Transformer Ampacity

Status
Not open for further replies.

dasarmin

Member
Location
Texas
A 3-phase 30kva transformer will supply 83.3amps @ 208Vac, when determining the available amps at 120Vac (say from a 208/120 Loadcenter), the calculation or chart would be based on 1-phase. So the same 30kva transformer will supply 250amps @ 120Vac. Correct?
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
Not really.
There is no way to add the 120V circuits into one single value.

The transformer would be capable of supplying (3) 83.3A 120V circuits.
 

dasarmin

Member
Location
Texas
Ok, so for a 208V circuit at full current a 2-pole breaker rated at 80amps.
For 120V circuit, a 1-pole breaker at full current rated at 80amps.
 

kwired

Electron manager
Location
NE Nebraska
A 3-phase 30kva transformer will supply 83.3amps @ 208Vac, when determining the available amps at 120Vac (say from a 208/120 Loadcenter), the calculation or chart would be based on 1-phase. So the same 30kva transformer will supply 250amps @ 120Vac. Correct?
this transformer can supply 250 amps of 120 volt single phase load - but it needs to be evenly divided in three and equally applied across all three phases - so still limits you to a single 120 volt load of 83.3 maximum.

You basically have three 10kVA 120 volt transformers, but they are networked together in a way they can utilize a balanced neutral instead of using three separate neutrals if all load applied is 120 volts, or they can supply 208 single or three phase loads, but each of those loads takes away some capacity from the applicable winding it is connected to, that is when you need to look at amps per line regardless of whether it is single phase 120, single phase 208 or three phase 208, as you could still have less then 30 kVA total but each leg of the system can only carry 10 kVA without temp rise above design.
 

mjmike

Senior Member
I typically see a 30KVA transformer used when going from 480V 3-phase to serve a 100A 208 3-phase panel. There is also debate on the primary breaker. When doing the math, the amps is 45.1A and per the NEC, you upsize to the next breaker which is 50A. Some people claim you drop the 0.1 but I cannot locate anything in the NEC saying to round down. As for the primary protection, provided the panel is close, you go from the transformer right to the 100A main breaker. Keep in mind that the main panel breaker does not count as transformer secondary protection so the primary can't exceed the 125% (or next size up breaker).
 

kwired

Electron manager
Location
NE Nebraska
I typically see a 30KVA transformer used when going from 480V 3-phase to serve a 100A 208 3-phase panel. There is also debate on the primary breaker. When doing the math, the amps is 45.1A and per the NEC, you upsize to the next breaker which is 50A. Some people claim you drop the 0.1 but I cannot locate anything in the NEC saying to round down. As for the primary protection, provided the panel is close, you go from the transformer right to the 100A main breaker. Keep in mind that the main panel breaker does not count as transformer secondary protection so the primary can't exceed the 125% (or next size up breaker).

Where in NEC does it say you can't feet that same transformer with a 30 amp breaker?

You will trip that breaker if you are trying to get all 30KVA but if the load is not there and the 30 will hold when energizing the transformer - nothing is wrong with it from an NEC perspective.

I would need to verify, but pretty sure if you round that 45.1 amps up to 50 - your load calculations still need to be 45.1 or less or you are overloading the transformer.
Also (would need to verify) I believe if that 50 amp breaker will not hold when energizing the transformer you can have higher setting, the secondary side breaker setting becomes even more important at protecting the transformer from overload.
 
Last edited:

david luchini

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
I typically see a 30KVA transformer used when going from 480V 3-phase to serve a 100A 208 3-phase panel. There is also debate on the primary breaker. When doing the math, the amps is 45.1A and per the NEC, you upsize to the next breaker which is 50A. Some people claim you drop the 0.1 but I cannot locate anything in the NEC saying to round down. As for the primary protection, provided the panel is close, you go from the transformer right to the 100A main breaker. Keep in mind that the main panel breaker does not count as transformer secondary protection so the primary can't exceed the 125% (or next size up breaker).

Why wouldn't the 100A main panel breaker count as the secondary protection? Seems to me a 30kVA, 480-208/120V transformer with 100A protection on the primary and 100A protection on the secondary would be fine from a Code perspective.
 

Smart $

Esteemed Member
Location
Ohio
I typically see a 30KVA transformer used when going from 480V 3-phase to serve a 100A 208 3-phase panel. There is also debate on the primary breaker. When doing the math, the amps is 45.1A and per the NEC, you upsize to the next breaker which is 50A. Some people claim you drop the 0.1 but I cannot locate anything in the NEC saying to round down. As for the primary protection, provided the panel is close, you go from the transformer right to the 100A main breaker. Keep in mind that the main panel breaker does not count as transformer secondary protection so the primary can't exceed the 125% (or next size up breaker).
Rounding of calculation amperes [220.5(B)] is permitted, not required.

The main breaker can serve as transformer secondary protection and transformer secondary conductor protection, in addition to serving as panel protection.
 

kingpb

Senior Member
Location
SE USA as far as you can go
Occupation
Engineer, Registered
A 3-phase 30kva transformer will supply 83.3amps @ 208Vac
Yes.

when determining the available amps at 120Vac (say from a 208/120 Loadcenter), the calculation or chart would be based on 1-phase. So the same 30kva transformer will supply 250amps @ 120Vac. Correct?
No.

3 phase power formula is: KVA3ph = sqrt3*VLL*I

1-phase power formula is: KVA1ph = VLG*I

Alternatively, KVA3ph = 3*KVA1ph; where confusion usually comes in is; where the sqrt3 comes from.

It is derived by substitution that:

VLG = VLL/sqrt3, but because mathematically it is not appropriate to have sqrt in the denominator, you multiply the top and bottom by the sqrt3, thus you get:

VLG = sqrt3*VLL/3

Since the KVA3ph = 3*KVA1ph ; then KVA3ph = 3*VLG*I

You can see by observation that the 3's cancel;

KVA3ph = (3*sqrt3*VLL/3)*I

Thus you are left with the 3 phase formula for power.

With that said the current in both the formulas is the same value, and therefore the current in a phase of a 3 phase system is the same as the current in the single phase that was derived from the 3-phase system.

83.3A is the current in each phase, the sum of the total single phase loads could add up to 83.3A x 3, but as other stated that would be a completely balanced system, good luck with that. The largest load on any phase could not draw more than 83.3A, or 10KVA.
 
Status
Not open for further replies.
Top