Sizing single phase transformer for 1200 amp single phase multi-meter project

[CD1980]

I got a recent inquiry from a customer in our service area who's asking for service to a 1200 amp panel for single phase 120/240v service. Typically all designs we have done thus far is 800 amps and below which we typically serve with a 100KVA 120/240v transformer. I'm curious if our limit is 100kVA is this customer request feasible from a practicality point of view? It's for a multi-residential set up so there will be 8-9 meters installed vs. one single 1200 amp service but like I mentioned we haven't done anything this size to date. My take is if we assume a load factor of 60% we would need to install a 150-200kVA unit which we do not stock. Our largest unit being 100KVA makes me believe this request may not be feasible.

I wanted to ask if there's any utility designers here who have serviced a 1000amp or larger single phase multi-meter complex with 120/240v and if so what size transformer was utilized for the service? Any help would be greatly welcomed, thank you.

 

[Dsg319]

I have no direct input on this but are load calculations of the residences taken into consideration?

 

[CD1980]

I have no direct input on this but are load calculations of the residences taken into consideration?

The assumptions made so far is each residential customer connected to this panel will have a 200amp service. My take at this point is to use our typical 50/60% load factor since it's residential load. Per my numbers I'm thinking for 1200 amps of maximum service at 240v and a 60% load factor I'd need a 170KVA transformer. the single line I've been provided shows the main being 1200 amps with 9 units at 200amp and a 'house panel' at 125amp. I wasn't sure if my approach is off and need to reconsider what the proper KVA size should be. At this time our maximum size underground transformer is 100 KVA for 120/240v single phase and I've never designed anything like this before nor has anyone in our department. The customer seems to want to maintain their proposal but I'm trying to validate if it's even possible. I'm trying to validate the transformer KVA sizing would work since we can always run multiple secondary wire to the panel to accommodate the amps. I hope I don't sound stupid asking these questions, I just am in a weird spot trying to validate a design we've never done before. I was hoping someone from a utility company has experience with this kind of single phase design and shares what size unit they'd call for. Thanks for any and all help.

 

[augie47]

Our POCOs would not even talk about the job without seeing a load calculation.
9 units on a 100ka pad is not at all unusual here but our individual units aren't often on 200 amp feeders.

 

[CD1980]

I'm reviewing some preliminary information that was provided. The 1200 amp panel is showing their total calcs for a 9-unit complex to be about 1,060 amps. So at 240v and 1,060 amps with a 60% load factor I'm coming up with 152 KVA. Since our largest unit is a 100 KVA for single phase 120/240v I'm at the point where I'd have to suggest to them to split this into (2) smaller 600 amp panels, right? If I'm way off on my logic that's what I was hoping to verify here. I appreciate the feedback so thanks everyone.

 

[winnie]

I am not a utility engineer, just going on basic understanding.

1) The greater the number of users, the more things tend to average out. So I'd expect a better load factor for the single 1200A service than for 2 600A services.

2) The larger the transformer the greater the available fault current. I'd expect the fault current at the leads of a 100 kVA transformer to be <10kA but a 200 kVA to be >10kA. Dividing the loads probably means lower fault current and lower cost equipment.

Perhaps build out the system with a single 100 kVA transformer, but design from the outset to easily add another.

Jon

 

[petersonra]

most utilities do not allow the customer to make these decisions. They have a few choices and the end user has to use one of those options.

personally, I would not allow anything special. it just makes for a mess when something fails and there are no spare parts.

 

[CD1980]

most utilities do not allow the customer to make these decisions. They have a few choices and the end user has to use one of those options.

personally, I would not allow anything special. it just makes for a mess when something fails and there are no spare parts.

I agree 100%. I'm just asking if someone here agrees with my assessment that with our maximum size 100 kVA single phase 120/240v transformer that their request is not feasible unless we get a new larger transformer which is something we will not accommodate. This setup is unique and I didn't have any previous projects to use as reference to ensure if this was ever accommodate in the area we serve. I'm of the opinion that it appears not doable at this point per the numbers so I was just trying to double check and make sure i'm not wrong with the logic.

 

[petersonra]

why would they even want to do this? it seems like it would be more cost effective to have a 208/120 service.

 

[winnie]

Could you please clarify: are you a designer from the utility side of this installation, or from the customer side?

-Jon

 

[CD1980]

Could you please clarify: are you a designer from the utility side of this installation, or from the customer side?

-Jon

utility side. which is why I was trying to find out if other utilities have serviced such a large single phase customer in this manner. Usually we get a request for an apartment complex with single phase multi meter panels around 400-600 amp 240/120v. I've never seen one requested of this magnitude and I'm of the stance that it would exceed our largest available transformer. I wanted to do a sanity check here to see if I'm not mistaken was all so that way I can tell the customer their request won't be feasible.

 

[CD1980]

why would they even want to do this? it seems like it would be more cost effective to have a 208/120 service.

The customer said in a previous email:

'The power needs are larger (for each unit) than next door as well since this project is being designed per the new t-24 code update which becomes effective Jan. 1st.

The new requirements force us to provide 220V at the wtr. Htr, dryer, range and EV in each garage even if they are not used. Previously we only supplied gas and 110v at these locations.

All the buildings are 800 amps or less except for building 23 (9 plex) as previously mentioned.'

I'm in california and I'm starting to see newer developments designed with no gas hence they are requesting larger services from the utility. I'm trying to get ahead of a situation I see becoming more frequently requested in upcoming years.

 

[petersonra]

are you allowed to parallel two LV xfmrs?

 

[jaggedben]

Speaking as someone who has never sized a transformer but has years of experience doing load calcs and looking at residential unit meter data (among other reasons, to validate main breaker downsizes for solar systems)...

Most likely the actual NEC load calcs for these units are still under 150A even though they're all electric. And it's quite rare for a residential dwelling unit without high air conditioning load to consume over 10kW (average over an hour long interval). Even with high A/C load, it's extremely unlikely that all units would simultaneously consume the 20kW or so needed to overload an 800A service. So, if the 200A per unit and 1200A for the building is just based on customer assumptions, and not actual load calcs, then using those numbers is likely quite conservative. I'd bet an 800A service is still adequate, and therefore your same transformer is too.

So going back to post #2. Do they have an actual load calc?

That said, it may indeed be on the bubble, and I don't see why they should object to going to 208/120 and running single phase to each unit. Seems like they just don't want to change what they've been doing. But if they're going to keep doing this, or if the NEC load calc really says they need over 100kVA, maybe it's time to learn some new tricks.

 

[Hv&Lv]

We have a couple of larger places with 167 kVA transformers. Apartments, churches, etc..

For that load we would put a 167.

 

[JoeStillman]

Why not feed them at 208Y/120V 3Ø? They can still feed the individual dwelling units at 120/208 1Ø. We do this all the time.

 

[electrofelon]

I wanted to ask if there's any utility designers here who have serviced a 1000amp or larger single phase multi-meter complex with 120/240v and if so what size transformer was utilized for the service? Any help would be greatly welcomed, thank you.

Don't take this the wrong way but are you new at this? Don't you have models and data that you use to size your equipment? 60% load factor seems real high to me for residential. Based on what I see utilities do and my experience, I would think 9 dwelling units fed from a 100kva would be totally fine.