Voltage Drop for 460V, 3 phase motor

[Dale001289]

Can someone please answer, once and for all, should 460V be used or 480V? (using ETAP). Also, does Motor Service Factor (1.15) have to be considered?

 

[templdl]

Can someone please answer, once and for all, should 460V be used or 480V? (using ETAP). Also, does Motor Service Factor (1.15) have to be considered?

The common motor designed per NEMA standards will operate satisfactory at +- 10% of the name plate voltage,

 

[kwired]

Can someone please answer, once and for all, should 460V be used or 480V? (using ETAP). Also, does Motor Service Factor (1.15) have to be considered?

480 volts is nominal supply voltage. In general it can vary by 5% and is considered acceptable so that puts us anywhere between 455 and 504 being "normal". Many places will see that voltage be on the high instead of the low side, at least when in a minimal or no load condition, that way normal voltage drop that will occur as load increases will hopefully not drop below the lower end of the acceptable range.

Motor nameplates are usually 460 volts. Say you had a 10 hp motor that is doing 10 hp of work - and actual input voltage was 460 - that means the amps, efficiency, power factor should all match the nameplate values. Change the voltage a little bit and all those other values will change a little as well.

Service factor over 1.0 just means the winding insulation can take the added heating that results if you load it beyond nameplate ratings better then a 1.0 SF motor can.

 

[ron]

110.4 Voltages. Throughout this Code, the voltage considered shall be that at which the circuit operates. The voltage rating of electrical equipment shall not be less than the nominal voltage of a circuit to which it is connected.

Voltage, Nominal. A nominal value assigned to a circuit or system for the purpose of conveniently designating its voltage class (e.g., 120/240 volts, 480Y/277 volts, 600 volts).The actual voltage at which a circuit operates can vary from the nominal within a range that permits satisfactory operation of equipment.

 

[Dale001289]

480 volts is nominal supply voltage. In general it can vary by 5% and is considered acceptable so that puts us anywhere between 455 and 504 being "normal". Many places will see that voltage be on the high instead of the low side, at least when in a minimal or no load condition, that way normal voltage drop that will occur as load increases will hopefully not drop below the lower end of the acceptable range.

Motor nameplates are usually 460 volts. Say you had a 10 hp motor that is doing 10 hp of work - and actual input voltage was 460 - that means the amps, efficiency, power factor should all match the nameplate values. Change the voltage a little bit and all those other values will change a little as well.

Service factor over 1.0 just means the winding insulation can take the added heating that results if you load it beyond nameplate ratings better then a 1.0 SF motor can.

Ok. Here's another question, Ref 430.6(A)(1): Why does the Code insist on using the FLC's (Table 430.250) instead of nameplate FLA? We have a 60hp application at 460V. Nameplate says 68A. NEC says 77A must be used per the table. This causes the VD to kick the cable up one size. To me, doesn't make sense if the nameplate data is available, why not use it?

 

[ron]

Ok. Here's another question, Ref 430.6(A)(1): Why does the Code insist on using the FLC's (Table 430.250) instead of nameplate FLA? We have a 60hp application at 460V. Nameplate says 68A. NEC says 77A must be used per the table. This causes the VD to kick the cable up one size. To me, doesn't make sense if the nameplate data is available, why not use it?

I've used exception#3 to 430.6(A)(1) to reference the nameplate

 

[kwired]

Ok. Here's another question, Ref 430.6(A)(1): Why does the Code insist on using the FLC's (Table 430.250) instead of nameplate FLA? We have a 60hp application at 460V. Nameplate says 68A. NEC says 77A must be used per the table. This causes the VD to kick the cable up one size. To me, doesn't make sense if the nameplate data is available, why not use it?

Those tables are based on worst case situation of efficiency and power factor. They want to ensure that if the motor fails and is replaced with same HP rating that the conductors are still sufficiently sized should the replacement be less efficient then the original.

You typically will not find new motors that have FLA as high as those tables, especially your general purpose motors. You may find some OEM or definite purpose motors though that may push the limits.

 

[Dale001289]

Those tables are based on worst case situation of efficiency and power factor. They want to ensure that if the motor fails and is replaced with same HP rating that the conductors are still sufficiently sized should the replacement be less efficient then the original.

You typically will not find new motors that have FLA as high as those tables, especially your general purpose motors. You may find some OEM or definite purpose motors though that may push the limits.

thanks to all for this valuable input.

 

[drcampbell]

If the nameplate says 460 volts, the motor should run just fine with 460 volts at the motor terminals. The overall system is designed with 480-volt service and 460-volt motors to accommodate the inevitable voltage drop.

But you may find it economical to use larger conductors than the minimum needed for adequate ampacity. Supplying a 68-amp load with 20 volts of drop would mean that your feeder is a 2.4-kW electric heater. While this won't cause any harm, you probably don't want to pay for it. (and if it runs through a conditioned space, pay for it twice -- once to turn electricity into heat and a second time for the air conditioner to pump the heat out of the space)

 

[Dale001289]

If the nameplate says 460 volts, the motor should run just fine with 460 volts at the motor terminals. The overall system is designed with 480-volt service and 460-volt motors to accommodate the inevitable voltage drop.

But you may find it economical to use larger conductors than the minimum needed for adequate ampacity. Supplying a 68-amp load with 20 volts of drop would mean that your feeder is a 2.4-kW electric heater. While this won't cause any harm, you probably don't want to pay for it. (and if it runs through a conditioned space, pay for it twice -- once to turn electricity into heat and a second time for the air conditioner to pump the heat out of the space)

Excellent point - 2.4kW puts it all in perspective; in this case, we're dealing with a $5B LNG plant grassroots expansion of existing terminal. I'm just one player amongst dozens trying to get it right. Thanks again.

 

[Jraef]

ANSI defines voltages as two "classes";

"Distribution Voltage" is what is distributed and delivered to incoming terminals of end user facilities. So 480V is a Distribution Voltage, you will have 480V +- 5% at the line side of your service entrance: between 504 and 432V

"Utilization Voltage" is the class of voltage used to design the end point use age, ie what the motor is designed for. It is lower because it is EXPECTED to have a certain amount of voltage drop. Utilization Voltages are part of the equipment design spec, and for motors designed per NEMA MG-1 design specs will have a +-10% tolerance. So a 460V motor will run fine between 506 and 414V, so well within the line tolerances.

For doing an ETAP evaluation for something like a transient motor starting analysis you should use 460V, but for evaluating fault current or arc flash, you should use 480V.

 

[Dale001289]

ANSI defines voltages as two "classes";

"Distribution Voltage" is what is distributed and delivered to incoming terminals of end user facilities. So 480V is a Distribution Voltage, you will have 480V +- 5% at the line side of your service entrance: between 504 and 432V

"Utilization Voltage" is the class of voltage used to design the end point use age, ie what the motor is designed for. It is lower because it is EXPECTED to have a certain amount of voltage drop. Utilization Voltages are part of the equipment design spec, and for motors designed per NEMA MG-1 design specs will have a +-10% tolerance. So a 460V motor will run fine between 506 and 414V, so well within the line tolerances.

For doing an ETAP evaluation for something like a transient motor starting analysis you should use 460V, but for evaluating fault current or arc flash, you should use 480V.

makes great sense to me, thanks for the info!

 

[GoldDigger]

... Nameplate says 68A. NEC says 77A must be used per the table. This causes the VD to kick the cable up one size. To me, doesn't make sense if the nameplate data is available, why not use it?

Since VD limits and VD calculation are not actually part of the NEC in the first place, you are free to use whichever current you prefer in your voltage drop calculations.
If you have knowledge that the motor will never be fully loaded you can use a partial load current for VD calculations. You cannot do that for required ampacity calculations.

If a motor has, for example, a 1.15 service factor, and you know that the equipment designers are deliberately using the motor above its nominal power rating you should calculate VD based on the higher, above full load, current as that will affect the operating voltage of the already highly stressed motor.

 

[templdl]

Interesting replys.
Back to my initial post staying simply NEMA designs are +- 10% of the NP voltage.
NEMA MG-1 12.44.1a defines voltage tolerances for alternating voltage motors under running conditions at rated load as +/- 10% of rated voltage.
That is a range of 506-414v based upon a utilization voltage of 460v.

 

[Smart $]

I've used exception#3 to 430.6(A)(1) to reference the nameplate

That exception only applies to motor-operated appliances.

 

[Dale001289]

That exception only applies to motor-operated appliances.

Could a 60hp Blower motor in a LNG Plant be considered a very large appliance?:lol:

 

[Smart $]

Could a 60hp Blower motor in a LNG Plant be considered a very large appliance?:lol:

Sure. Located in an industrial facility makes it a bit iffy according to the NEC definition of appliance, but I know of no Code requirement which removes it from consideration.

However, getting the AHJ to agree is a completely different matter. :happyyes:

 

[templdl]

That exception only applies to motor-operated appliances.

Mmmmm, 460v appliances.

 

[Dale001289]

Mmmmm, 460v appliances.

Our contract reads, "Voltage drop not to exceed 5% total, typically split with 2% for the feeder and 3% for the branch". Our transformers sit less than 50 feet from the 480V bus (Switchgear), so virtually no V-drop there. I assume therefore, nearly the entire 5% can be applied to the branch - Comments?

I tend to think 460V at the terminals is the way to go; from a mathematical standpoint it doesn't seem to make much difference.

 

[Smart $]

Our contract reads, "Voltage drop not to exceed 5% total, typically split with 2% for the feeder and 3% for the branch". Our transformers sit less than 50 feet from the 480V bus (Switchgear), so virtually no V-drop there. I assume therefore, nearly the entire 5% can be applied to the branch - Comments?

I tend to think 460V at the terminals is the way to go; from a mathematical standpoint it doesn't seem to make much difference.

If you include tolerances you get

(480-5%)–(460-10%)=42V_DROP

That's 8.75%.

One thing to keep in mind with motors in general though... voltage and current are inversely proportional. Using the above percentage as a limit for the V_DROP calculation means you should use a current value that is 5% above nominal.

Then there's the stipulation in your contract to use a V_{DROP limit of 5%... so consider the preceding as mere speculation.}