30 KVA XFMR Sizing

[Palmer]

I have to take a test for school and here is what I know. It’s a 480 - 120/208 Y 3 phase transformer to be sized for continuous load.

It’s a 30 KVA Transformer is primary overload I came up with 45 amps or just use a 40amp breaker (they give you a 30 a 40 and a 50) using #8 wire. Now the secondary i’m coming up with 104 amps so a 100 amp breaker, using #3 copper. It’s in FMC and I’m not sure if it’s going to be 24” or under where I would not need to derate the conductors. But my question is do I need to up the size of the wire because of the code saying under 100 amp needs to be sized to 60 degrees? Any input? I think I have this all figured out.

edit if i do have to derate and use 60c column then my #8 is still fine derated for 40amps but my secondary would have to be #1 which is 150 at 90c derated by 80% is 120 ( and at 60 is good for 110)

 

[rlundsrud]

Could you show your math please.

 

[Palmer]

Could you show your math please.

for wire sizes or xfmr amperage ?

 

[augie47]

lets look at both

 

[augie47]

No more input ??

FWIW, unless there are added restrictions you can certainly use primary breker larger than 40 and unless they mention non-linear loads there is no reason to derate, There is a lot of relevant info not given that would effect your answer.

 

[Palmer]

No more input ??

FWIW, unless there are added restrictions you can certainly use primary breker larger than 40 and unless they mention non-linear loads there is no reason to derate, There is a lot of relevant info not given that would effect your answer.

I will show the math when I get hole from work. Derange because over 3 current carrying conductors in a conduit

 

[augie47]

I will show the math when I get hole from work. Derange because over 3 current carrying conductors in a conduit

Keep in mind de-rating is for more than 3 current-carrying conductors... equipment grounds don't count and neutral conductors don't count unless a majority of the load is non-linear, a rare occurrence.

 

[Palmer]

Keep in mind de-rating is for more than 3 current-carrying conductors... equipment grounds don't count and neutral conductors don't count unless a majority of the load is non-linear, a rare occurrence.

what do you mean by non linear?

 

[Besoeker3]

what do you mean by non linear?

Pretty much anything electronic. Like dimmers, variable speed drives, televisions, computers...................
The current is not sinusoidal.

 

[Palmer]

Pretty much anything electronic. Like dimmers, variable speed drives, televisions, computers...................
The current is not sinusoidal.

It says right on the test that that load is always 100% so I think that tells me I should derange the conductors.

 

[augie47]

You size the conductors based on load and protect them based on their ampacity. IMO, you don't have enough information to provide an adequate answer. FWIW, a 30 kva 480Y120 transformer with a FLA of 36 amps and could be protected by a 45 amp breaker or less. IF you elected to use the 45 amp breaker and were not provided with termination temperature ratings you would need a #6 (In reality the terminations are likely 75° and a #8 would be sufficient). The transformer output of 83 amps could handle a 104 amp continuous load but the conductors would be sized to match your overcurrent device at 75° ratings.

 

[Palmer]

You size the conductors based on load and protect them based on their ampacity. IMO, you don't have enough information to provide an adequate answer. FWIW, a 30 kva 480Y120 transformer with a FLA of 36 amps and could be protected by a 45 amp breaker or less. IF you elected to use the 45 amp breaker and were not provided with termination temperature ratings you would need a #6 (In reality the terminations are likely 75° and a #8 would be sufficient). The transformer output of 83 amps could handle a 104 amp continuous load but the conductors would be sized to match your overcurrent device at 75° ratings.

Temps are 75c absolutely. I’m confident on the #8 primary side but am more so questioning the secondary of #1,
#2 or #3

 

[augie47]

Needs to follow 240.21(C) and match your secondary overcurret device

 

[Palmer]

Needs to follow 240.21(C) and match your secondary overcurret device

30000 / (480 x 1.732) = 831.36 30000 / 831.36 = 36.0850 36.0850 x 125% = 45 amp primary

30000 / (208 x 1.732) = 360.256 30000 / 360.256 = 83.2741 83.2741 x 125 % = 104 amp secondary

This is where I get confused looking at table 310.16. I've seen in the code that you need to use the 60c column for everything under 100 amps, but then I've also seen that I can size it to the temperature of the terminals which is 75c. And another factor is derating the wire after 3 current carrying conductors. In the field I would just use the higher size wire.

With a OCPD on the primary side of 40 amp, #8 not derated at 75c is good for 50.. Derated from 90c by 20% is good for 44 and if not derated and using the 60c column it's still good for 40. Primary I'm very confident in using the #8.

Secondary with a OCPD of 100, #3 not derated at 75c is good for 100 amps. Derated from 90c by 20%, #2 is good for 104 amps. And finally using the 60c column, #1 is good for 110 amps. This is where I'm really entirely overthinking things. I'm good once I get the secondary side on the bonding jumper and the gec to building steel based off of 250.66 and the primary side ground using table 250.122.

If any of this looks wrong or questionable please explain further, I feel like I'm trying to split an atom here haha but all your help is honestly so appreciated.

EDIT: The 60c column for under 100 amp is only used when there is no temperature listed. So I'm either going with #3 copper or #2.. #1 is way out of the question.

 

[topgone]

30000 / (480 x 1.732) = 831.36 30000 / 831.36 = 36.0850 36.0850 x 125% = 45 amp primary

30000 / (208 x 1.732) = 360.256 30000 / 360.256 = 83.2741 83.2741 x 125 % = 104 amp secondary

This is where I get confused looking at table 310.16. I've seen in the code that you need to use the 60c column for everything under 100 amps, but then I've also seen that I can size it to the temperature of the terminals which is 75c. And another factor is derating the wire after 3 current carrying conductors. In the field I would just use the higher size wire.

With a OCPD on the primary side of 40 amp, #8 not derated at 75c is good for 50.. Derated from 90c by 20% is good for 44 and if not derated and using the 60c column it's still good for 40. Primary I'm very confident in using the #8.

Secondary with a OCPD of 100, #3 not derated at 75c is good for 100 amps. Derated from 90c by 20%, #2 is good for 104 amps. And finally using the 60c column, #1 is good for 110 amps. This is where I'm really entirely overthinking things. I'm good once I get the secondary side on the bonding jumper and the gec to building steel based off of 250.66 and the primary side ground using table 250.122.

If any of this looks wrong or questionable please explain further, I feel like I'm trying to split an atom here haha but all your help is honestly so appreciated.

EDIT: The 60c column for under 100 amp is only used when there is no temperature listed. So I'm either going with #3 copper or #2.. #1 is way out of the question.

Are we talking about transformer protection here? 125% fudge factor is the required protection on the secondary of the transformer. The primary protection might trip if you are using 125%. See 450.3 (B).

 

[kwired]

It says right on the test that that load is always 100% so I think that tells me I should derange the conductors.

If the load is continuous and you are not using 100% rated overcurrent devices (which is somewhat rare) then your minimum conductor ampacity will be 125% of the continuous load plus any non continuous load - this is for termination temperature rating more than anything else. So for this part of design you need to know actual load, not just that you have a 30 kVA transformer supplying unknown loads. If you have 30kVA of continuous load then you do need 104 amps conductor ampacity and 104 amps (or next size higher which is 110) overcurrent protection.

30000 / (480 x 1.732) = 831.36 30000 / 831.36 = 36.0850 36.0850 x 125% = 45 amp primary

30000 / (208 x 1.732) = 360.256 30000 / 360.256 = 83.2741 83.2741 x 125 % = 104 amp secondary

This is where I get confused looking at table 310.16. I've seen in the code that you need to use the 60c column for everything under 100 amps, but then I've also seen that I can size it to the temperature of the terminals which is 75c. And another factor is derating the wire after 3 current carrying conductors. In the field I would just use the higher size wire.

With a OCPD on the primary side of 40 amp, #8 not derated at 75c is good for 50.. Derated from 90c by 20% is good for 44 and if not derated and using the 60c column it's still good for 40. Primary I'm very confident in using the #8.

Secondary with a OCPD of 100, #3 not derated at 75c is good for 100 amps. Derated from 90c by 20%, #2 is good for 104 amps. And finally using the 60c column, #1 is good for 110 amps. This is where I'm really entirely overthinking things. I'm good once I get the secondary side on the bonding jumper and the gec to building steel based off of 250.66 and the primary side ground using table 250.122.

If any of this looks wrong or questionable please explain further, I feel like I'm trying to split an atom here haha but all your help is honestly so appreciated.

EDIT: The 60c column for under 100 amp is only used when there is no temperature listed. So I'm either going with #3 copper or #2.. #1 is way out of the question.

That 60C rule for under 100 amps is because older equipment under 100 amps was often only 60C rated. About the only time you need to pay attention to this is if you have older equipment (like more than 30 years old now) as pretty much all smaller breakers, contactors, etc. all have had 75C terminations on them for 30 years or so.

The need to adjust ampacity because of more than 3 current carrying conductors would only come into play if your neutral is deemed to be a current carrying conductor. With linear loading the neutral only carries unbalanced current of the ungrounded conductors and is not considered a current carrying conductor when it comes to determining it's heat contribution to the raceway or cable containing it. However when you have a majority of the neutral load being from non linear loads - the harmonic currents from the non linear loads are additive in the neutral of a three phase wye system, and you can find situations where the neutral conductor actually needs to be larger than the ungrounded conductors because of this. A large data center is primarily the main place you will run into needing to worry about this in the real world.

I see you found some of what I mentioned on your own - good for you.

 

[Palmer]

If the load is continuous and you are not using 100% rated overcurrent devices (which is somewhat rare) then your minimum conductor ampacity will be 125% of the continuous load plus any non continuous load - this is for termination temperature rating more than anything else. So for this part of design you need to know actual load, not just that you have a 30 kVA transformer supplying unknown loads. If you have 30kVA of continuous load then you do need 104 amps conductor ampacity and 104 amps (or next size higher which is 110) overcurrent protection.


That 60C rule for under 100 amps is because older equipment under 100 amps was often only 60C rated. About the only time you need to pay attention to this is if you have older equipment (like more than 30 years old now) as pretty much all smaller breakers, contactors, etc. all have had 75C terminations on them for 30 years or so.

The need to adjust ampacity because of more than 3 current carrying conductors would only come into play if your neutral is deemed to be a current carrying conductor. With linear loading the neutral only carries unbalanced current of the ungrounded conductors and is not considered a current carrying conductor when it comes to determining it's heat contribution to the raceway or cable containing it. However when you have a majority of the neutral load being from non linear loads - the harmonic currents from the non linear loads are additive in the neutral of a three phase wye system, and you can find situations where the neutral conductor actually needs to be larger than the ungrounded conductors because of this. A large data center is primarily the main place you will run into needing to worry about this in the real world.

I see you found some of what I mentioned on your own - good for you.

So basically you’re saying I shouldn’t derate my wires then, the only options for secondary ocpd are 70 90 and 100. And I know they have 8,6,1 and 1/0 wire to choose from. So with a 100 amp secondary ocpd I should use #3 wire. It’s just for a transformer for a small 1/3 hp motor I think 3 pole 15 208 volt

 

[augie47]

With the choices you are given I' choose the 100 amp conductor since the transformer is capable of 104 and accordingly a #1 conductor even though a#3 would work,

 

[kwired]

So basically you’re saying I shouldn’t derate my wires then, the only options for secondary ocpd are 70 90 and 100. And I know they have 8,6,1 and 1/0 wire to choose from. So with a 100 amp secondary ocpd I should use #3 wire. It’s just for a transformer for a small 1/3 hp motor I think 3 pole 15 208 volt

A lot of transformer for a 1/3 Hp motor.

Nothing says you must supply that transformer with it's rated capacity, especially if not using it's rated capacity. You still need overcurrent device on primary that will hold during energizing or it will at very least be a nuisance.

Secondary you must consider feeder tap/transformer secondary rules in 240.21, but otherwise don't need to utilize full kVA rating either