Rationale for 220.82 not permitting 3-phase services?

[derekk]

I'm sorry if this comes across as ranting, but I'm legitimately interested if anyone can give me some insight into this question.

This comes up over and over again on our projects and I feel like I'm constantly wasting people's money and it drives me crazy. I'm just curious if anyone knows the reasoning behind limiting the application of 220.82 (Optional Calc's, Dwelling unit) to 3-wire services? In my experience, the larger the house, the larger the difference between the Standard and Optional calcs because so many dedicated loads are counted at 100% (or 75%) under the Standard method. There is always a "tipping point" where the local utility will require 3-phase service, maybe 400-600A.

I've searched and just have not been able to find any reason for this limitation. The only thing I can come up with is that when comparing an 400A 3-phase service vs a 600A 1-phase service, there is more chance that a single phase could become overloaded on the lower ampacity 3-phase service, but most larger loads are not going to be 120V, so this is not really a good argument.

Anyone with insight before I waste time on another NEC change proposal?

Thanks.

 

[jaggedben]

I think it's a great question, but I don't have any real insight. My only comment is that you don't need 120V loads to unbalance a phase; that can also happen with single phase L-L loads if not balanced. For example if loads are all connected to A-B and B-C and not C-A, then B could be overloaded.

Btw, are all your projects 10000 sq ft houses? And you're worried about wasting such people's money? I mean, most residential services are oversized anyway, in my experience.

 

[derekk]

Thanks for the response. For our residential division, the majority of our projects are pretty large, over 10K ft is common. They can also be smaller, but very "dense" electrically (large server rooms for example).

I probably should have added an example to my original post, but I did not want it to go too long. In terms of cost, it's mainly a case where I believe that the existing service is more than sufficient for the project. If the service is new anyway, going from an 800A to 1200A service is probably not going to be a big deal. I say "probably" because I have encountered cases where the larger service would have triggered the need for a new and very long primary. Those are the exceptions, of course.

On a current project (partial remodel and new structure), the existing service is 800A, 208Y/120V, 4W. Using the Standard method, our load is above 1000A, so a 1200A service is now required. Using the Optional method, our load drops to below 640A, so the existing 800A service is sufficient. The primary culprits are a lot of exterior ceiling heaters (spread among several structures) and several EV chargers. The new service hasn't been designed yet, but given our location, the cost will likely end up $100K to $200K. I should also mention that this is in an area of coastal CA with very mild weather and the exterior heating is controlled via the Lutron Homeworks system, with temp control of the heaters, not simply on/off. ON a few past projects, we were to able to satisfy the utility and inspector via automatic load-shedding. That would work well in this specific case too, but there is no guarantee it will be accepted and this is not an NEC-approved option. The other alternative would be to install recording meters on the property (after completion) for 30-days, 120-days, whatever and agree, in writing, to upgrade the service in the event that the results justify it.

Ethically, I feel like we're committing malpractice. I know full well that the existing service is more than sufficient to handle the load, but the NEC is requiring the client to essentially, "dump money into a hole" for no actual benefit. I'm an EE as well as an EC and I've been in this industry for many years. Situations like this continue to bother me. I would feel better if someone can come up with a rational reason for the code that I'm maybe just not considering. Load balancing 3P vs 1P would be a hard one to justify for me. We balance the loads in design and on larger jobs, we're going to analyze actual usage and adjust phase balancing if needed after completion anyway.

 

[jaggedben]

I hear you, especially on having to replace an existing service that is almost certainly just fine. Does 220.87 ever help?

 

[derekk]

We have a recording meter installed now. It's possible that 220.87 this may work, but I also have gotten pushback in the past when we are adding a lot of new load and using existing as the base.

 

[JoeStillman]

My POCO won't give you 3-phase for any dwelling unit, doesn't matter how big.

 

[tortuga]

220.82 was based on research that did not include any poly-phase dwellings, so its a unknown quantity.
You can search old code proposals online and see if someone already proposed adding 'multiphase'.
The rationale behind 310.12 was that you have 2CC in a raceway, but they contradicted that when they added 120/208 open wye.

 

[Elect117]

We have a recording meter installed now. It's possible that 220.87 this may work, but I also have gotten pushback in the past when we are adding a lot of new load and using existing as the base.

220.87 is the best method for adding load.

Some utilities will require 3PH (over 1PH) when the wire size gets to be large and the voltage drop can become a problem. Utilities have to combat something called pole loading, climbing space, and joint use with telecommunication members. All of those can make running numerous parallel runs and large conductor wire more difficult. Not to mention they typically only carry certain wire sizes. They don't custom order wire per job. And if you are in an underground district, then those conduits can become clogged up fast and there are heating considerations for them.

 

[tortuga]

the existing service is 800A, 208Y/120V, 4W. Using the Standard method, our load is above 1000A, so a 1200A service is now required. Using the Optional method, our load drops to below 640A, so the existing 800A service is sufficient.

Mind posting the load calc's? It would be interesting to see a single family dwelling service over 1000A.
What size is the utility transformer?