50kva vs 75kva transformer - new construction 400A service

[5mall5nail5]

Hey guys - so my mother called me today to ask me something as I know a decent amount of stuff, but this one is outside of my immediate knowledge. My parents are building a new house on a 16 acre lot with a builder they've known for years, etc. Currently they have a 200A service at their house but have a completely full 40 space panel due. Their current house is roughly 5000 sq ft with a 2000 sq ft finished basement or so. My mother runs a computer consulting company out of the house and as such uses a lot of electricity for that via servers, etc.

They are looking to have more power at their next house because she wants to make the garage more of a shop for her metal jewelry making, etc. and want to future proof if they add a detached building, pool, etc. etc. So, knowing my parents current setup and all the builder recommend 300-400A service. My mother said Ok sounds good lets see what the electric company needs to do for that, etc. So, the builder sent her the email he received from the utility company stating they can do that, etc. and two prices one for a 50kva transformer that they would need or a 75kva transformer they would need.

I guess my questions, firstly, are 1) 50kva vs 75kva - whats the difference, why, and should they? 2) 300-400A service - the only time I've hear or experienced those types of numbers has been in commercial buildings like the one I work at with 480v 3 phase - is there a > 200A solution for residential 240/120v setups (single phase?)? And... would a 400A solution be two 200A feeds or, what? These are all the figures and things I am left with yet - my mother asked the utility for more info and details but the guys email essentially said something about "Is 320-325A enough?" to the builder. Can anyone fill in the gaps so I can help my parents figure out what they'd need to support this?

Thanks,
Jon

 

[deter]

you probably need to consult an electrical engineer

 

[5mall5nail5]

Yeah you're probably right but I figured I might be able to help them out

Are there 300 - 400A residential services? I haven't noticed (though I haven't been looking) any 300+ A panels.

 

[Strouty]

My local utility has a 60% rule of thumb. If you are using 60% or more of the total service amps they say you should upgrade.

As for the KVA, what was the price difference? I actually just had mine upgraded, but they don't charge for it because it is on the pole. I am pretty sure mine was a 12.5KVA and they installed a 25 KVA unit. Most houses share 12.5 KVA transformers.

What type of loads do the servers run?

By the way I am not an expert and I did not stay at a Holiday Inn Express last night.

 

[5mall5nail5]

The difference between 50kva and 75kva was only $500 difference. I'd have to see how much the true load is on servers, desktops, AC, etc. etc. The main concern is wanting to have overhead for future proofing since running new utility from the street will likely be out of question with the length of this driveway. Right now they have 0 spaces for circuits in their 40 space panel and yes they could do a sub but are pretty close to being at capacity load wise as is. This new property will have septic pumps, well pumps, and geo thermal pumps, etc. So it might have more constant load as well.

 

[A_Pmech]

The numbers you're batting around sound very big for a home-based computer business.

What is the current electric bill for the business?

The house may require a fair amount of power as it sounds like it will be all-electric, but even then, the numbers sound high. Most electric companies have in-house engineering staff which can help you perform a load analysis. Over-sized transformers cost the electric company money to heat the great outdoors.

 

[CJCar]

The next step up after 200A is typically 320A for residential. Typically a 400A meter base with load center is set and then feeds multiple panels.

How are you calculating the current load? Keep in mind that your breakers and load centers are for distribution - just because you have a physically full panel doesn't mean you are at electrical capacity.

In the office here we burn just over 30A for 30+ desktops and a full rack of servers.

The loads you mention for the new place are not high current loads. Resistive heat loads, water heaters, electric range, hot tubs, pool heaters would be large loads.

The PoCo usually requires a work sheet be completed listing the prospective loads and will size the service accordingly. I can't fathom the PoCo asking the customer what size transformer they want.

 

[ranger302]

FYI 50kVA is 208A at 240V line to line single phase
and 75kVA is 312A at 240V line to line single phase
Amps=(kVA*1000)/voltage single phase

 

[Mustang51js]

Yeah you're probably right but I figured I might be able to help them out

Are there 300 - 400A residential services? I haven't noticed (though I haven't been looking) any 300+ A panels.

My old boss put a 300 amp service on his house,was two 150 amp panels. You need a 300 amp meter though. His was underground service

 

[5mall5nail5]

The numbers you're batting around sound very big for a home-based computer business.

What is the current electric bill for the business?

The house may require a fair amount of power as it sounds like it will be all-electric, but even then, the numbers sound high. Most electric companies have in-house engineering staff which can help you perform a load analysis. Over-sized transformers cost the electric company money to heat the great outdoors.

The bill is nasty. Its a 7000+ sq ft home so the utilities are already pretty high, but they use a lot of electricity at that as well. I would have to calc the load, its my parents house so I am not there this is per the recommendation of the people involved in planning their next setup I was only confused because when I started seeing 75kva+ I started thinking 3 phase and I don't think they want to go down that path.

In the office here we burn just over 30A for 30+ desktops and a full rack of servers.

That's true but most modern desktops have small 250 - 300w psus at 30% load if that. A single Dell R720 comes with dual 1100w PSUs with a bunch more CPU load and HDDs, but yeah.

I am not here to debate the necessity only that it'd been recommended by the builder knowing their use today and intentions, I am only trying to find out why one would prefer a 50kva vs 75kva transformer and it seems to me that the 75kva supports 320A service

 

[Charles (in GA)]

If the POCO is willing to do it, and the price difference is not that much, go for all you can. Install a 400 amp meter socket and two 40 space 200 amp panels, each supplied by a 200 amp disconnect at the meter. This is a common installation in larger homes.

The NEC used to limit panels to 42 circuits per panelboard, but that was changed in (I think) the 2011 version of the code and now there is no limit. Now you see panels, that look like the largest of the 200 amp panels, that will hold 80 of the small half-width breakers.

I have yet to figure out what two people do in 7000 ft of house. I rattle around in the 2000 ft I have, sometimes not setting foot in some parts of the house for days or weeks on end. It just boggles my mind the utility bills and everything that goes with a house that large.

Charles

 

[91bronc300]

What does the utility company say about what service the two different transformers will support? I mean, can you put a 300 amp meter and main panel on either transformer?

Here is an interesting article that talks about transformer efficiencies http://www.copper.org/environment/sustainable-energy/transformers/education/archive/dist_trans.html

I'm going to guess that the power company puts standard 150C temp rise transformers on the power poles and according to that article those peak in efficiency at 35% of rated load or for a 50KVA transformer about 73 amps or about 17,500 watts. I'm guessing that you want to size a transformer more for putting the sweet spot of efficiency right where most day to day power consumption level is and not so much for ultimate capacity. 17.5 thousand watts is a pretty high consumption rate for one house but certainly doable. A 50kva would still be able to supply over 200 amps all day every day like mentioned above and over that amount for certain amounts of time (it won't overheat instantly). If the power company says that you can use a 300 amp meter and main panel with a 50KVA transformer then that's probably better than paying for a 75KVA one. The 75KVA one would be less efficient for your day to day loads over the 50KVA one unless you are pulling about 110 amps all the time.

Peak efficiency 50KVA = 73 amp draw
Peak efficiency 75KVA = 110 amp draw

 

[CJCar]

That's true but most modern desktops have small 250 - 300w psus at 30% load if that. A single Dell R720 comes with dual 1100w PSUs with a bunch more CPU load and HDDs, but yeah.

That would be output watts on the server supplies. Also unless the servers are running at max 24/7 they will throttle way back. Our dual Xeons (IBM x346) typically hover around 150 watt draw at 20% load.

Only a full calc and capacity planning (including HVAC) for the server room will tell you what is needed. That process should include generator and ATS considerations.

I agree with Charles above though. Go for the max the PoCo will allow; you only want to do it once.

I believe bronco answer your direct question as to why one would prefer the smaller transformer.

 

[gatchel]

What does transformer efficiency have to do with it if the load is measured after the transformer at the meter? (Just curious) Wouldn't that be covered by the Poco and not the end user?

 

[91bronc300]

What does transformer efficiency have to do with it if the load is measured after the transformer at the meter? (Just curious) Wouldn't that be covered by the Poco and not the end user?

That is probably true. But the power company is probably going to know if you are consistently running in a range that is less efficient for a transformer you wanted installed during construction and may start adjusting your bill for it. Or maybe not, I dunno. But I would still say it's better to only have the 75KVA installed if you are sure you're really going to need it, rather than 'aw hell, we might need it in the future so let's pay for it now'. Because if the 50KVA ends up sufficient then the 75KVA was wasted money initially and a wasted few points efficiency from then on out. Even if you're not paying for the lost efficiency yourself it still matters, energy is still wasted. Like PMech said there should be someone at the PoCo that can help nail this stuff down right the first time. The OP should probably call them instead of going through email.

 

[Stuart in MN]

I've never heard of the customer telling the utility what size transformer to use. Typically, the utility is going to want you to fill out a load sheet, and they'll size the transformer based on that. They'll size the transformer based on what they think the average load is going to be - even if you have a 400 amp service, the load sheet may show that you're not going to use anyway near the full 400 amps so they'll install a smaller transformer.

 

[CJCar]

What does transformer efficiency have to do with it if the load is measured after the transformer at the meter? (Just curious) Wouldn't that be covered by the Poco and not the end user?

This what I was getting at on my first post: why is the PoCo even asking? I went back and reread the OP. Here's my take now: he's relaying info from his parent's, who is relaying from the contractor, who is relaying from the PoCo. I'm thinking the PoCo is saying they can do a 50kva (200A) or a 75kva (320A) service.

If this the case, his parents just need to decide for the larger service, and their contractor should see to it that the EC install appropriately sized gear.

 

[5mall5nail5]

They're asking the builder and offering two price schemes, the builder passed it along to my family with two amperage ratings, 50kva being 208A and 75kva being 312A, the difference is $500, but is this recommended/ideal/normal?

 

[CJCar]

They're asking the builder and offering two price schemes, the builder passed it along to my family with two amperage ratings, 50kva being 208A and 75kva being 312A, the difference is $500, but is this recommended/ideal/normal?

If they need the larger service, yes its perfectly normal.

Do they need the larger service is the question I think we are indirectly asking you.

 

[5mall5nail5]

If they need the larger service, yes its perfectly normal.

Do they need the larger service is the question I think we are indirectly asking you.

It's irrelevant as I have no figures - I am not involved in that, its been assessed and recommended - I can't evaluate the necessity.

 

[CJCar]

It's irrelevant as I have no figures - I am not involved in that, its been assessed and recommended - I can't evaluate the necessity.

Well then you are asking a question without providing the needed info to receive a sound answer.

Yes, again, it is fine for that size service on a residential install, assuming everything is sized correctly and installed per applicable codes.

For that size property and planning on future outbuildings I would do it.

 

[ddawg16]

The question is.....50 or 75KVA?

It's not up to us to decide if he needs that much power.

Me? I'd go 75KVA for the buffer.

But I also have to ask.....what is being done to make the house more efficient? It would seem to me that spending money on good windows....thicker walls with more insulation...LED lighting...sun tubes...., Gas heating or Heat pump....etc would make a drastic improvement on energy usage.

As for the garage....even the most avid woodworker doesn't really need more than 40A/240 in the garage.

 

[CJCar]

The question is.....50 or 75KVA?

It's not up to us to decide if he needs that much power.

Me? I'd go 75KVA for the buffer.

But I also have to ask.....what is being done to make the house more efficient? It would seem to me that spending money on good windows....thicker walls with more insulation...LED lighting...sun tubes...., Gas heating or Heat pump....etc would make a drastic improvement on energy usage.

As for the garage....even the most avid woodworker doesn't really need more than 40A/240 in the garage.

Agree to all.

Also is that $500 difference just the transformer? Keep in mind the added costs for the larger service feeders. That can add up quickly.

 

[Rookie2]

I would call PENNELEC and or a local Inspection service.

https://www.firstenergycorp.com/content/customer/service_requests/Builders_Contractors.html

 

[Blk88GT]

For what it's worth, I have two 200A (one for my house, one for my 2000sqft shop) services running off a 25KVA transformer (as sized by my poco). All my heat is electric.

No issues so far.

 

[ddawg16]

For what it's worth, I have two 200A (one for my house, one for my 2000sqft shop) services running off a 25KVA transformer (as sized by my poco). All my heat is electric.

No issues so far.

Hmmmm......your a little under powered if you only have 25KVA.

 

[Nexussian]

It depends on the primary goal.

If the primary is "headroom" for future expansion, get the bigger transformer.

As others have mentioned, check additional costs.

If the extra $500 is only for the transformer, what does the line to the meter base and meter base itself cost (extra cost there or no?).

While you are asking, what does the power cost?

Locally, a larger service comes with a higher rate per KWH, I suspect due to the loss mentioned earlier, from heating the great outdoors with the larger transformer.

With those expenses in mind, have they looked into alternative energy sources?

I know a few businesses that power their server racks off batteries and solar, with a grid tie for crummy weather.

It not only reduces what they have to pay the utility, it gets the business a tax break, and the power is spotless, 60 Cycle, no spikes, no surge even during a power failure as the "alternative" power supply acts as a giant UPS.

 

[Blk88GT]

Hmmmm......your a little under powered if you only have 25KVA.

Maybe so, but it's the power company's problem if it ever becomes an issue.

 

[gatchel]

Maybe so, but it's the power company's problem if it ever becomes an issue.

Very True and the best way to look at it. I had an issue where our pole pig was under sized for the houses that share our feed (based on another lineman's suggestion). I requested that a recording voltmeter be installed for testing. 2 weeks later the pole x-fmr was brand spanking new, much larger, and they also changed the neighborhood high voltage line feed from the main rod it comes from.
Problem solved.

 

[Strouty]

Very True and the best way to look at it. I had an issue where our pole pig was under sized for the houses that share our feed (based on another lineman's suggestion). I requested that a recording voltmeter be installed for testing. 2 weeks later the pole x-fmr was brand spanking new, much larger, and they also changed the neighborhood high voltage line feed from the main rod it comes from.
Problem solved.

I had similar issues, my compressor was dimming the lights. I had no idea that my commercial shop shared a transformer with a residential house. So every time my lights dimmed, they had the same issue in the house. Must have driven them crazy.

Power company installed a larger transformer, but I have yet to really test it.

 

[walrus]

Hmmmm......your a little under powered if you only have 25KVA.

He is only under powered if he actually uses more than 25 KVA, just cause he has has 2 200 services doesn't mean he is using that much