NEC interpretation
NEC interpretation
(OP)
Dear colleagues,
It is interesting to hear some opinions on the issue that seems to be confusing. Below is a quotation from NEC (2014 Edition):
430.122 Conductors — Minimum Size and Ampacity.
(A) Branch/Feeder Circuit Conductors. Circuit conductors
supplying power conversion equipment included as
part of an adjustable-speed drive system shall have an ampacity
not less than 125 percent of the rated input current to
the power conversion equipment.
Consider Adjustable Speed Drive (ASD) system that always includes stand alone Drive Isolation Transformer (DIT)in the scope of supply of the ASD system. We generally state that the DIT primary terminals is a "border" of the ASD system, customer has to provide adequately sized and protected feeder from the grid to DIT primary. I see the following interpretations of how to apply the above article:
1. 125% factor applies ONLY to the conductors from the grid to DIT primary (power conversion equipment is defined as inclusive the DIT)
2. 125% factor applies ONLY to conductors from the DIT secondary winding terminals to the drive cabinets (power conversion equipment includes the drive cabinet(s) itself.
3. 125% factor applies to BOTH conductor runs (grid - primary and secondary - drive cabinet)
What variant would you consider correct? This issue has significant economical implications for large ASD systems.
It is interesting to hear some opinions on the issue that seems to be confusing. Below is a quotation from NEC (2014 Edition):
430.122 Conductors — Minimum Size and Ampacity.
(A) Branch/Feeder Circuit Conductors. Circuit conductors
supplying power conversion equipment included as
part of an adjustable-speed drive system shall have an ampacity
not less than 125 percent of the rated input current to
the power conversion equipment.
Consider Adjustable Speed Drive (ASD) system that always includes stand alone Drive Isolation Transformer (DIT)in the scope of supply of the ASD system. We generally state that the DIT primary terminals is a "border" of the ASD system, customer has to provide adequately sized and protected feeder from the grid to DIT primary. I see the following interpretations of how to apply the above article:
1. 125% factor applies ONLY to the conductors from the grid to DIT primary (power conversion equipment is defined as inclusive the DIT)
2. 125% factor applies ONLY to conductors from the DIT secondary winding terminals to the drive cabinets (power conversion equipment includes the drive cabinet(s) itself.
3. 125% factor applies to BOTH conductor runs (grid - primary and secondary - drive cabinet)
What variant would you consider correct? This issue has significant economical implications for large ASD systems.





RE: NEC interpretation
Bill
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"Why not the best?"
Jimmy Carter
RE: NEC interpretation
You are approaching this in a similar manner; the DIT is separate so the rules don't apply. Think of this in the intent; the DIT is just an accessory to the VFD, it's all one system. Skimping on the conductors saves a little but risks a lot.
"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington
RE: NEC interpretation
It is not (and never has been) my intention to cut corners in wire sizing. In my opinion, there is nothing wrong if I size secondary DIT conductors based on 100% of the drive converter rated current, and 125% factor would apply only to primary DIT conductors. In this case I do not consider DIT as separate item, on the contrary, DIT is a functional part of converter that accepts feeding conductors.
RE: NEC interpretation
I believe that Art.430.21 gives the answer to your question. You size the conductors by your motor disconnect (either fused switch or circuit breaker) sized 125% FLA all the way to the motor. The isolation transformer is considered "by the code" as motor branch-circuit. Therefore #3 is your answer.
Hope this helps,
Dave
ps: The way you state the "grid" you can use the service disconnect as your motor disconnect if that is your case.
RE: NEC interpretation
I respectfully disagree with you about "all the way to the motor". The Article 430.6(C) allows for AC motors used in adjustable voltage drive systems, to establish ampacity of motor conductors based on motor nameplate current (or 100% of the rated current). We received a formal confirmation from Authority Having Jurisdiction (State Bureau of Construction Codes)that motor conductors in such application can be sized based on 100% of nameplate current, without 125% factor. We use this approach for many years in our projects, and never had any issues with acceptance of our systems.
RE: NEC interpretation
Dave
RE: NEC interpretation
430.6 tells you how many amps the motor draws
430.22 tells you what factor you need to multiply the motor amps (from 430.6) by to size the conductors
Basically, 430.6(C) allows you to use the name plate amps versus the table value in the back of article 430. 430.22 still applies.
RE: NEC interpretation
As I stated above it depends on the local AHJ only. This needs to reviewed and possible changes before the next NEC-2017. cycle is over.
Dave
RE: NEC interpretation
RE: NEC interpretation
May I kindly note that the original posting asked for opinions on applicability of 125% factor to transformer secondary conductors?
RE: NEC interpretation
RE: NEC interpretation
RE: NEC interpretation
RE: NEC interpretation
djs you are correct about the UL 508 when the DIT part of panel label.
If the DIT is a separate device than the 125% applies again from the drive to DIT in and out, and to the motor.
If the motor disconnect is a low voltage fused switch, it must be HP rated.
The motor circuit also includes at least one lockable open disconnect means in the open position.
Hope this helps,
Dave
All of the above is for a single motor installation only.
RE: NEC interpretation
You need 125% all the way from the breaker to the motor.
EE