NEC interpretation

[thinker]

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.

 

[waross]

For a drive isolation transformer you may want to check the transformer section of the code. You may find that transformer primaries also require a 125% factor for the conductors. However there may be exceptions which may or may not apply.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

It might help to understand the reasoning behind 430.122. It (and its predecessor in 2002) were enacted because people were installing drive that were larger than the motor, but sizing the conductors per the motor branch rules. Then later someone would see that the drive was larger and use a larger motor, without thinking through the supply conductor issue, because technically they were only responsible for the motor lead sizing, thinking of the VFD as almost a separately derived system, ergo not their problem. So by assuring that the conductors feeding the drive were sized for what the drive is designed for, it removed that possible conflict.

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

 

[thinker]

Jraef,
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.

 

[melspuds]

thinker,
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.

 

[thinker]

Melspuds,

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.

 

[melspuds]

If the AHJ has given their blessing, then so be it. A few years ago, the AHJ didn't buy it that way for my project. Everything is up to the local AHJ. You are correct about Art 430.6(C). That was the way I wanted to go, but it did not fly with the AHJ.
Dave

 

[wroggent]

The AHJ may have approved that practice, but it is not correct.

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.

 

[melspuds]

wroggent,
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

 

[thinker]

Disagree again. Art.430.22 refers to a single motor (generally, any motor). Art.430.6(C) gives specifics for the motor that is a part of adjustable voltage SYSTEM. AHJ was correct in clarifying this part of our [so interpretive] Code.

 

[thinker]

Gentlemen,

May I kindly note that the original posting asked for opinions on applicability of 125% factor to transformer secondary conductors?

 

[melspuds]

If you go by Art 430.6(C) it states, that if no nameplate or max Amps you go by table 430-250 and apply 150% for the size of the conductors. The motor circuit always starts at the feeder to the motor disconnect and the transformer is part of the motor circuit.

 

[thinker]

In my case, I do have a nameplate rated current, so the last sentence of 430.6(C) does not apply.

 

[djs]

If the DIT and the drive are enclosed in the same enclosure, then per code would be considered a single unit. The wiring inside the enclosure would then be covered by different standards possibly UL 508. If the DIT is a separate item, then the NEC would apply to all of the wiring to/from the DIT and VFD.

 

[melspuds]

The motor circuit always starts with the motor disconnect switch at 125% of FLA and downstream to all other devices in the motor load circuit. That includes DIT, starters, drives, other lockouts, and ends at the motor leads.
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.

 

[eeprom]

The 125% allows you to safely run a motor into the service factor, which can be as high as 1.25. If you sized the conductors at 100%, and then your motor died after repeated overloads, the next engineer might think he would be better off buying a replacement with a higher service factor. Then you would be overloading the cables.

You need 125% all the way from the breaker to the motor.

EE