, that's great.

Just curious: This 50A CB, does it connect to the line side of the 1600A Main CB or to the Load side? Is there one Main CB (the 1600A) or two (the 1600A and the 50A)? It's okay either way. I'm just curious how the mfg is internally connecting the CBs.

ice

Dear Iceworm,
Thanks for your input. Yes, I have also never seen one that is connected to a 1600A (or 1200A, etc) service without going through a distribution switchboard. The owner wants to rent this place to four tenants, one of them will have a 1600A service, two with 1200A each and one with an 800A service based on their square footage. He wants to give them minimum power at this time since each tenant will prepare their own Tenant Improvement (TI) design based on their needs. Therefore he wants to connect those transformers (27KVA each) and sub-panels (100A each) to each service.

Main Switchboard (MS) has already been purchased by the owner, as shown on the design. I am wondering how best I can design (or even possible to design) so that he gets what he wants. Can I use a Gutter (or something) on the load side of the 1600A switch (Same for other switches as well) and connect a 50A breaker from the Gutter for the primary side of the transformer, to minimize cost? Or, is there any better suggestion? I am attaching the drawing again for ready ref. Thanks again for all your help.

 

[iceworm]

... He wants to give them minimum power at this time since each tenant will prepare their own Tenant Improvement (TI) design based on their needs. Therefore he wants to connect those transformers (27KVA each) and sub-panels (100A each) to each service.

Main Switchboard (MS) has already been purchased by the owner, as shown on the design. I am wondering how best I can design (or even possible to design) so that he gets what he wants. ....

I don't have any help. Maybe ask whoever speced out the MS. Perhaps they have some idea about they were going to do this. Although it is more likely they were just brain dead suppliers meeting minimum spec.

If the 1600A - 800A CB have replacable trip units, perhaps trip units capable of being set to 50A are available. Likely have to change the sensors and lugs as well. Save the existing trip units to change all back when the Improvement Plan is developed.

ice

 

[mbrooke]

I know this may be off topic, but in your prints, in regards to the grounding and bonding blurb; why isn't the building UFER/ building steel bonded? And why the ground ring?