Motor Feeder OCPD sizing

[Kratky07]

Hey Gents,
I am designing an electrical service that will exclusively feed (2) 3PH 480V motors with VFDS. Here in Houston working with the local utility is a nightmare, and the larger the service size is the more ridiculous requirements are imposed by the utility providers making it very difficult to build out the services due to easements, based off thier poles/pad mount transformers needed to supply the customers load demand. That being said, I am trying to keep the service as small as possible to not have to have a huge pad mount transformer which is required once you exceed 1000 Amps I believe. So anything on my load calc over 800A means I need huge easement requirements with utiltiy basically making the customers install impossible.

Pump #1 name plate 480V3PH/400HP VFD INPUT CURRENT FLA-461 (Heavy Duty) the VFD spec sheet also reads 598.1A Normal Duty (no idea what that means exactly)

Pump#2 name plate 480V3PH/200HP VFD INPUT CURRENT FLA 243A (Heavy Duty) 315.3A (Normal Duty)

So for my load calc I selected the Normal Duty FLA on the VFD's spec sheet and my largest motor is 461Ax1.15%= 530A + my smaller pump 243A gives me 773A total. this works great because it keeps my customers service size at 800A, allowing us to build them an overhead service requiring no underground easement, allowing us to run underground conduits to each respective VFD in its desired location.

According to 430.128 I can size my disconnecting means to 115% and not 125% minimum or Max 175% with fusible disconnect per T430.52 which would make my service size massive.

Am I in the wrong for selecting the lower ampacity listed on the VFD (Heavy duty) vs Normal duty, and why is Normal duty so much higher of an input?

Also, My output currents are far lower than my inputs, but im assuming I need to use the input currents for my feeder/service size over output correct?

I dont work with VFD's much, so any input on this is super appreciated. THANKS!

 

[Sparky2791]

Here in PA the local POCO bases the transfomer size off of the loads provided, not off the ampacity of the service. With motors of that size the local POCO may dictate the need for a pad mount, maybe not, that is up to them. Before you go to crazy worrying about the service size perhaps it may not mater in the long run. Do you have to complete a load sheet when applying for service in Houston? Most POCO's I deal with you are required to list the specifics of any motor 50HP or greater including number of starts per hour (Estimated) and starter type. Also are these motors considered a continuous load? Need to add 25% to the total current for any continuous load in order to properly size the service. So that may push you over 800A anyway. Last per 430.130 (A) (1) you are required to use 430.6 for the circuit protection size. 430.6 requires you to use motor load values listed in Table 430.247, 430.248, 430.249 or 430.250, which ever table applies to your motor. This means you must use 477A for the 400A motor and 240A for the 200HP motor. That seems to throws everything off for you as well.

 

[Eddie702]

You have to size it based on the drive input rating and not the motor rating

 

[Kratky07]

You have to size it based on the drive input rating and not the motor rating

I did, the value I listed are the VFD drive input.

 

[Kratky07]

Here in PA the local POCO bases the transfomer size off of the loads provided, not off the ampacity of the service. With motors of that size the local POCO may dictate the need for a pad mount, maybe not, that is up to them. Before you go to crazy worrying about the service size perhaps it may not mater in the long run. Do you have to complete a load sheet when applying for service in Houston? Most POCO's I deal with you are required to list the specifics of any motor 50HP or greater including number of starts per hour (Estimated) and starter type. Also are these motors considered a continuous load? Need to add 25% to the total current for any continuous load in order to properly size the service. So that may push you over 800A anyway. Last per 430.130 (A) (1) you are required to use 430.6 for the circuit protection size. 430.6 requires you to use motor load values listed in Table 430.247, 430.248, 430.249 or 430.250, which ever table applies to your motor. This means you must use 477A for the 400A motor and 240A for the 200HP motor. That seems to throws everything off for you as well.

Thanks for the input, and you do bring up good Points. Reading back through 430.128, and 430.130 It does steer you back to T430.52, and also these motors would run for 3+ hours so 25% on top of my VFD input would kick me up to higher that 800A on my load calculation. Also, yes I have to submit load sheet, and one-line to the utility company. In 430 it states that I can use the VFD's input Ampacity over the Table values.

 

[jim dungar]

You use the drive input data for the source to the VFD, you use the NEC tables for the drive output to the motor.

Why is there a VFD feeding this motor? If it is to slow down the motor, maybe never letting it get to 100% speed at all, so it could thst your load is not continuous per the NEC definition.

You should confirm your VFD manufacturer's use of heavy and normal duty. Sometimes these refer to constant torque or variable torque loads.

 

[kwired]

Here in PA the local POCO bases the transfomer size off of the loads provided, not off the ampacity of the service.

That's generally how it goes most places.

Lets say the 200 HP motor is rated say 185 kVA and the 400 HP motor is 370 kVA. They will vary depending on actual efficiency and power factor but is likely somewhat close to what their full load rating is. That means they likely will want about a minimum of 750 kVA transformer unless there is good evidence these motors wouldn't normally operate at full load rating. Even if they went with a 500 kVA, most POCO's are going to want to do this with pad mount unit.

POCO's here usually won't put more than three 50 kVA pole mount type transformers on a single pole- that only lets them build a 150 kVA bank on the pole.

 

[ptonsparky]

You do not need to add an additional 25% for continuous load.

 

[ptonsparky]

My short and quick says 1063 for the higher amps and 819 for the lower.

 

[Kratky07]

You use the drive input data for the source to the VFD, you use the NEC tables for the drive output to the motor.

Why is there a VFD feeding this motor? If it is to slow down the motor, maybe never letting it get to 100% speed at all, so it could thst your load is not continuous per the NEC definition.

You should confirm your VFD manufacturer's use of heavy and normal duty. Sometimes these refer to constant torque or variable torque loads.

Good Point. I'm not sure, I am going to check with the engineers that I am designing this for.

 

[Jraef]

Normal and heavy duty ratings are about the overload capability of the VFD, because overloading affects the ability of the transistors to stay cool enough. "Normal Duty", aka "Variable Torque", is for centrifugal pumps and fans, because they physically cannot be overloaded without needing to shut down, so the VFD overload capability is lower. Everything that is not a CENTRIFUGAL pump or fan requires a "Heavy Duty" (a.k.a. Constant Torque) rated drive, which is generally the "next size up" in order to attain that added thermal capacity of the transistors. You did say "pumps", but what KIND of pumps are they? Positive Displacement Pumps like like gear pumps, screw pumps, progressive cavity pumps and and piston pumps are Heavy Duty loads. Turbine, axial and impeller pumps are Normal Duty.

The VFD, depending on age, will have a listed MAXIMUM over current protective device, either fuses or a circuit breaker of some type, and pay attention to the details, because if they do not give a circuit breaker size, then you CANNOT assume they are listed to be used with a circuit breaker, it might be fuses ONLY. It happens a lot more than you might think. You can go smaller than maximum on the OCPD size, but the smaller you get, the easier it is to get nuisance tripping. If the VFD is one designed and listed after 2020, it must now meet a new UL standard that dictates a MINIMUM size as well. So again, read the details.

The major mfr. that uses Normal and Heavy Duty instead of VT & CT for the ratings, is Rockwell. All of the Rockwell drives in this size are listed with breakers, but the newer PowerFlex 755T Series are listed under the new rules that have a minimum size rating of 125% of the MOTOR FLA for circuit breakers. But if you use the PowerFlex 753, which is older, you have no minimum size.