Conductor sizing and bonding questions

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csoc64

Senior Member
Location
northeast
Installing 22 panel, roof mounted system using REC245 panels and Enphase 215 micro-inverters. System is utility interactive. AC combiner box will be inside barn that panels are mounted on. From there, I have approx 290' to interconnect point. After doing voltage drop calculation (based on 20A load), I'm looking at #1 AL wire. 1. Does the code allow me to undersize my neutral conductor for this application? Also, my understanding is that I have to proportionally up-size my egc (which would be #12 cu for 20A circuit), but I'm not quite sure how to calculate the new size. Finally, looking at 690.47(A). Do I need to install a new g.e. system at the array site? If so, how is that most efficiently accomplished. Hope these questions make some sense. I'm relatively new to the industry (as well as being newly licensed), so my head is swimming a little.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
The neutral cannot be smaller than the equipment grounding conductor and unfortunately for sizes 14-10 the equipment grounding conductor is directly proportional to the upsize ungrounded conductor. Thus if you need a #1 AL conductor for power then both your neutral and equipment grounding conductor will need to be sized at #1 AL
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
The neutral cannot be smaller than the equipment grounding conductor and unfortunately for sizes 14-10 the equipment grounding conductor is directly proportional to the upsize ungrounded conductor. Thus if you need a #1 AL conductor for power then both your neutral and equipment grounding conductor will need to be sized at #1 AL
Maybe this is saying the same thing, but my understanding is that first you size your ungrounded conductors for ampacity disregarding voltage drop and size your ground wire per 250.122. Then if you upsize your ungrounded conductors to reduce voltage drop you must upsize the ground wire by the same proportional amount. If the neutral wire is used only for voltage sensing it can be the same size as the ground wire. Is this not correct?
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
Maybe this is saying the same thing, but my understanding is that first you size your ungrounded conductors for ampacity disregarding voltage drop and size your ground wire per 250.122. Then if you upsize your ungrounded conductors to reduce voltage drop you must upsize the ground wire by the same proportional amount. If the neutral wire is used only for voltage sensing it can be the same size as the ground wire. Is this not correct?

Sounds the same to me. :D
 

SolarPro

Senior Member
Location
Austin, TX
See also 690.45(A):

"Increases in equipment-grounding conductor size to address voltage drop considerations shall not be required."

This supersedes 250.122(B) on the dc side of the system only.
 
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Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
See 690.45(A):

"Increases in equipment-grounding conductor size to address voltage drop considerations shall not be required."
That is assuming no overcurrent protective device is used. If there is an overcurrent protective device ahead of the run then I believe 250.122(B) applies.
 

csoc64

Senior Member
Location
northeast
Maybe this is saying the same thing, but my understanding is that first you size your ungrounded conductors for ampacity disregarding voltage drop and size your ground wire per 250.122. Then if you upsize your ungrounded conductors to reduce voltage drop you must upsize the ground wire by the same proportional amount. If the neutral wire is used only for voltage sensing it can be the same size as the ground wire. Is this not correct?

I was thinking along the same lines, but I am just now reading through 690.45 (A) which reads "Increases in equipment grounding conductor size to address voltage drop considerations shall not be required". If this is the case, am I correct in assuming that I am ok with a #12 egc for my 20A load? I am not sure what you mean by voltage sensing, so the neutral comment confuses me somewhat.
 

csoc64

Senior Member
Location
northeast
See also 690.45(A):

"Increases in equipment-grounding conductor size to address voltage drop considerations shall not be required."

This supersedes 250.122(B) on the dc side of the system only.

This would be all AC from the micro-inverters on the roof.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
I was thinking along the same lines, but I am just now reading through 690.45 (A) which reads "Increases in equipment grounding conductor size to address voltage drop considerations shall not be required". If this is the case, am I correct in assuming that I am ok with a #12 egc for my 20A load? I am not sure what you mean by voltage sensing, so the neutral comment confuses me somewhat.

My take on this is if there is overcurrent protective device ahead of the run then it is required otherwise not.
 

SolarPro

Senior Member
Location
Austin, TX
This would be all AC from the micro-inverters on the roof.

In that case, if the micro is transformer-isolated, then you need a dc GEC, which can't be smaller than #8 AWG. But the #8 wire can serve as both the dc CEG and the ac EGC per 690.47(C)(3).

(Sorry, just referred to the original post. Enphase is transformer-isolated. Technically, you need a #8 copper as described above.)
 
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SolarPro

Senior Member
Location
Austin, TX
Study 690.47(C)(3) as this is likely your best option. Note that the #8 combined dc GEC/ac EGC must be unspliced back to the ac grounding busbar that connects to the existing ac GEC. That may mean you have to crimp pigtails onto the conductor as it passes through other equipment—like a utility mandated ac disconnect or a revenue meter—between the array and the ac panel.
 

SolarPro

Senior Member
Location
Austin, TX
Here's an excerpt from an article on PV system grounding that we're about to send to the printer (it will hit the street in about a month):

Ken Gardner, the principal engineer at Gardner Engineering
Alternative Energy Services, explains his design
approach for Enphase microinverter systems: “We like to use
the SnapNrack mounting system because then we can use
WEEBs to bond the solar modules and microinverters. We
then run 6 AWG solid copper wire between WEEB bonding
lugs on the rails to a SolaDeck junction box, at which point
we transition to 8 AWG stranded wire using an irreversible
splice. The 8 AWG ground is required because this is the
minimum size conductor allowed for use as a dc GEC for the
microinverters per Section 250.166. The 8 AWG ground also
serves as the EGC and is therefore bonded to the rooftop
junction box. We install the 8 AWG ground with the current-
carrying conductors—typically 8 or 10 AWG for residential
systems—in metal flex, usually run to the main service panel
where we land the ground on the main grounding lug per
Section 690.47(C)(3). In theory, we could use Type MC cable,
but the grounding conductor inside is usually a size smaller
than the main current-carrying conductors and is too small
to serve as a GEC.”
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
Installing 22 panel, roof mounted system using REC245 panels and Enphase 215 micro-inverters. System is utility interactive. AC combiner box will be inside barn that panels are mounted on. From there, I have approx 290' to interconnect point. After doing voltage drop calculation (based on 20A load), I'm looking at #1 AL wire. 1. ... Also, my understanding is that I have to proportionally up-size my egc (which would be #12 cu for 20A circuit), but I'm not quite sure how to calculate the new size.

First of all, your feed and breaker for your AC combiner are required to be sized for 25A or more. (22*0.9*1.25=24.75). So you're looking at a minimum #10 Cu or #8 Al EGC in any case, from the combiner to the interconnect point.

The neutral cannot be smaller than the equipment grounding conductor and unfortunately for sizes 14-10 the equipment grounding conductor is directly proportional to the upsize ungrounded conductor. Thus if you need a #1 AL conductor for power then both your neutral and equipment grounding conductor will need to be sized at #1 AL

It seems to me that the breaker and the nominal amperage of the AC combiner circuit can be higher than required for the solar output. Say you fed it with a 60A breaker, and called it a 60A feed. Since this would require #4 AL conductors for ampacity, but only a #8 AL EGC, the upsized neutral and EGC could be a couple sizes smaller than #1. Maybe a #4, but I haven't done the math. I've also left derating out of this. But I think the point stands.
 
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