30kva 3 phase 480 277 transformer calc question..

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highpowered

Member
Location
los angeles
Client has asked a question without giving me all info needed but here goes

160 amp appleton 4pinw/sleeve 240v 4 wire=200 amp rated fused disconnect(125%)
-Existing 30kva transformer line is 480/277v
-also have a mobil office needs 120v 20 amp im assuming

my calculation is 40 amp line breaker 80 amp load breaker@120/208. this dosnt make sense?

i dont think the 30kva is enough for that type of load? is there any way to configure existing transformer to carry a 200 amp240v load plus the mobil office?

id appreciate any help im tired and its been a long time since i did a transformer calc
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
I believe thwe actual secondary would be 83 amps, which for transformer secondary protectioin would allow you to use a 110 amp breaker.
You really do need to know the actual load as the plug could be oversized.
 
Location
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Look at Table 450.3(B) Currents of 9 Amperes or more with primary and secondary protection. The primary OCP could be as high as 100 amps and secondary as high as 110 for a three phase transformer. A single phase unit could be as high as 175 primary and 200 secondary. (208v secondary)

and now that I have taken long enough to type this out... to quote Augie "You really need to know the actual load as the plug could be oversized."
 
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highpowered

Member
Location
los angeles
my kva on this my calc is 24kva(120vx200a)...they plan on adding ac units to the mobil office the 200amp panel should cover this load + there advertising trailer@80%. to protect the line and load is 100amp breaker(line) 110amp breaker (load)? Thanks this site is new to me and it is very useful i 've been browsing every night sucking in info
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
I believe there is a flaw in your calculation. If the load is truly 200 amps on all three phases, it would add up to 72 kw (24 x 3).
A true 200 amp 3 phase load (200 amps per phase) would need around 75 kva.
In addition keep in mind on unbalanced loads, you need to watch the transformer loading per phase. A 30 kva transformer is not happy with 20 kva of 120 volts on one phase.
 

erickench

Senior Member
Location
Brooklyn, NY
The calculation would be thus:

30KVA/3=10KVA for each phase

The rated secondary current from line to line would be 10KVA/240=41.67A

You may have to install a larger transformer to get the higher KVA.
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
Full load current at 240V 3ph (line to line) for a 30KVA is 72.2A (amperes x voltage x sq rt of 3/1000), am I missing something?

I think the only other point is that he needs to watch the load per phase to neutral so as not to overload any winding (each winding being 10 kva)
 

kwired

Electron manager
Location
NE Nebraska
my kva on this my calc is 24kva(120vx200a)...they plan on adding ac units to the mobil office the 200amp panel should cover this load + there advertising trailer@80%. to protect the line and load is 100amp breaker(line) 110amp breaker (load)? Thanks this site is new to me and it is very useful i 've been browsing every night sucking in info

I assumed the 120v x 200a means you have 200 amps of 120 volt loads to supply. Balance that across three phases you have a load of about 67 amps per phase. Everyone is calculating the capacity of the transformer when the bottom line is what does the load require.

In a wye system the line to neutral current is higher than the line to line current. The reverse is true for voltages.

Neutral current depends on the connected load.

If you are talking about neutral capacity of the transformer itself one phase to neutral rating is same for both as they both carry same current. Two phases to neutral with maximum load on the phases still results in approximately same current on the neutral. Throw some non linear loads in there and the harmonics on the neutral will start to add up though instead of cancelling each other.
 

tryinghard

Senior Member
Location
California
i would change the transformer to 120,120,120. 160 amps would be on 2 phases the third would have another 15 amps on it.
You need to calculate the load per Art 220 the method you?re using is not Art 220. Your method to divide the ampere rating of the subpanel gives you some theoretical values if these were the calculated amperes the additional 15A should be added to all three phases for sizing

120+15=135x1.732=77.94x208x1.732=28.08kva
So 30kva with 45A supply breaker would be fine.

The load determines the needed circuit using the actual equipment nameplates, non-continuous, and continuous loads calculated per Art 220, see examples in Annex D to help (maybe D3).
 
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