Help on Commercial Load Calcs

[Smart $]

...

I was originally thinking a 600A service, based on a similar building in the complex. They have an 800A service, but with 8 single-phase AC units on a 400A service that is pulled off of the main.

Throw a few of the other typical loads you mentioned per unit in on the ones already posted, allow something for servers... and the AHJ should approve a 600A service.

 

[BMacky]

Commercial Load Calcs

Commercial Load Calcs

OK, so last question. I have seen 3-phase services where the single-phase systems that have been pulled off the 3-phase gear were done using three-phase breakers and just running a pair of conductors off the two 120 legs. Seems like a waste of a breaker to me. Is there any switchgear you are aware of where the system main busing is three pahase, with a combination of busing where you can pull single-phase secondary feeders off the system?

Seems like somewhere someone should have a need for that and the mfrs have come up with something like this.

Thanks.

Bob

 

[Smart $]

OK, so last question. I have seen 3-phase services where the single-phase systems that have been pulled off the 3-phase gear were done using three-phase breakers and just running a pair of conductors off the two 120 legs. Seems like a waste of a breaker to me. Is there any switchgear you are aware of where the system main busing is three pahase, with a combination of busing where you can pull single-phase secondary feeders off the system?

Seems like somewhere someone should have a need for that and the mfrs have come up with something like this.

Thanks.

Bob

If we are talking about panelboard type distribution equipment on a hi-leg system, and you want to connect a 1? load, you can simply use a 2-pole breaker. However, for 120/240V split-phase circuits, they must be in the C-A positions (B being the code-compliant hi-leg)... and can be 'slash'-rated breakers. Any breaker connected to the B leg must be full 240V rated. And more to the point, I don't know of any two-pole, 3-space breakers offhand...

This issue is part of the reason I wanted clarification on 120 or 240V lighting. With the latter, you could pull circuits off with either 2-pole breakers and better balance the loads on a 3? service, and run 3? subpanels.

An alternative is to supply two mains with the SE conductors: one 3?, the other 1?... but will likely have to use MCB if panelboard type. Using MLO would likely put you over the six-disconnect rule.

Another approach is to get approved for two services: one 240V 3? 4W and one 120/240V 1? 3W. You could then use MLO mains with 6 breakers each. But POCO and/or AHJ may be uncooperative in this approach. And monitoring usage per suite is another issue if the AC units are divvied up.

 

[Smart $]

OK, so last question. ...

PS: I won't hold you to that.

 

[kwired]

Bob,

First tip, hire an engineer. You will need to engage one at some point and it is a lot easier to do it from the get go. Plus, you can get him to do these load calcs instead of trying to learn everything in a day or two. Second tip, I do commercial office buildings anywhere from 12 - 18 w/sf which puts you around a 400A service. I would try to make this work but would go to 600A if you have a heavy computer/electronics load component. You are looking at a 400 or 600A service entrance rated main distribution panel and then some smaller subpanels that will be fed out of the main panel. Good Luck!

I will second the question of why is an engineer needed. This is not that complex of an installation from what has been mentioned so far.

I also question where you are getting 12-18 watts/sq ft from? Or is that a figure that involves all loading and not just general lighting by the square foot?

Going with 3.5 VA per square foot for all of the 8500 square feet only gives us a 124 amp load @ 120/240 single phase - we still need to add HVAC and other fixed loads, but I don't anticipate there will be that much load that it must have a 600 amp service - especially if it is three phase. Now the fact that it is a 240 volt delta with a high leg will limit us on what we can connect to the high leg unless there is additional transformation installed to be able to utilize that leg more effectively.

 

[Smart $]

... I don't anticipate there will be that much load that it must have a 600 amp service - especially if it is three phase. Now the fact that it is a 240 volt delta with a high leg will limit us on what we can connect to the high leg unless there is additional transformation installed to be able to utilize that leg more effectively.

That's another option that I didn't think to mention...

Separately derived 120/240V from 240V legs connected A-B and A-C could help balance the 3? loading, perhaps dropping the calculated current on A and C legs enough to install a 400A service. The question is whether the added cost of the transformers will offset the savings on the service equipment... I'm doubting it at present.

 

[kwired]

That's another option that I didn't think to mention...

Separately derived 120/240V from 240V legs connected A-B and A-C could help balance the 3? loading, perhaps dropping the calculated current on A and C legs enough to install a 400A service. The question is whether the added cost of the transformers will offset the savings on the service equipment... I'm doubting it at present.

Since you bring up cost of equipment, the capacity of the delta high leg transformer needs some consideration also. Even though it is somewhat the problem of the POCO, if it is not enough capacity they may have some charges to upgrade to whatever is necessary also. If it would happen to be an open delta transformer bank it very well may not be capable of delivering needed power on all three phases. If POCO needs to change anything then it would seem that changing to a 208/120 wye would be the better option in most cases.

 

[JoeStillman]

An alternative is to supply two mains with the SE conductors: one 3?, the other 1?... but will likely have to use MCB if panelboard type.

This is just what I like to do when I have to use 4W delta. Just take two hits off the CT cabinet or meter - one to the 1?, 3W panel and one to the 3?, 3W panel. I always do this because the crazy leg (208V to neutral) scares me. I don't want someone to come along later and put a 1P breaker on the B-phase (or C-phase here in Philadelphia Electric territory.)

 

[kwired]

This is just what I like to do when I have to use 4W delta. Just take two hits off the CT cabinet or meter - one to the 1?, 3W panel and one to the 3?, 3W panel. I always do this because the crazy leg (208V to neutral) scares me. I don't want someone to come along later and put a 1P breaker on the B-phase (or C-phase here in Philadelphia Electric territory.)

How do you keep them from putting the 1P breaker in the three phase panel, and possibly hitting the high leg?

 

[Smart $]

How do you keep them from putting the 1P breaker in the three phase panel, and possibly hitting the high leg?

3? 3W panel... there's no neutral to connect a L-N load.

 

[Bjenks]

OK, so last question. I have seen 3-phase services where the single-phase systems that have been pulled off the 3-phase gear were done using three-phase breakers and just running a pair of conductors off the two 120 legs. Seems like a waste of a breaker to me. Is there any switchgear you are aware of where the system main busing is three pahase, with a combination of busing where you can pull single-phase secondary feeders off the system?

Seems like somewhere someone should have a need for that and the mfrs have come up with something like this.

Thanks.

Bob

Someone asked earlier, "Why hire an EE in Florida?", well here is a good reason (I just happen to live in Florida). I have been doing high level studying of NEC load calcs for at least 8 years now and still miss a few points even with my complicated spreadsheet. It also reminds me that when I do get hired to do these load calculations, it is usually because it is being required by the AHJ. This means that they should also be requiring an electrical one-line showing basic equipment specifications. Also should be included is a grounding system. I have seen a lot of EC supplied load calculations/one-line and 80% of the time there is at least a medium size error in there somewhere, especially on large dwelling units. The fact you are asking about a delta high leg panel that includes single phase loads shows you should be hiring an EE. We have a lot of open delta 3 phase systems in my area and you won't believe how many times I have to come in and show why they are having problem with voltage drop on A-C or B phase, because most just take the total load and divide it by sqrt(3). Most of the EC who do hire me are either high performing EC or a small guy who got a job that he is worried he will mess up on. Then there is the 60% that try and do it on their own and will usually oversize the service and pass the cost on to the client or get away with it till the client finds out later and pays to fix it on his own. Understanding the demands can get confusing if you ask me and that is where many get it wrong.

Of course this is coming from an EE on an EC forum who is just trying to justify his existence . I also know that the reason many are on this forum is because they are probably one of the two EC that I mentioned who hire me.

 

[kwired]

3? 3W panel... there's no neutral to connect a L-N load.

If there is open spaces, I'm willing to bet $1.05 that someone will still try to install a 120 volt circuit in that panel sometime down the road, and they will likely connect the grounded conductor of their branch circuit to the equipment grounding bus.

 

[Smart $]

If there is open spaces, I'm willing to bet $1.05 that someone will still try to install a 120 volt circuit in that panel sometime down the road, and they will likely connect the grounded conductor of their branch circuit to the equipment grounding bus.

I agree there is a chance... but people have tried, and you cannot prevent stupidity. :happyno: