Service Vs Disconnect

[larryjoma]

Hello All,

I have a panel with a 7 1/2hp motor and electric drive. Along with other peripherals like a power supply and relays. The total calculated FLA for the panel is 42amps. In the past, this panel is being built with a 60A disconnect. Recently, it has been brought up that the Electric Drive is fused at 90A, according to manufacturer, therefore the disconnect has to be atleast 115A.

I changed it, now the customer is saying he has to put in larger service since the panel is above 60A. Is the customer correct? If he only needs 42amps to run the panel, why should he put more than the 60a service he has currently? Is there code on this? Why would less service be bad?

Thanks, Larry

 

[kwired]

Hello All,

I have a panel with a 7 1/2hp motor and electric drive. Along with other peripherals like a power supply and relays. The total calculated FLA for the panel is 42amps. In the past, this panel is being built with a 60A disconnect. Recently, it has been brought up that the Electric Drive is fused at 90A, according to manufacturer, therefore the disconnect has to be atleast 115A.

I changed it, now the customer is saying he has to put in larger service since the panel is above 60A. Is the customer correct? If he only needs 42amps to run the panel, why should he put more than the 60a service he has currently? Is there code on this? Why would less service be bad?

Thanks, Larry

Please clarify what this "electric drive" is. Is it a VFD for the 7.5 hp motor?

 

[charlie b]

I also need some help understanding your terms.

• When you say, "service" are you saying that the panel is fed directly from the utility company. If so, where does the utility meter come into this discussion? If not, then what is providing power to the panel?
• When you say "panel," are you talking about a circuit breaker panelboard, or are you talking about an enclosure, inside of which there is a disconnect switch, an "electric drive" (whatever that is), and those other components?
• Is the disconnect to which you refer fused or unfused? If fused, then please explain how the electric drive is fused separately from the fuse associated with the disconnect.
• Is the disconnect to which you refer included within the enclosure? If so, then it should come with the right size disconnect from the manufacturer. Why is it not?

I have other questions, but let's start there.

 

[larryjoma]

Please clarify what this "electric drive" is. Is it a VFD for the 7.5 hp motor?

Yes. Sorry, it is a VFD, yaskawa V1000 series

 

[larryjoma]

I also need some help understanding your terms.

• When you say, "service" are you saying that the panel is fed directly from the utility company. If so, where does the utility meter come into this discussion? If not, then what is providing power to the panel?
• When you say "panel," are you talking about a circuit breaker panelboard, or are you talking about an enclosure, inside of which there is a disconnect switch, an "electric drive" (whatever that is), and those other components?
• Is the disconnect to which you refer fused or unfused? If fused, then please explain how the electric drive is fused separately from the fuse associated with the disconnect.
• Is the disconnect to which you refer included within the enclosure? If so, then it should come with the right size disconnect from the manufacturer. Why is it not?

I have other questions, but let's start there.

1. Service. Service "drop" of 60A from the building to the main enclosure (panel)
2. Panel - is the main enclosure which houses the VFD (electric Drive), power supply, disconnect, etc.
3. 60A Fused disconnect. The disconnect is then ran to a distribution block. The Distribution block is ran to the VFD, and 120vac transformer. Each of those components have separate Line fusing.
4. Yes. The disconnect is inside the main enclosure. We build the main enclosure in house, and has been built the same way the last 5-10 years.

 

[charlie b]

OK, thanks for the information. But let me split a hair or two. If power comes to this equipment from inside a building, and that building gets its power from the utility, then you are not dealing with a "service." You are dealing with a "feeder." It's not just semantics. The rules are different if you connect directly to a utility or if you connect to a panel inside a building.

For my part, I strongly suspect that you don't need to upsize your disconnect to 115 amps. And even if you did, the power coming into your equipment need not be rated higher than the load requires.

But let's get deeper into the details.

1. What voltage does the motor run at?
2. If it is 208V, then the full load current of the motor would be less than 25 amps. That leaves 17 amps (out of your calculated 42) for other stuff. Even if you are including an extra 25% in your load calculation for the "largest motor" factor, that still accounts for only 6 of the 17. Control circuits and relays don't use much current. So where are the other amps coming from?
3. IF the motor runs at 480V, the change the 25 above to 11, and the disparity becomes worse.
4. Was it the motor manufacturer or the VFD manufacturer that changed the fuse from 60 to 90? Or are they the same manufacturer?
5. Did the manufacturer specify a Minimum Circuit Ampacity of 90 amps, or did they merely change the fuse size?

 

[larryjoma]

But let's get deeper into the details.

1. What voltage does the motor run at?
2. If it is 208V, then the full load current of the motor would be less than 25 amps. That leaves 17 amps (out of your calculated 42) for other stuff. Even if you are including an extra 25% in your load calculation for the "largest motor" factor, that still accounts for only 6 of the 17. Control circuits and relays don't use much current. So where are the other amps coming from?
3. IF the motor runs at 480V, the change the 25 above to 11, and the disparity becomes worse.
4. Was it the motor manufacturer or the VFD manufacturer that changed the fuse from 60 to 90? Or are they the same manufacturer?
5. Did the manufacturer specify a Minimum Circuit Ampacity of 90 amps, or did they merely change the fuse size?

1. 460vac
2. There is another couple of VFDs in the main enclosure. They are smaller (5hp) they were fused correctly.
3-5. Yaskawa did not change the fusing. It appears we have been fusing it wrong. I have only been at this current company 10 months and this was brought to my attention.

I only mentioned the total FLA of 42 to distinguish that 42A is only 72% of 60A. I had read that if your FLA is 80% of your current disconnect you must take it up a size. I want to tell the customer that his 60A Feeder is sufficient but I cant find a code that states something along the lines "If you disconnect is fused at 100A, you dont have to run 100A feeder to it. Anything smaller is ok."

 

[Jraef]

Yaskawa did not change the fusing. It appears we have been fusing it wrong. I have only been at this current company 10 months and this was brought to my attention.

I don't understand why a total load of 42A now suddenly requires 90A fusing. You said "It appears we have been fusing it wrong", that is where I think the bigger issue is. Where does that come from? Details of ALL of the devices might help, but you said there were 2 x 5HP VFDs and a small CPT, so assuming those loads totaled 17A out of 42A total, I would guess your motor is 25A so probably 20HP at worst. A 20HP V1000 VFD, assuming a Heavy Duty rating (38A) will need a MAXIMUM fuse rating of 90A, but that does NOT mean a 60A fuse will not work. The NEC will call for the CONDUCTORS feeding that VFD to be 125% of the VFD max amps, so the WIRE needs to be sized for no LESS than 47A, but that still does not mean anything with regard to the OCPD. If you were not blowing the 60A fuses in the past, I see no reason to change them to 90A now.

 

[kwired]

The confusing part is the 90 amp fuses mentioned. Doesn't seem to be enough load to justify the need for that much supply capacity. Before more details were brought up I thought possibly a 90 amp for a single phase 240 volt supply - that would be getting closer to what would be needed if we were using that to convert to three phase output with the drive, but still would probably work on a 50 or 60 amp fuse in almost all cases involving a VFD. Across the line starting may need 90 amp in a phase conversion situation though.

Otherwise some basic calculations with a 7.5 and two 5 hp 480 volt three phase motors - you should only need about 29 amp conductors (maybe you have more motors then I mentioned?) and overcurrent protection for a common feed could be as little as 35 amps and you would have little starting issues in most instances, even with across the line starting.

 

[Smart $]

1. Service. Service "drop" of 60A from the building to the main enclosure (panel)
2. Panel - is the main enclosure which houses the VFD (electric Drive), power supply, disconnect, etc.
3. 60A Fused disconnect. The disconnect is then ran to a distribution block. The Distribution block is ran to the VFD, and 120vac transformer. Each of those components have separate Line fusing.
4. Yes. The disconnect is inside the main enclosure. We build the main enclosure in house, and has been built the same way the last 5-10 years.

1. Service drops do not come from a building. Service drops come from utility poles. Once it reaches a bulding, they become service entrance conductors, but still technically the "service".

A service disconnect... actually all service equipment has to be listed [230.66] as service equipment or suitable for use as service equipment.

Where's the meter?

What model VFD?

...
I only mentioned the total FLA of 42 to distinguish that 42A is only 72% of 60A. I had read that if your FLA is 80% of your current disconnect you must take it up a size. ...

Depends on what you mean by FLA when discussing several motors and other non-motor loads. What you want specifically is an Article 220 service (or feeder) calculation. That'll usually be 125% of the largest motor FLA, plus 100% of the other motors' FLA, plus the other loads. Demand factors may apply to the other loads, and then to determine ratings you must factor the continuous load portion of the other loads at 125%.

 

[Smart $]

I don't understand why a total load of 42A now suddenly requires 90A fusing. ...

The confusing part is the 90 amp fuses mentioned. ...

The VFD supply fuse is sized to the motor it drives, not the VFD input rating.... and cannot exceed the maximum permitted for the motor.

If this is a 460V 3Ø 7.5HP motor, the FLA per Table 430.250 is 11A. That's means a typical maximum breaker of 250%... so 27.5A... next higher standard OCP is 30A.

 

[kwired]

The VFD supply fuse is sized to the motor it drives, not the VFD input rating.... and cannot exceed the maximum permitted for the motor.

If this is a 460V 3Ø 7.5HP motor, the FLA per Table 430.250 is 11A. That's means a typical maximum breaker of 250%... so 27.5A... next higher standard OCP is 30A.

In my example with a drive with single phase in/three phase out if you sized the overcurrent device to the motor you very possibly trip that device if it is carrying same kVA but only as single phase. Such drive would either need specifically designed as single phase in three phase out or derated if it is a three phase drive only supplied with single phase.

I'm pretty certain you size input protection to the drive to drive specifications, the drive's internal overload components and software protects the motor.

 

[Aleman]

Sounds like you have an oversized drive? If the 60 works fine then stick with the 60 amp. And keep your drive protection where it was.
I can't see protecting a drive at 8X FLA of the motor. And also don't put the drive on a 90 amp breaker if the 90 is higher than the protection
for the panel.

 

[Smart $]

...
I'm pretty certain you size input protection to the drive to drive specifications, the drive's internal overload components and software protects the motor.

Completely revised in 2014. Here's what 430.130 and 131 say...

430.130 Branch-Circuit Short-Circuit and Ground-
Fault Protection for Single Motor Circuits Containing
Power Conversion Equipment.

(A) Circuits Containing Power Conversion Equipment.
Circuits containing power conversion equipment
shall be protected by a branch-circuit short-circuit and ground-fault
protective device in accordance with the following:

(1) The rating and type of protection shall be determined
by 430.52(C)(1), (C)(3), (C)(5), or (C)(6), using the full-load current rating of the motor load as
determined by 430.6.

(2) Where maximum branch-circuit short-circuit and ground-fault protective ratings are
stipulated for spe- cific device types in the manufacturer’s instructions for the power conversion
equipment or are otherwise marked on the equipment, they shall not be exceeded even if higher
values are permitted by 430.130(A)(1).

(3) A self-protected combination controller shall only be permitted where specifically identified
in the manufac- turer’s instructions for the power conversion equipment or if otherwise marked on
the equipment.


Informational Note: The type of protective device, its rat- ing, and its setting are often marked
on or provided with the power conversion equipment.


(B) Bypass Circuit/Device. Branch-circuit short-circuit and ground-fault protection shall also
be provided for a bypass circuit/device(s). Where a single branch-circuit short-circuit and
ground-fault protective device is provided for circuits containing both power conversion equipment
and a bypass circuit, the branch-circuit protective device type and its rating or setting shall be
in accordance with those determined for the power conversion equipment and for the bypass
circuit/device(s) equipment.


430.131 Several Motors or Loads on One Branch Cir-
cuit Including Power Conversion Equipment. For
instal- lations meeting all the requirements of 430.53 that include one or more power converters,
the branch-circuit short- circuit and ground-fault protective fuses or inverse time circuit
breakers shall be of a type and rating or setting permitted for use with the power conversion
equipment using the full-load current rating of the connected motor load in accordance with 430.53.
For the purposes of 430.53 and 430.131, power conversion equipment shall be consid-
ered to be a motor controller.

 

[kwired]

Completely revised in 2014. Here's what 430.130 and 131 say...

I was not aware of that change, a change that does make sense though, I thought it was wrong before when you had an oversized drive that you needed to supply the thing with a larger circuit then the thing would ever draw. Not so bad when drive is only slightly larger then needed but I once had a 250 HP drive running a 50 HP motor - just because the drive was available. I was not going to run a 400 amp circuit when the 100 amp circuit already feeding the motor was never going to be overloaded.

 

[Smart $]

I was not aware of that change.

430.130 and 430.131 are completely new in 2014. In 2011 it was essentially supposed to be what the new sections require, but not explicitly stated.