xfmr/ panel sizing

T

TexasEE14

Member
Location
texas
Occupation
EE
Please review my calculations and let me know if this is correct or way off. I will try to include everything i have. The goal is to add a transformer and panel for a lunch room with the following circuits.
(1) two-120v outlets tied together
(2) five separate 120v outlets for 5 refrigerators
(3) 240v outlet for oven-40A breaker (4300watt)
(4) 240v outlet for stove-50A breaker (3700watt)
(5) 240v outlet for small under counter water heater
(6) 120v outlet for vent-a-hood
(7) two-120v outlets tied together
(8) 120v outlet for a microwave
(9) two-120v outlets tied together with a microwave on one of the outlets.
Total of 13 separate circuits. (planned for 15-20A breakers for the rest of the circuits)

Originally the plan was to have a 15kVA single phase 480-120/240V xfmr, 70A primary ocp & 70A secondary ocp. However the question was posed if it was too small. Below i have done the calcs for a larger 37.5kVA xfmr. are my calcs correct? or is this way over sized?


  • 37.5KVA 480V-120/240V single phase 3-wire transformer.
  • Primary amps- 37.5KVA / 480V = 78.125A
    • Primary OCP per table 450.3 (B) can be up to 2.5 X the primary amps.
    • In this scenario, the XFMR is close to MCC so I will do higher % . I'll use 224% (2.24X) so the primary OCP breaker will be 175A
  • Secondary amps - 37.5KVA / 2 = 18.75KVA ( this is because it’s a 3-wire system so 2 120V legs split the KVA value.
    • So 18.75KVA / 120V = 156.25A or (37kVA / 480 = 156.25A)
    • Secondary OCP per table 450.3 (B) can be up to 1.25 X the secondary amps
    • For this application we will be using 240V and 120V circuits so at 1.25 * 156.25A = 195.3125A OCP
      • This can be rounded down to a 175A breaker for the secondary
  • The conductors for the primary and secondary will be rated 1.25 X the primary and secondary OCP to get the amperage needed for the wire (215.3).
    • So ( both are 175A) 175A X 1.25 = 218.75A .
    • The conductors used in this situation must be rated at or above 218.75A. (uglys book is a 3/0 cable)

Any feed back on this would be greatly appreciated. I dont have the current draw on the appliances but hoping there may be some more experience here to have a better idea of what is realistic. The cable sizing seems a bit large for this type of application but thats just my initial thoughts. i dont want to oversize this if possible. The next size down transformer is a 25kVA i could use if needed.
 
So this small building already has a small panel with a 15kVA xfmr feeding it which already trips when say the oven and microwave is used at the same time. So the goal when renovating this lunch room was to add an additional panel/xfmr to the lunch room separately and power all the circuits, which would also pull some of the circuits from the old overloaded panel and split the load up some.

The original thought was to just add another 15kVA xfmr which should be sufficient since the old panel works, it just keeps tripping breakers when multiple appliances are used at once, so with the split load on the different panels we would mitigate the tripping issues we have.

But a question came up about the new 15kVA still being undersized the worry of tripping when all of the above circuits are in use. So from the manufacture we are using the largest xfmr we can fit into our mcc is a 37.5kVA (same dimensions as the 15kVA) .

long story short- we can use a 15kVA, 25kVA or a 37.5kVA. which ever is sufficient for the loads mentioned in the original post, plus some extra space for future loads.
 
You do not need to size your conductors any larger than the size of their overcurrent protection. For example, you used 125% of the transformer secondary to choose its protective device of 175A, you do not need to add an additional 125% for the conductor.

As others have said, start with your load then work backwards to the transformer and the source.
 
Yes i agree to typically start by adding loads etc, then calc backwards to understand the full current needed for the main and to the transformer, however this in this situation im not able to really do that. I do not have the information on the appliances that will be used. Also, per NFPA 70 - (215.3) - it shows to have the 1.25 X the OCP for the conductors. Is that not correct?

Another note: If i was able to get the loads for the appliances, since im using a single phase 3-wire xfmr 120/240v, would you calculate the 240v loads and 120 volt loads separately and just add up? since this is a single phase 3 wire the kVA should be the full value for 240 but half it for the 120 volt.
 
Thank you for the input. So if we are just looking at my transformer calculations, are these correct not worrying about the load that it will have. Are just the calcs for the conductor sizing, and ocp's correct based on a single phase 120/240 split load ?
 
Your 1.25 factor applies to the load when you are sizing conductors, not the OCP size. You've already applied that factor when you sized the breakers. Conductors get protected at their ampacity, so #3/0, 75°C wire is oversized on a 175A breaker. The minimum wire size in this design is #2/0, assuming you are using THHN/THWN-2 (the most common type of wire.)
 
Thank you for the input! so it sounds like my calculations are correct but the conductor size is not.

BTW just did rough estimation of 28.25kVA load total.

2 microwaves - 8.3A each = 16.6A (120V)
5 fridges - 5A each = 25A (120V)
1 stove - 15.4A (240V)
1 oven - 18A (240V)
8 outlets @ 10A each = 80A (over estimation) (120V)
1 vent-a-hood 7A (120V)
1 water heater - 20A (240V)
 
Right, so the MCB for the panel (xfmr secondary) should be calculated from my original calculations, and not calculated from the loads?

This is what i originally assumed but the mention of needing to know the loads made it sound as if you calc the MCB from the load you have and not the xfmr.

Again, appreciate all the feedback
 
TexasEE14 said:
This is what i originally assumed but the mention of needing to know the loads made it sound as if you calc the MCB from the load you have and not the xfmr.
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It's both....From your loads, your panel can have an MCB of not less than 125A. From your transformer size, your panel can have an MCB not greater than 200A. Select an MCB size within that range that makes the most sense for your project.
 
TexasEE14 said:
Right, so the MCB for the panel (xfmr secondary) should be calculated from my original calculations, and not calculated from the loads?

This is what i originally assumed but the mention of needing to know the loads made it sound as if you calc the MCB from the load you have and not the xfmr.

Again, appreciate all the feedback
Click to expand...
If you want to serve a 100 A load, you would need a 125A breaker. If you want to serve a 150 load you will need a 200A breaker and conductors. The size of your transformer is not relevant.

If you want to serve a 150A 120/240V load you will need a transformer that is at least 36kVA transformer.
 
jim dungar said:
If you want to serve a 100 A load, you would need a 125A breaker. If you want to serve a 150 load you will need a 200A breaker and conductors. The size of your transformer is not relevant.

If you want to serve a 150A 120/240V load you will need a transformer that is at least 36kVA transformer.
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I disagree that the transformer size is not relevant, but only because I don't like to leave transformer capacity unavailable; it's just the way I was taught to design stuff. The quantum nature of equipment sizes make it almost impossible to put in a breaker/feeder/transformer/panel that is exactly the right size and no larger. It's like trying to buy a pair of pants with a 37" waist. Your choices are limited to 36" and 38". Undersize things and you'll regret it.
 
JoeStillman said:
I disagree that the transformer size is not relevant, but only because I don't like to leave transformer capacity unavailable; it's just the way I was taught to design stuff. The quantum nature of equipment sizes make it almost impossible to put in a breaker/feeder/transformer/panel that is exactly the right size and no larger. It's like trying to buy a pair of pants with a 37" waist. Your choices are limited to 36" and 38". Undersize things and you'll regret it.
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Yours is a design choice, not an NEC requirement.
Say your load requirement is 38kVA, you would need to install a 50kVA unit. But why also spend the money on a larger breaker and conductors than what your load is? The extra capacity will not be used by the planned load.
 
As an aside, when selecting your OCP device for your transformer routinely selecting 250%, albeit allowed by Code, can significantly increase your installation costs. Not only having to select a conductor sized to that OCP device but the 250% can often lead you to a larger frame, more expensive breaker.
 
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