Ungrounded system

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mwm1752

mwm1752

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When applying the provisions of 690.47( C ) (3) are both the AC system & DC system required to be ungrounded? Most structures are supplied by the utility with an AC grounded System, or are we just dealing with the inverter feeder to the AC point of attachment?
 
These resources may be of some assistance:

Grounding Compendium for PV Systems

Ungrounded PV Power Systems in the NEC

Also, this excerpt from our NEC 2014 code revisions article may help:

Section 690.47 ?Grounding Electrode System?

This section includes some relatively minor revisions for clarity, as well as one major addition that some Code experts have condemned as unsafe.

Ungrounded PV systems. In prior Code cycles, there was some confusion regarding grounding-electrode conductors for ungrounded systems. Section 690.47(B) now specifically allows use of an ac equipment-grounding system as the ground-fault?detection reference for ungrounded PV systems. In addition, Section 690.47(C)(3) states that a combined dc grounding-electrode conductor (GEC) and ac equipment-grounding conductor (EGC) can be installed for ungrounded systems. This combined dc GEC and ac EGC is sized in accordance with Section 250.122?as an EGC, in other words?and is not required to be larger than the largest ungrounded phase conductor. Note that this combined grounding conductor must still be unspliced or irreversibly spliced.

As an example, if a non-isolated inverter?s ac output circuit back-feeds a 30 A circuit breaker, the combined dc GEC and ac EGC between the inverter and the service panel can be sized per Table 250.122, which specifies 10 AWG copper based on the OCPD rating. Further, the combined dc GEC and ac EGC is never required to be larger than the ungrounded phase conductors of the PV system. This allows the combined grounding conductor to be sized smaller than the minimum dc GEC size of 8 AWG that Section 250.166 would otherwise require. When these sections are applied together, it is clear that non-isolated (transformerless) inverters used in ungrounded PV systems do not require a dc GEC sized per Section 250.166 to be run to a dc grounding electrode. Instead, an unspliced conductor acting as the ac EGC can be used to meet the grounding electrode system requirements for ungrounded PV systems.
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mwm1752 said:
When applying the provisions of 690.47( C ) (3) are both the AC system & DC system required to be ungrounded? Most structures are supplied by the utility with an AC grounded System, or are we just dealing with the inverter feeder to the AC point of attachment?
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Ungrounded PV means that the DC side is not referenced to ground, though the rules for EGC conductors are the same. You still have to ground all the exposed metal parts (racking, etc.).

The AC side can be either grounded or not (there may or may not be a neutral), depending on the inverter design, but that's not what the language is referring to. You still need full EGC on the AC side as well.
 
Unless the language very clearly states or implies otherwise (e.g. mentions 'AC' or inverter output), you should assume that most requirements and any general 'system' references in Article 690 are for the DC side. Keep in mind that PV systems covered in 690 may or may not even have an inverter (even though in the real world most systems do).
 
ggunn said:
Ungrounded PV means that the DC side is not referenced to ground, though the rules for EGC conductors are the same. You still have to ground all the exposed metal parts (racking, etc.).

The AC side can be either grounded or not (there may or may not be a neutral), depending on the inverter design, but that's not what the language is referring to. You still need full EGC on the AC side as well.
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My take is that the DC EGC can be sized per 250.122 " ). For ungrounded systems, this conductor shall be sized in accordance with 250.122 and shall not be required to be larger than the largest ungrounded phase conductor." as it is within 690.47. Inverters are being used listed for ungrounded systems, voltage between conductors is the same as voltage to ground in ungrounded systems, (NEC commentary-Where an ungrounded system is utilized, the voltage to ground (by definition) is the greatest voltage between the given conductor and any other conductor of the circuit. For example, the voltage to ground for a 480-V ungrounded delta system is 480 V.) does this also apply to DC systems. If so,690.47 ( C ) (3) appears to allow the DC EGC to be smaller than 8#. The grounding buss bar in the associated equipment seem to be located in the inverter.250.121 exception.
 
mwm1752 said:
My take is that the DC EGC can be sized per 250.122 " ). For ungrounded systems, this conductor shall be sized in accordance with 250.122 and shall not be required to be larger than the largest ungrounded phase conductor." as it is within 690.47. Inverters are being used listed for ungrounded systems, voltage between conductors is the same as voltage to ground in ungrounded systems, (NEC commentary-Where an ungrounded system is utilized, the voltage to ground (by definition) is the greatest voltage between the given conductor and any other conductor of the circuit. For example, the voltage to ground for a 480-V ungrounded delta system is 480 V.) does this also apply to DC systems. If so,690.47 ( C ) (3) appears to allow the DC EGC to be smaller than 8#. The grounding buss bar in the associated equipment seem to be located in the inverter.250.121 exception.
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The parts you are quoting are actually for the grounding electrode system. (Equipment grounding is in 690.43 thru .46. )

For ungrounded systems (i.e. no DC conductor grounded) you can indeed use 250.122 for the GEC under that section in the 2014 NEC, and thus smaller than #8 Cu up to a 60A circuit. I'm not exactly sure how to interpret 'phase conductor' in this context but I would think it means the AC conductors, since DC doesn't have 'phases'.

The voltage to ground is the same definition for DC systems, although I'm not sure how that's relevant to your question.
 
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