Use of LFMC on roof for enphase AC branch connections?

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Hello folks,

I am a solar designer with a solar contractor, and we are getting electrical bids from a subcontractor for a small commercial ballasted array on a flat roof with enphase microinverters.

The electrician we are talking to wants to use LFMC to connect the AC branch lines from the junction boxes at the end of each row, to our AC combiner on the roof. I am usually speccing at least IMC for roof conduit.

These runs are about 6 feet at most between points of connection, it is not a big roof. I'm not really comfortable with LFMC, not used to seeing it used this way, but seems like it is ok per NEC if properly supported.

Opinons?

Thanks for your input.
 

Zee

Senior Member
Location
CA
LFMC may be OK per NEC literally, but rooftops are HARSH environs due to UV.
PVC degrades severely in sunlight and heat. I think this degradation may apply to the coating on the LFMC, but i cannot say i have seen it, as i usually see steel pipe (EMT) on a roof.
It certainly is true for sched 40 or 80, PVC conduit which i have seen shrink and tear out of glued connections, LBs, boxes etc. on roofs.,

IMC is overkill.
EMT is fine.
LFMC is underkill (so to speak).
 
Last edited:

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
It seems to me that the NEC permits LFMC.

In response to Zee's points about PVC:
The NEC defines LFMC as sunlight resistant, whereas it does not define PVC as such.
Moreover, it prohibits use of PVC in ambient temperatures above 50C, which effectively prohibits the use of PVC on roofs. (See also 310.15(B)(3)(c).) Whereas there is no such temperature limitation for LFMC.

With that said, I personally have never used anything for rooftop conduit except EMT. I agree that IMC is overkill.

One final thing: In case you are thinking that there are any restrictions on wiring methods for Enphase output circuits that don't apply to all AC circuits ... there aren't. This question could be asked in the general NEC forum as "LFMC on rooftop...or not?", without mentioning solar at all.
 

acrwc10

Master Code Professional
Location
CA
Occupation
Building inspector
It seems to me that the NEC permits LFMC.

In response to Zee's points about PVC:
The NEC defines LFMC as sunlight resistant, whereas it does not define PVC as such.
Moreover, it prohibits use of PVC in ambient temperatures above 50C, which effectively prohibits the use of PVC on roofs. (See also 310.15(B)(3)(c).) Whereas there is no such temperature limitation for LFMC.

With that said, I personally have never used anything for rooftop conduit except EMT. I agree that IMC is overkill.

One final thing: In case you are thinking that there are any restrictions on wiring methods for Enphase output circuits that don't apply to all AC circuits ... there aren't. This question could be asked in the general NEC forum as "LFMC on rooftop...or not?", without mentioning solar at all.

I disagree with this, 690.31(E) would not allow you to use NM on the output circuit until it landed on the first readily accessible disconnecting means. (if run inside the building).
 

PWDickerson

Senior Member
Location
Clinton, WA
Occupation
Solar Contractor
690.31(E) applies to DC conductors, not AC. The output circuit of an enphase micro-inverter can be run using NM-B cable inside a building.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
690.31(E) applies to DC conductors, not AC. The output circuit of an enphase micro-inverter can be run using NM-B cable inside a building.

This is correct. And it is precisely because this is commonly misunderstood that I made the comment above.

(The only complication with using NM for an inverter output circuit is that you will usually need an additional ground wire, because the NM ground is unlikely to be adequate for a solar GEC.)
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Does anybody have a link to a chart showing the average high and low design temperatures for around the country?

ASHRAE numbers are at http://www.solarabcs.org/about/publications/reports/expedited-permit/map/index.html

All time and monthly record high and low temps can be found at http://www.weather.com/

These numbers can differ by quite a bit and it's a judgement call which ones to use. I usually use the weather.com record low and highest average high, but I tend to the conservative side on such matters, especially with cold temps (high voltage) which can blow up an inverter.
 

acrwc10

Master Code Professional
Location
CA
Occupation
Building inspector
690.31(E) applies to DC conductors, not AC. The output circuit of an enphase micro-inverter can be run using NM-B cable inside a building.


How do you figure 690.31(E) doesn't apply to AC? It starts off by saying "Direct-current photovoltaic source AND output circuits inside a building" The word AND is the reason for the confusion, if the section was worded "output circuits and direct current photovoltaic source circuits inside a building" it would not be so misunderstood. When you look at the use of "output circuits" it is the output of an inverter and all most always AC.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
How do you figure 690.31(E) doesn't apply to AC? It starts off by saying "Direct-current photovoltaic source AND output circuits inside a building" The word AND is the reason for the confusion, if the section was worded "output circuits and direct current photovoltaic source circuits inside a building" it would not be so misunderstood. When you look at the use of "output circuits" it is the output of an inverter and all most always AC.
Yeah, it's confusing, but if you look at the definitions section of 690, you will see that "output circuit" refers to the DC output of a combiner box, which is the input to a string inverter. In the language you quoted the modifier "direct-current" applies to both source and output circuits and it's not relevant to microinverters.

We have talked about this in here before; it's one of those instances when the code could have been written more clearly.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
How do you figure 690.31(E) doesn't apply to AC? It starts off by saying "Direct-current photovoltaic source AND output circuits inside a building" The word AND is the reason for the confusion, if the section was worded "output circuits and direct current photovoltaic source circuits inside a building" it would not be so misunderstood. When you look at the use of "output circuits" it is the output of an inverter and all most always AC.

Look up the definition of a photovoltaic output circuit in the beginning of 690. A "photovoltaic output circuit" is a DC circuit. An "inverter output circuit" is an AC circuit. This is clearly defined. I do not agree with your comment about the use of 'output circuits'.

The sentence you quote above means "Direct-current photovoltaic source circuits and direct-current photovoltaic source output circuits." That's how parallelism works in grammar. If it were to mean what you think it does, it would have to read something like "Direct-current photovoltaic source circuits AND [inverter] output circuits." If it meant to apply to inverter output circuits (also defined in the beginning of 690), it would say so. Moreover, the words 'direct-current' were added precisely to clear up this confusion; if the section simply applied to all solar related circuits, then this extra language wouldn't be necessary.
 
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