GEC size reduction allowed by NEC?

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The project I'm working on has a automatic door opener that will require 120V/1PH Life Safety power.
The nearest and most practical Life Safety power source is 480Y/277V.
The plan so far is to provide a small, 750VA step-down transformer near the source (using #12 primary and secondary conductors) just to serve the small, automatic door.
The intent is to provide a GEC from the secondary neutral to the nearest structural steel.

Are there any allowances in NEC that will will let the GEC from the secondary side of the transformer be smaller than #8 copper (250.66)? It seems odd to have a GEC several sizes larger than the phase conductors.

System bonding jumpers aren't required to be larger than the neutral conductor (250.30(A)(1)except. 2), so I was curious if something similar was allowed for small step-down transformers that aren't supplying Class 1, 2, or 3 circuits.
 

Dennis Alwon

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As I read 250.30(A)(5) it states the gec shall be sized with 250.66 for the derived ungrounded conductors. The smallest size is #8 and I don't know of any exceptions. Hang on perhaps someone here knows an exception somewhere.
 

infinity

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What differentiates this from a Class 1 transformer? Given the 750va size couldn't this be considered a Class 1 transformer? If so, then no GEC is even required.

(3) Grounding Electrode Conductor, Single Separately Derived System. A grounding electrode conductor for a single separately derived system shall be sized in accordance with 250.66 for the derived phase conductors and shall be used to connect the grounded conductor of the derived system to the grounding electrode as specified in 250.30(A)(7). This connection shall be made at the same point on the separately derived system where the system bonding jumper is connected.
Exception No. 3: A grounding electrode conductor shall not be required for a system that supplies a Class 1, Class 2, or Class 3 circuit and is derived from a transformer rated not more than 1000 volt-amperes, provided the grounded conductor is bonded to the transformer frame or enclosure by a jumper sized in accordance with 250.30(A)(1), Exception No. 3, and the transformer frame or enclosure is grounded by one of the means specified in 250.134.
 

don_resqcapt19

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What differentiates this from a Class 1 transformer? Given the 750va size couldn't this be considered a Class 1 transformer? If so, then no GEC is even required.
This is not a remote control or signaling circuit. If it is a Class 1 circuit, it would have to be a Class 1, power limited circuit and the voltage on that type of circuit is limited to 30 volts. 725.41(A).
 

jim dungar

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This is not a remote control or signaling circuit. If it is a Class 1 circuit, it would have to be a Class 1, power limited circuit and the voltage on that type of circuit is limited to 30 volts. 725.41(A).
If there is a 'remote' switch for this door operator or if it is tied into the fire alarm system, it might be considered 'Safety Control Equipment' in which case it would not be subject to the 30V power limited restrictions.
 

don_resqcapt19

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If there is a 'remote' switch for this door operator or if it is tied into the fire alarm system, it might be considered 'Safety Control Equipment' in which case it would not be subject to the 30V power limited restrictions.
I can't see how the power source that actually operates the door would be a remote control or signaling circuit.
 

kwired

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725.41:

>
>

(A)
Class 1 Power-Limited Circuits. These circuits shall be supplied from a source that has a rated output of not more than 30 volts and 1000 volt-amperes.

>
>
>

(B) Class 1 Remote-Control and Signaling Circuits. These circuits shall not exceed 600 volts. The power output of the source shall not be required to be limited.

Only power limited class 1 circuits are limited to 30 volts.

If it operates the door it may or may not be class 1. The motor on this is likely less than 1/10 hp.
 

hurk27

Senior Member
Take a look at 250.30(A)(3) Ex. #3

Close but I think 250.30(A)(8)(a) is a bit closer:
8) Grounded Conductor. Where a grounded conductor is
installed and the system bonding jumper connection is not
located at the source of the separately derived system,
250.30(A)(8)(a), (A)(8)(b), and (A)(8)(c) shall apply.
(a) Routing and Sizing. This conductor shall be routed
with the derived phase conductors and shall not be smaller
than the required grounding electrode conductor specified
in Table 250.66 but shall not be required to be larger than
the largest ungrounded derived phase conductor.
In addition,
for phase conductors larger than 1100 kcmil copper or
1750 kcmil aluminum, the grounded conductor shall not be
smaller than 121⁄2 percent of the area of the largest derived
phase conductor. The grounded conductor of a 3-phase,
3-wire delta system shall have an ampacity not less than
that of the ungrounded conductors.

Not even sure about the above but it makes no sense that the GEC would be required to be larger then the derived circuit conductors?
In small point of use SDS's I have never had to run a seperate GEC as long as the X0) was bonded to the supply EGC, maybe I have been doing it wrong? but never been flagged on it?
 
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iwire

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(B) Class 1 Remote-Control and Signaling Circuits. These circuits shall not exceed 600 volts. The power output of the source shall not be required to be limited.

Only power limited class 1 circuits are limited to 30 volts.

If it operates the door it may or may not be class 1. The motor on this is likely less than 1/10 hp.

IMO If the circuit provides the power that moves the door the circuit is not a Remote-Control or Signaling Circuit.
 

Smart $

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Close but I think 250.30(A)(8)(a) is a bit closer:


Not even sure about the above but it makes no sense that the GEC would be required to be larger then the derived circuit conductors?
The GEC isn't about just ground-fault current. What about dissipating lightning surge current?

In small point of use SDS's I have never had to run a seperate GEC as long as the X0) was bonded to the supply EGC, maybe I have been doing it wrong? but never been flagged on it?
Depends on size of your EGC/GEC... and the "continuous" requirement for GEC's. I suspect you have been doing it wrong. ;)
 

don_resqcapt19

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This is one of those cases where there is no real need for a grounding electrode, but the code requries one. There is no issue with lighting inside the building that an additional grounding electrode would help with and the only possible contact with a higher voltage system is the conductors that feed the primary side of the transfomer. The EGC is sized to clear a primary fault. I submitted a proposal that would have permitted the primary EGC to also be the secondary GEC, but CMP5 rejected it.
I don't see any technical reason for a GEC for any transformer that is installed in the same building as the primary feeder OCPD. Yes, a larger transformer may require local bonding to the building steel or metal water piping, but I just don't see an need for a grounding electrode system.
 

kwired

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This is one of those cases where there is no real need for a grounding electrode, but the code requries one. There is no issue with lighting inside the building that an additional grounding electrode would help with and the only possible contact with a higher voltage system is the conductors that feed the primary side of the transfomer. The EGC is sized to clear a primary fault. I submitted a proposal that would have permitted the primary EGC to also be the secondary GEC, but CMP5 rejected it.
I don't see any technical reason for a GEC for any transformer that is installed in the same building as the primary feeder OCPD. Yes, a larger transformer may require local bonding to the building steel or metal water piping, but I just don't see an need for a grounding electrode system.

Kind of always been my way of thinking. I have always used the same conductor for GEC of the secondary and as EGC for primary on small (5kVA and less especially) transformers. Majority of them I have installed are within 5 feet of the service equipment. Maybe only thing I have been doing wrong is not using 8 AWG minimum for the conductor size.
 

dkidd

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New for NEC 2011

250.121 Use of Equipment Grounding Conductors.
An equipment grounding conductor shall not be used as a
grounding electrode conductor.
 

kwired

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NE Nebraska
New for NEC 2011

250.121 Use of Equipment Grounding Conductors.
An equipment grounding conductor shall not be used as a
grounding electrode conductor.

What about use of grounding electrode conductor for equipment grounding?

In cases I mentioned before we typically had 277/480 volt service panel, and a 480-120 transformer to supply general purpose 120 volt receptacle right next to it. I can run GEC for the transformer secondary to the neutral bus in the 277/480 panel where the GEC for the service is terminated can't I? There is less than 5 feet of conductor involved here. Or do others feel I need a separate GEC and EGC to this transformer that will both tie to same point on each end and really serve no useful purpose to install the second conductor? Only thing I see that I may have done wrong in past on this install is the conductor that was run should have been 8 AWG minimum even though it is larger than the ungrounded conductors associated with it.
 

jim dungar

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IMO If the circuit provides the power that moves the door the circuit is not a Remote-Control or Signaling Circuit.
Yet, the 2008 NEC handbook exhibit 725.3 shows an oil burner motor being part of a Class 1 circuit. If these automatic doors are controlled as part of the fire system, I think the AHJ could apply 725.31.
 
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