Transformer Calculations

[JSR62382]

I have a friend building a building. His contractor left the job unfinished and he ask me to help finish it. We have a 480 panel feeding a 150kva transformer so here's the calcs. Primary is 150000 / 480 / 1.732 = 180 amps times 1.25 equals 225 amps. The 480 panel currently has a 200 amp breaker feeding this transformer. Is it permissible to have a breaker smaller than calculated? The secondary is 120/208. The exist 2 - 400 amp rated MLO panels. There needs to be overcurrent protection for these panels. So 150000 / 208 / 1.732 = 413 amps. Most of the loads in these panels will not be continuous. Can I use a single 400 amp overcurrent device to protect both of these panels?

 

[Hv&Lv]

I could be wrong, but I don’t think it’s ever against code to use a protective device that’s smaller than required.
You have 4/0 wire and want to protect it at 60 amps? What’s it going to hurt.

as far as your transformer, I wouldn’t go so low as to cause nuisance tripping, but why wouldn’t you be able to lower the breaker?

 

[Carultch]

It is OK to have a primary breaker less than the amps associated with the KVA rating of the transformer at the primary voltage, assuming you aren't fully utilizing all of the kVA of the transformer, and the breaker is sized according to what is required for the load. There is a maximum rating of the primary breaker in order to protect the transformer against overload as discussed in NEC 450.3, but there is no minimum. You could put a 150kVA transformer on a 100A primary breaker at 480V 3-phase, if you only had 100A or less of primary-side load that sizes the breaker.

What is (usually) not OK, is using a primary side breaker to indirectly protect the secondary. In your example, that would be the assumption that your 200A primary breaker protects the secondary at 462A, because 462 = 200*480/208. In this topology, it doesn't comply. This is only allowed in certain topologies that qualify, like delta-delta, and single phase 2-wire on both sides. Topologies that line up currents directly across a pair of windings, with no option to redistribute the amps. Anytime there is a WYE system on either side, or a center-tapped part of the transformer, you need overcurrent protection on both sides.

In any case, both the primary and secondary breakers get sized according to the calculated load, at that corresponding position in the circuit. Think of transformers like gearboxes. Gearboxes trade torque and speed, transformers trade voltage and current. V_pri*I_pri = V_sec*I_sec, if you neglect losses. If you account for losses, it is V_pri*I_pri*efficiency = V_sec*I_sec.

 

[JSR62382]

It is OK to have a primary breaker less than the amps associated with the KVA rating of the transformer at the primary voltage, assuming you aren't fully utilizing all of the kVA of the transformer, and the breaker is sized according to what is required for the load. There is a maximum rating of the primary breaker in order to protect the transformer against overload as discussed in NEC 450.3, but there is no minimum. You could put a 150kVA transformer on a 100A primary breaker at 480V 3-phase, if you only had 100A or less of primary-side load that sizes the breaker.

What is (usually) not OK, is using a primary side breaker to indirectly protect the secondary. In your example, that would be the assumption that your 200A primary breaker protects the secondary at 462A, because 462 = 200*480/208. In this topology, it doesn't comply. This is only allowed in certain topologies that qualify, like delta-delta, and single phase 2-wire on both sides. Topologies that line up currents directly across a pair of windings, with no option to redistribute the amps. Anytime there is a WYE system on either side, or a center-tapped part of the transformer, you need overcurrent protection on both sides.

In any case, both the primary and secondary breakers get sized according to the calculated load, at that corresponding position in the circuit. Think of transformers like gearboxes. Gearboxes trade torque and speed, transformers trade voltage and current. V_pri*I_pri = V_sec*I_sec, if you neglect losses. If you account for losses, it is V_pri*I_pri*efficiency = V_sec*I_sec.

I inherited this installation and I know I need to make a change on the secondary to add overcurrent protection. My question is can it be less than the maximum calculated. Can I have a 400 amp overcurrent protection when the max calculated is 413 amps x 125%.

 

[Carultch]

I inherited this installation and I know I need to make a change on the secondary to add overcurrent protection. My question is can it be less than the maximum calculated. Can I have a 400 amp overcurrent protection when the max calculated is 413 amps x 125%.

"The max calculated" does not necessarily need to be based off the 150 kVA rating, which is how I suspect you determined the 413A number (which really calculates as 416A). It is based on the load calculation from the accumulation of loads that will fill the panelboard. Or that will fill the panelboard + all its subpanels.

In otherwords, suppose the transformer weren't there in the first place, but you derived this from a different source of the 120/208V grid. What OCPD would you be putting on this panel as the main breaker? That becomes your secondary-side OCPD. The difference due to the fact that this is a transformer, is that 240.21(C) applies to conductor sizing, making 240.4(B) no longer apply. But the sizing algorithm of the secondary breaker doesn't need to even "think about" the transformer KVA rating.

The KAIC rating of the breaker does depend on the KVA of the transformer, but that's a topic for another time.

 

[augie47]

You need to perform a load calculation to assure the project is adequate,
The 200 amp breaker will likely be ok on the primary.
You can have to six secondary panels but you do need secondary protection.
(Panel require main OCP in or ahead of them 408.36)
Since they are 400 amp MLOs you can use one breaker to protect both or you can use a 400 amp in each since your transformer has 125% primary protection.

 

[SSDriver]

Also if you size the primary breaker a lot smaller (like 50% smaller) than the KVA rating of the transformer it may trip the breaker the first or second time you energize the breaker on a new install. This is ok and normal.

Yes you can use a single 400 amp over-current device to protect both panels. If you go with more than one over-current device the total of the OCPD's cannot be higher than what one single device would be if you do not have primary protection or protection over 125%.

EDIT: augie beat me to it.

 

[frofro19]

I have a friend building a building. His contractor left the job unfinished and he ask me to help finish it. We have a 480 panel feeding a 150kva transformer so here's the calcs. Primary is 150000 / 480 / 1.732 = 180 amps times 1.25 equals 225 amps. The 480 panel currently has a 200 amp breaker feeding this transformer. Is it permissible to have a breaker smaller than calculated? The secondary is 120/208. The exist 2 - 400 amp rated MLO panels. There needs to be overcurrent protection for these panels. So 150000 / 208 / 1.732 = 413 amps. Most of the loads in these panels will not be continuous. Can I use a single 400 amp overcurrent device to protect both of these panels?

180 amps times 1.25 equals 225 amps. Is this 1.25 for continuous load?

 

[jim dungar]

I think it can be confusing to size conductors and protective devices by starting at the transformer and then moving in two directions. I like to start at the load and then work towards the source.

1. Start at the loads and determine the branch circuit sizes and protective devices.
2. Then go to the panel feeding these branch circuits to determine its protective device (NEC 408) and the size of the conductors feeding it. Repeat if there are several panels in series.
3. Once you reach the transformer you will know the conductors on the secondary side of your transformer. Use NEC 240.21(C) to make sizing any adjustments based on which rule is applicable. Per NEC 408, the panel protective device does not have to be in the panel, it can be remote which can help when there are long runs on the secondary.
4. Now size the transformer to handle the load of you secondary equipment (surprisingly this is not really an NEC requirement). It can be larger than your load.
5. Select the primary side protective device based on the maximum load seen by the transformer (some transformers maybe be overloaded). Going much below the NEC 450 limit of 125% of the transformer nameplate may cause problems when energizing the transformer.
6 If the chosen primary side protective device is larger than 125% of the nameplate, check and adjust the secondary device(s) to be less than 125% of the nameplate secondary rating. Multiple circuits may be fed directly off the transformer lugs per NEC 450.
7. Choose primary side conductors based on the chosen primary side protective device. Note, rounding up to the next highest size protective device is not always allowed for transformer circuits.

 

[JSR62382]

I think it can be confusing to size conductors and protective devices by starting at the transformer and then moving in two directions. I like to start at the load and then work towards the source.

1. Start at the loads and determine the branch circuit sizes and protective devices.
2. Then go to the panel feeding these branch circuits to determine its protective device (NEC 408) and the size of the conductors feeding it. Repeat if there are several panels in series.
3. Once you reach the transformer you will know the conductors on the secondary side of your transformer. Use NEC 240.21(C) to make sizing any adjustments based on which rule is applicable. Per NEC 408, the panel protective device does not have to be in the panel, it can be remote which can help when there are long runs on the secondary.
4. Now size the transformer to handle the load of you secondary equipment (surprisingly this is not really an NEC requirement). It can be larger than your load.
5. Select the primary side protective device based on the maximum load seen by the transformer (some transformers maybe be overloaded). Going much below the NEC 450 limit of 125% of the transformer nameplate may cause problems when energizing the transformer.
6 If the chosen primary side protective device is larger than 125% of the nameplate, check and adjust the secondary device(s) to be less than 125% of the nameplate secondary rating. Multiple circuits may be fed directly off the transformer lugs per NEC 450.
7. Choose primary side conductors based on the chosen primary side protective device. Note, rounding up to the next highest size protective device is not always allowed for transformer circuits.

Very good advice I'm sure. In my scenario however the equipment I mention is already installed by a contractor that walked off the job. My task is to make the existing equipment code compliant with the least amount of changes ($$). Thanks

 

[jim dungar]

Very good advice I'm sure. In my scenario however the equipment I mention is already installed by a contractor that walked off the job. My task is to make the existing equipment code compliant with the least amount of changes ($$). Thanks

So at what step are your starting? If the secondary panels are already installed you still need to do steps 4-7. If the transformer is already in place, jump in at step in at 5.

 

[kwired]

180 amps times 1.25 equals 225 amps. Is this 1.25 for continuous load?

Pretty sure it is coming from T450.3. With no secondary protection you can protect primary at 125%. Could be up to 250% if there is secondary protection. OP's situation however does require secondary protection.

 

[jim dungar]

Pretty sure it is coming from T450.3. With no secondary protection you can protect primary at 125%. Could be up to 250% if there is secondary protection. OP's situation however does require secondary protection.

We do not know that secondary protection is needed. For sure there will be protective devices on the secondary to address 408 and 240.21(C) requirements, but those may not be required to meet 450.3.

 

[kwired]

We do not know that secondary protection is needed. For sure there will be protective devices on the secondary to address 408 and 240.21(C) requirements, but those may not be required to meet 450.3.

Correct is not required by 450.3, but likely is for other reasons. I think you still see a lot of people that do go with 125% even though they can go up to 250% for many typical 480x208/120 applications.