Control panel conductors

[overamped]

I'm building a motor control panel and I have a question about calculating the 480 volt feeder size. When adding the sum of the loads (motors, heaters, tranformers etc.) do I use the amperage on the primary or the secondary side of the transformer? So if I have a 2 KVA transformer my primary side would be 4.2 amps but on my secondary would be 16.7. If I add the secondary my feeders would be much larger then if I add the primary.

 

[Smart $]

I'm building a motor control panel and I have a question about calculating the 480 volt feeder size. When adding the sum of the loads (motors, heaters, tranformers etc.) do I use the amperage on the primary or the secondary side of the transformer? So if I have a 2 KVA transformer my primary side would be 4.2 amps but on my secondary would be 16.7. If I add the secondary my feeders would be much larger then if I add the primary.

Figure all your loads in VA, add 25% for largest motor and any non-motor continuous loads, then divide by 480V (and square root of 3 if 3?). The transformer is not considered a load.

 

[petersonra]

I'm building a motor control panel and I have a question about calculating the 480 volt feeder size. When adding the sum of the loads (motors, heaters, tranformers etc.) do I use the amperage on the primary or the secondary side of the transformer? So if I have a 2 KVA transformer my primary side would be 4.2 amps but on my secondary would be 16.7. If I add the secondary my feeders would be much larger then if I add the primary.

Figure all your loads in VA, add 25% for largest motor and any non-motor continuous loads, then divide by 480V (and square root of 3 if 3?). The transformer is not considered a load.

There is often confusion about this.

The incoming feeder FLA is the sum of all of the loads attached to it. In the case of a single phase transformer the FLA is only on 2 of the lines but that is what the FLA is calculated at.

The ampacity of the incoming feeder wires is based on the FLA of the panel plus 25% of the FLA of the largest motor. That is why the control panel nameplate has this information on the door. In any case, it is not the responsibility of the control panel builder or designer to size the incoming feeder, and in general should not even attempt to do so.

 

[overamped]

There is often confusion about this.

The incoming feeder FLA is the sum of all of the loads attached to it. In the case of a single phase transformer the FLA is only on 2 of the lines but that is what the FLA is calculated at.

The ampacity of the incoming feeder wires is based on the FLA of the panel plus 25% of the FLA of the largest motor. That is why the control panel nameplate has this information on the door. In any case, it is not the responsibility of the control panel builder or designer to size the incoming feeder, and in general should not even attempt to do so.

The reason I'm asking is I going to have a step down transformer feeding a 3 phase 230 volt motor that is rated at 5 amps. I'm assuming on my primary 480 volt side it will be 10 amps so will that load be calculated at 5 amps or 10 amps?

 

[petersonra]

The reason I'm asking is I going to have a step down transformer feeding a 3 phase 230 volt motor that is rated at 5 amps. I'm assuming on my primary 480 volt side it will be 10 amps so will that load be calculated at 5 amps or 10 amps?

The actual FLA of the transformer primary has to be used. It does not matter what the secondary load actually is.

 

[Smart $]

The actual FLA of the transformer primary has to be used. It does not matter what the secondary load actually is.

It's going to depend on several issues. Not sure whether other loads are primary or secondary, but he has a 3? motor on the secondary. I believe that will require secondary protection not greater than 125% of the secondary rated current... and that has to be high enough to allow for starting inrush of the motor. Motor rated at 5A, says a ITCB could be as high as 250%...

5A ? 250% = 12.5A

If ocpd is at that rating, the secondary rated current must equal or exceed...

12.5A ? 125% = 10A

I'm sure you can see where this is going. Don't want to go any further at this point until overamped specifies other actual loads, whether continuous and whether connected to primary or secondary. Another factor is whether the secondary rated current will be or has to be 9A or greater. If secondary rated current can be or is less than 9A, secondary protection can go as high as 167%.

 

[petersonra]

It's going to depend on several issues. Not sure whether other loads are primary or secondary, but he has a 3? motor on the secondary. I believe that will require secondary protection not greater than 125% of the secondary rated current... and that has to be high enough to allow for starting inrush of the motor. Motor rated at 5A, says a ITCB could be as high as 250%...

5A ? 250% = 12.5A

If ocpd is at that rating, the secondary rated current must equal or exceed...

12.5A ? 125% = 10A

I'm sure you can see where this is going. Don't want to go any further at this point until overamped specifies other actual loads, whether continuous and whether connected to primary or secondary. Another factor is whether the secondary rated current will be or has to be 9A or greater. If secondary rated current can be or is less than 9A, secondary protection can go as high as 167%.

None of this has any effect on the feeder conductors coming into the panel.

 

[Smart $]

None of this has any effect on the feeder conductors coming into the panel.

True... just has to be sized for the max load per line conductor.

But seems to me the OP'er is implying there is more involved than just the panel feeder...

 

[Smart $]

The reason I'm asking is I going to have a step down transformer feeding a 3 phase 230 volt motor that is rated at 5 amps. I'm assuming on my primary 480 volt side it will be 10 amps so will that load be calculated at 5 amps or 10 amps?

The 480 side current will be lower. .. secondary current multiplied by the inverse of the transformer voltage ratio 480:240. Simply put, the primary current will be one-half the secondary current.

 

[petersonra]

True... just has to be sized for the max load per line conductor.

But seems to me the OP'er is implying there is more involved than just the panel feeder...

That is not so. It has to be sized using the FLC of the xfmr, even if it is totally unloaded.

 

[don_resqcapt19]

That is not so. It has to be sized using the FLC of the xfmr, even if it is totally unloaded.

Why? The transformer is not really a load.

 

[petersonra]

Why? The transformer is not really a load.

Because ul508a says so.

 

[Smart $]

Because ul508a says so.

UL508 has no "jurisdiction" on feeder sizing... though instructions may come to bear for NEC compliance. However, consider motor control panels are not required to be UL listed or NRTL tested, and may be of custom design and fabrication.

 

[petersonra]

UL508 has no "jurisdiction" on feeder sizing... though instructions may come to bear for NEC compliance. However, consider motor control panels are not required to be UL listed or NRTL tested, and may be of custom design and fabrication.

That is sort of true. Many jurisdictions require control panels be listed, even though it is often not enforced.

This is what UL508a says about determining the FLA of a control panel.

49.2 The full-load ampere rating of the panel shall, at a minimum, include the sum of the ampere ratings
of all loads that are able to be operated simultaneously plus the primary ampere rating of all control
transformers connected to the input voltage.

I have to modify what I said because the OP has now changed his tune a little. He now has said that the transformer supplies only a motor. I concur that only the motor load has to be considered in this case because the transformer is not a control transformer since it has a motor connected to it.

I do not see how you can size a feeder to a control panel without taking into account what the nameplate on the control panel says. If the nameplate says the FLA is 100 and the largest motor FLA is 20, than the required ampacity of the feeder wires has to be at least 105 A.

 

[Smart $]

That is sort of true. Many jurisdictions require control panels be listed, even though it is often not enforced.

This is what UL508a says about determining the FLA of a control panel.



I have to modify what I said because the OP has now changed his tune a little. He now has said that the transformer supplies only a motor. I concur that only the motor load has to be considered in this case because the transformer is not a control transformer since it has a motor connected to it.

I do not see how you can size a feeder to a control panel without taking into account what the nameplate on the control panel says. If the nameplate says the FLA is 100 and the largest motor FLA is 20, than the required ampacity of the feeder wires has to be at least 105 A.

I have to admit I did not know UL required the primary rated current for control transformers to be used in the nameplate FLA determination, and not the actual load... but I do not determine the FLA that gets put on the nameplate. When installing a listed control panel, I go by what is on the nameplate.

However, unless the motor is an integral part of the listing, I do not see how a control panel can specify the FLA of both the controller and motor.

 

[overamped]

The 480 side current will be lower. .. secondary current multiplied by the inverse of the transformer voltage ratio 480:240. Simply put, the primary current will be one-half the secondary current.

This was what I was looking for. I was just not sure if I size my feeders by adding the load on the secondary (which would make for a bigger feeder) or the load on the primary (which would make for a smaller feeder).

 

[GoldDigger]

This was what I was looking for. I was just not sure if I size my feeders by adding the load on the secondary (which would make for a bigger feeder) or the load on the primary (which would make for a smaller feeder).

The conductors feeding the primary side are sized based on the primary load which is a combination of transformer idling current at near 0 PF and the reflected load on the secondary (that is, how much primary current has to flow to supply the secondary side load.)
The conductors leaving the secondary need to be sized based on the potential output of the transformer or the actual load depending on how you provide secondary overcurrent protection.

 

[Jraef]

The reason I'm asking is I going to have a step down transformer feeding a 3 phase 230 volt motor that is rated at 5 amps. I'm assuming on my primary 480 volt side it will be 10 amps so will that load be calculated at 5 amps or 10 amps?

I think you either have your description backward or your math is backward. If you have a 230V motor rated for 5A being fed from a 480-240V transformer, the current on the 480V side would be 2.5A, not 10A.

If however you have a 480V motor being fed from a 230V control panel and the transformer is used to step UP the voltage for the motor, then the opposite is true. A 5A 480V motor will pull 10A from the 240V source.

Either way, your feeder size TO the control panel is based on the power that will flow through that feeder, so the PRIMARY side of that transformer, whichever way it is.

 

[overamped]

I think you either have your description backward or your math is backward. If you have a 230V motor rated for 5A being fed from a 480-240V transformer, the current on the 480V side would be 2.5A, not 10A.

If however you have a 480V motor being fed from a 230V control panel and the transformer is used to step UP the voltage for the motor, then the opposite is true. A 5A 480V motor will pull 10A from the 240V source.

Either way, your feeder size TO the control panel is based on the power that will flow through that feeder, so the PRIMARY side of that transformer, whichever way it is.

Sorry!!! I described it backwards!!! It is a step down transformer.