Three phase service questions

Coppersmith

Senior Member
Location
Tampa, FL, USA
Occupation
Electrical Contractor
Please excuse my ignorance. I may be designing a three phase service and it's my first one. Client says they want a 400 amp three phase 240v service for a small workshop. I'll be looking at it tomorrow. I haven't contacted the POCO yet to see if they can supply this voltage directly, or I need to install a transformer. (480 to 240 delta/delta?)

Q1: Does 400 amps mean the total of the phase to phase connections (AB + BC + CA) is 400 amps, or each phase to phase connection is 400 amps?
Q2: The delta transformers I have looked at have a 120 v tap, but it's limited to 5% of the KVA. If I wanted more 120V, would I have to install a single phase transformer connected to two phases of my 240V output?
Q3: If the POCO can supply 240V three phase directly, I assume this would be four wires. Is there the same 5% of KVA limit on 120V?
Q4: If I supply a 480 to 240 transformer, how many KVA would I need for 400 amps?
 
Location
Saint Louis
Occupation
Electrician
Are you sure it’s not a 120/208

the only straight 240 3phase and grounded b phase which is really uncommon

And each phase is good for 400 amps.
 
Last edited:

Coppersmith

Senior Member
Location
Tampa, FL, USA
Occupation
Electrical Contractor
Are you sure it’s not a 120/208

the only straight 240 3phase and grounded b phase which is really uncommon

And each phase is good for 400 amps.
I asked that question. Client swears his equipment uses 240v three phase. I'll be looking at equipment dataplates tomorrow if I can. He also has some 240v single phase equipment.
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
IMO, it depends on 2 factors, what voltage is acceptable to the equipment being connected and what voltage is available from POCO.
In many areas 3 phase 240 is no longer common with POCO and they may want to supply you with 208Y/120. Confirm that 1st and check the manufacturer info and see if it's critical with the equipment. A great deal of equipment is comparable with both voltages,
In either case the 5% limit is not a concern on a POCO system.
If POCO can only supply 208 and the equipment needs 240 there are options we can discuss at that time/
 

Cow

Senior Member
Location
Eastern Oregon
Occupation
Electrician
If a customer wanted 400 amp 240v 3 phase, I would most likely install one 240v 3 phase panel strictly for 3 phase loads, and a 240v single phase panel for line-neutral 120v loads.

The last time we needed 240v 3 phase, I had to step it down from 480v and ran into the same 5% issue. I ended up using a specific transformer from Sq D that didn't have any limitations on the neutral current.
 

texie

Senior Member
Location
Fort Collins, Colorado
Occupation
Electrician, Contractor, Inspector
You indicate a "small" workshop. I take that as limited 3 phase loads @ 240 volt. Many POCO's, even today, would supply this with a 120/240 center tapped delta system, AKA high leg system. Many POCO's will require at least 100 KW or so of demand load to give you a 208Y. You also need to find out if the POCO will even furnish any type of 3 phase service at the address. Often 3 phase 120/240 is easier for them as they only need 2 primaries at the location.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
If the available route to 3 phase is a 480V supply, then also check the ability of the equipment to use 480V directly.

A very common situation calling for 240V 3 phase is when you have a dual voltage motor that does 480 or 240, and the design range simply doesn't include 208V in the low voltage configuration.

Jon
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
Many POCO's, even today, would supply this with a 120/240 center tapped delta system, AKA high leg system.
Dominion Energy limits open-delta high-leg services to 200 amps, and generally limits them to existing installations.
 

Coppersmith

Senior Member
Location
Tampa, FL, USA
Occupation
Electrical Contractor
I'll know more tomorrow. I'll post an update. I just wanted to get enough information to be somewhat conversant for my meeting.
 
You indicate a "small" workshop. I take that as limited 3 phase loads @ 240 volt. Many POCO's, even today, would supply this with a 120/240 center tapped delta system, AKA high leg system. Many POCO's will require at least 100 KW or so of demand load to give you a 208Y. You also need to find out if the POCO will even furnish any type of 3 phase service at the address. Often 3 phase 120/240 is easier for them as they only need 2 primaries at the location.
Certainly going to be regional differences, but my experience is different and none of the POCOS I work with will supply 240 Delta anymore. They will be happy to supply a 208Y/120 if they have three phase distribution there.
 

Coppersmith

Senior Member
Location
Tampa, FL, USA
Occupation
Electrical Contractor
Ok so I saw the shop today. It's in an industrial park. The building is 10,000 sqf divided into three businesses. The shop is 3000 sqf. Currently all tenants have 1p 120/240v service. The shop has a 100 amp panel. I called the POCO, but it's going to be 3-5 days before an engineer calls me back. I can see 3p power on the poles one block over. There didn't appear to be 3p power next to this building.

He has a bunch of old machines in there. I couldn't get data on most of them, but he swears everything is 220v. There's a 3p 220v milling machine (currently with a phase converter attached). There's 220v 1p welders. There is a 240v residential oven. I asked if he had a list of all the equipment and their electrical requirements so I could verify 400 amps was enough and he said no. And he didn't want to put the effort into generating one. He claims 400 amps is more than he needs and he plans to add more equipment in the future and wants room to grow.

I told him it's likely I could only get 208Y/120 to the building and that I would have to use a boost transformer to get the 220/240 he needs. I asked if the new equipment he purchased could be 208 and he said yes.

So here is what I'm thinking for a design assuming the POCO can supply 208Y/120:
400 amp meter (do these exist?) with two parallel sets of 250 KCMIL AL feeders.
One set goes to 200 amp main breaker, to a 200 amp boost transformer, to a 200 amp 240v MLO 3p 42 circuit panel.
Other set goes to a 200 amp 208Y/120 MB 3p 42 circuit panel.

The 208Y/120v panel will handle his future new 3p equipment, any existing equipment that can handle 208, and 208 or 120v lighting and receptacles. The 240v panel will handle his existing 240/220v equipment. I don't know for sure that this is enough 240v capacity. If we run short of capacity, we can add a 125 amp breaker to the 208v panel for another boost transformer feeding another 240v panel.

Thoughts?
 

Hv&Lv

Senior Member
Location
-
Occupation
Engineer/Technician
They do.
320 amp continuous, 400 amp maximum is a socket base.
400 & 600 amp continuous rated meter bases are K base bolt in meters.
Check with your poco first to see if they offer them.
we will if we are contacted first.

As far as poco banks, I’ve built banks as small as a 45 kVA 120/208, up to a 225 kVA open delta.
I’ve talked to utilities where they limit their voltages. I don’t understand why. It’s real easy to accommodate and build.
 
Using a class 320 self-contained meter base would certainly be simple and inexpensive, but two different power companies I have asked about using them says they do not allow them for non-residential and it would need to be a CT metered service for anything over 200 amps. Your mileage might vary of course, just don't forget to check with them on this.

Big question is how much they will charge to run 3 phase line over!
 

curt swartz

Electrical Contractor - San Jose, CA
Location
San Jose, CA
Occupation
Electrical Contractor
99.9% of the time when someone says 220V they don't have the slightest clue of what they are talking about. If the equipment is really 220V its probably 50HZ.

What makes the owner think 220V equipment should be fed with 240V (20V over rated input) vs. 208V (12V under rated input)?

Are you suggesting feeding a panel with a BB transformer? I definitely would not recommend that. If you want to adjust the voltage for an entire panel go with a 208V delta to 240/120V delta transformer. That way you will have a neutral available if some equipment might need it. The residentail oven mostlikely requires a neutral but it would also be fine on 208V. I have no idea how many pieces of equipment need voltage adjustments but would be inclined to install separate BB transformers for each piece.

How are you supposed to size panels, number of circuits, etc without a list of equipment? Are they just asking for you to install panels then someone else run all the circuits to the equipment?
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
First off: 220V is an OLD standard no longer in use, but if it is old equipment, it might indeed be designed for 220V. The NEW standard is 240V for Distribution, 230V for Utilization, meaning the MOTORS on machines will be designed for 230V, the utility will deliver 240V, expecting that there will be a voltage drop from the service entrance to the motors. It's been that way in this country for decades now.

Second: 90% of 230V equipment will be just fine with 208V as a supply. Motors 10HP and under that have been made in the last 20 years or so are usually made as "tri-voltage", meaning 208-230/460V connections are fine. Internally they have a little extra core steel to allow a variance of +10, -15%, so they can accept anything from 195V on the low end to 253V on the high end. Utilities are bound by rules to give you +-5% of their nominal voltage, so a 208V source can only go down to 197.6V, well within the "tri-voltage" motor rating.

Heating equipment, like their oven, will simply not get hot as quickly at 208V, most people never know the difference. The welder will draw a little more line current for the same output current, but they generally work fine.

The utility will INSIST on an accurate list of the expected loads and future expansion if it is a new service. Once they have that, they will present you with their available options. They can configure their transformers as separate single phase transformers in a 3 phase bank to give you more 120V single phase power than an off-the-shelf transformer would, so they can get around that 5% limit if they want to. But they also have to consider the load BALANCING on their system, so they will be pushing for the 208V service too.
 

Coppersmith

Senior Member
Location
Tampa, FL, USA
Occupation
Electrical Contractor
So I've spoken to the assistant to the engineer and she gave me lots of forms to fill out so they can decide if they are willing to extend the three phase power to the building and have us pay for it. I asked if they could give me a ballpark cost on the line extension and she was very cagey but eventually said she would ask the engineer.

She confirmed that it would be 208Y/120 and that no CTs are required for a 400 amp service. I'm still in the bidding stage, so I'm trying to limit my effort and just give him an estimate. To do that I need to know what the transformer is going to cost.

208V delta to 240/120V delta transformer
I called one transformer company and they said this would be a custom job. Does anybody know of a vendor for off-the-shelf transformers of this type? I'm also trying to figure out how big a transformer I need. If I want the primary to draw a max of 200 amps, how many KVA should I be looking for?

Is this right?:
VA = (1.732 x V x I)
72051 = (1.732 x 208 x 200) = 72KVA
Next standard size = 75 KVA.

What would the available secondary side amps be? Is this right?:
75000 / (240 x 1.732) = 180.43 amps (ignoring losses, How big are those losses?)
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
IMHO going from 208/120 up to 240 is a perfect job for an autotransformer system.

You do not need 240/120V delta, you just need the 240V L-L, so 240/139V wye would work just fine.

The 'idea' transformer for this job would be a set of 3 single phase 120:19V 3.8kVA transformers, arranged in a wye configuration. Expensive custom units.

But off the shelf are 120/240 in to 16/32V out 3 KVA units for under $500 each new (continuous rating of 188A) and if you really want to make sure you can provide that full 200A on a continuous basis, than 5 KVA units are less than $800 new.

This gets you a 236/126V wye nominal supply from your 208/120V wye, using off the shelf components.

-Jon
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
Without comment on the advisability thereof, NEC 215.11 Exception 1 appears to allow an open delta autotransformer arrangement with just two autotransformers. The grounded conductor would remain at 120V to the common untransformed leg, while the other two legs would rise in voltage to ground.

Thinking about how that is connected, can it be done with a single coil with the proper taps (or a single transformer that can be wired into a single coil with the proper taps(*))? Connect L1 to the center, L2 and L3 (208V to L1) to the two almost ends of the coil, and take L2* and L3* (~240V to L1) from the two ends of the coil?

Cheers, Wayne

(*) Something I'm unclear on: if using a two winding transformer as a single coil, by connecting one end of each coil, does the geometry of how the coils are wound on the core no longer matter? You can treat it is a big inductor, which is all you need for an autotransformer?
 

Hv&Lv

Senior Member
Location
-
Occupation
Engineer/Technician
So I've spoken to the assistant to the engineer and she gave me lots of forms to fill out so they can decide if they are willing to extend the three phase power to the building and have us pay for it. I asked if they could give me a ballpark cost on the line extension and she was very cagey but eventually said she would ask the engineer.

She confirmed that it would be 208Y/120 and that no CTs are required for a 400 amp service. I'm still in the bidding stage, so I'm trying to limit my effort and just give him an estimate. To do that I need to know what the transformer is going to cost.


I called one transformer company and they said this would be a custom job. Does anybody know of a vendor for off-the-shelf transformers of this type? I'm also trying to figure out how big a transformer I need. If I want the primary to draw a max of 200 amps, how many KVA should I be looking for?

Is this right?:
VA = (1.732 x V x I)
72051 = (1.732 x 208 x 200) = 72KVA
Next standard size = 75 KVA.

What would the available secondary side amps be? Is this right?:
75000 / (240 x 1.732) = 180.43 amps (ignoring losses, How big are those losses?)
That’s a 400 amp max, 320 amp continuous service unless they asked you to supply a K base meter base.

Did you get a chance to look at the nameplates?
 
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