240.4

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Carultch

Senior Member
Location
Massachusetts
is there something in article 690 that i am missing that would allow 10 awg / 42 amps

Read 690.8, and understand the procedure of sizing conductors for PV System applications. The answer for the inverter output circuits should be #6 Cu.

If the #10 wires are for the DC strings, which is common, these are not "protected" by the 60A overcurrent device that is connected to the inverter output circuit. Also, in this example, there is not 42A present on these conductors (because the inverter converts power format and trades voltage and current from input to output). Instead, these are conductors that would likely either have OCPD inside the inverter, or not require OCPD at all. Because the fault current is limited to only what the module Isc values produce, a single string doesn't require OCPD. And there is also a special exception for two strings in parallel that they can be "bare tapped" together.

For 42A of maximum continuous current (as you'll calculate or select in 690.8(A)), this means you'll need 60A worth of overcurrent protection device. 42A*125% = 52.5A.

For 690.8(B), there are two conditions that need to be met for the wire to be properly sized.

1. The max cont current * 125% must not exceed the ampacity of the termination conditions on both sides of the wire. This is usually the 75C column of 310.15(B)(16), but for 100A and less, you do need to check that terminations are listed and marked for 75C. See 110.14(C) for more detail.

2. The max cont current * 100% must not exceed the ampacity of the conductor at conditions of use. This is usually the 90C column of 310.15(B)(16), and then there are other factors discussed in Article 310 that come in to play. Usually temperature corrections and ampacity adjustments for bundling. These used to be called derate factors, and you will still see this term.

In 2011, there was a third condition. The OCPD where required, shall protect the wire as sized. All parts of Article 240 shall apply for conductor protection. This rule still exists in 2014, and just isn't explicitly stated.
 
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david

Senior Member
Location
Pennsylvania
Read 690.8, and understand the procedure of sizing conductors for PV System applications. The answer should be #6 Cu.


For 42A of maximum continuous current (as you'll calculate or select in 690.8(A)), this means you'll need 60A worth of overcurrent protection device. 42A*125% = 52.5A.

For 690.8(B), there are two conditions that need to be met for the wire to be properly sized.

1. The max cont current * 125% must not exceed the ampacity of the termination conditions on both sides of the wire. This is usually the 75C column of 310.15(B)(16), but for 100A and less, you do need to check that terminations are listed and marked for 75C. See 110.14(C) for more detail.

2. The max cont current * 100% must not exceed the ampacity of the conductor at conditions of use. This is usually the 90C column of 310.15(B)(16), and then there are other factors discussed in Article 310 that come in to play. Usually temperature corrections and ampacity adjustments for bundling. These used to be called derate factors, and you will still see this term.

In 2011, there was a third condition. The OCPD where required, shall protect the wire as sized. All parts of Article 240 shall apply for conductor protection. This rule still exists in 2014, and just isn't explicitly stated.

I'm doing the plan review the installer went to my boss and said he is an expert and i don't know what ia am talking about. The installer has installed really over a hundred this way .I need to be care full that i don't miss something.
Im hoping to learn trough this process
 

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Carultch

Senior Member
Location
Massachusetts
I'm doing the plan review the installer went to my boss and said he is an expert and i don't know what ia am talking about. The installer has installed really over a hundred this way .I need to be care full that i don't miss something.
Im hoping to learn trough this process

In the case of SolarEdge optimizer strings, your 690.8(A) circuit current (maximum continuous current) is 15A per series optimizer string. These don't work the way that direct module strings do. The DC-to-DC converter does something with the module's mix of voltage and current, and is ultimately current limited to 15A. Even under fault conditions, they are limited to 15A by the remaining units in series.

With direct module strings, we have two 125% factors that are prescribed by the NEC, and for different purposes. Obviously, the two of them together multiply to 156%. I think they included that informational note, so one wouldn't make the mistake thinking that 125% * 125% = 150%. The first factor accounts for the fact that the sunlight may be greater than our "nice round number" of 1000W/m^2, and reflective backgrounds can enhance the performance of the modules. More irradiance means more current. So we're preparing for potentially 1250 W/m^2 to be incident on the modules, when sizing the corresponding circuit. The second factor is the continuous load factor, and is consistent with what the NEC prescribes for all other continuous load applications.

For optimizer strings, the enhancement factor doesn't apply. Even if more than "one standard sun" is incident, the DC/DC converter will limit its output by its operation. SolarEdge optimizers are limited to 15A. The continuous load factor still does apply, so be prepared to multiply the 15A by 125%, but only once.
 
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david

Senior Member
Location
Pennsylvania
In the case of SolarEdge optimizer strings, your 690.8(A) circuit current (maximum continuous current) is 15A per series optimizer string. These don't work the way that direct module strings do. The DC-to-DC converter does something with the module's mix of voltage and current, and is ultimately current limited to 15A. Even under fault conditions, they are limited to 15A by the remaining units in series.

With direct module strings, we have two 125% factors that are prescribed by the NEC, and for different purposes. Obviously, the two of them together multiply to 156%. I think they included that informational note, so one wouldn't make the mistake thinking that 125% * 125% = 150%. The first factor accounts for the fact that the sunlight may be greater than our "nice round number" of 1000W/m^2, and reflective backgrounds can enhance the performance of the modules. More irradiance means more current. So we're preparing for potentially 1250 W/m^2 to be incident on the modules, when sizing the corresponding circuit. The second factor is the continuous load factor, and is consistent with what the NEC prescribes for all other continuous load applications.

For optimizer strings, the enhancement factor doesn't apply. Even if more than "one standard sun" is incident, the DC/DC converter will limit its output by its operation. SolarEdge optimizers are limited to 15A. The continuous load factor still does apply, so be prepared to multiply the 15A by 125%, but only once.

ok i need to rethink this from what you said. there are 2ea. (36) 280 Watt panels 2 strings of 18 panels/
I thank you and i need to look this over before i make any further comments
 

Carultch

Senior Member
Location
Massachusetts
ok i need to rethink this from what you said. there are 2ea. (36) 280 Watt panels 2 strings of 18 panels/
I thank you and i need to look this over before i make any further comments

Here's what you need to think about:
How are they routed?
Are they in direct sunlight?
What is the ASHRAE ambient min/max temperature for the site?
Are the strings spliced together in the array field, or not parallelled until inside the inverter?
Are they bundled together, or travelling in separate wiring structures?
Are they bundled with any unrelated current-carrying conductors? (unlikely, but it does happen)
What is the estimated length? Your final governing factor is voltage drop calculated from this, after you've thought about all the local issues that may apply.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
It's a rare case that 10awg THWN-2 is not sufficient for uncombined PV source circuits, and also Solaredge optimizer outputs, at least on residential systems. Unless more than 4 strings are run in the same raceway (less with Solaredge), and/or the ambient temperatures are extremely high (e.g. Death Valley), the math just won't disqualify that conductor.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Here's what you need to think about:
What is the ASHRAE ambient min/max temperature for the site?
The ASHRAE minimum temperature is not a factor in SolarEdge and microinverter installations because the modules are not directly connected in series.
 

Carultch

Senior Member
Location
Massachusetts
The ASHRAE minimum temperature is not a factor in SolarEdge and microinverter installations because the modules are not directly connected in series.

Correct. It is a factor for an individual optimizer input connection, albeit rare that it would matter, since they have a wide input voltage range.
 

Zee

Senior Member
Location
CA
Wasn't the question but it hurt my eyeballs:
Plans show: "10AWG USE-2 from panel to inverter" ???

One sentence three problems.
1. USE-2 is not allowable. PV WIRE is required for DC source circuit conductors on an ungrounded system. (Solaredge is always ungrounded.)

2. Also, technically the conductors are not from panels to inverter.....but from optimizers to inverter.

3. "Panel" is very misleading...sub-panel, Main Service Panel, or.....solar panels - plural???

Using USE-2 today in any case is a cheap move. But this last one is just IMHO, not CODE.:roll:
 
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jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
3. "Panel" is very misleading...sub-panel, Main Service Panel, or.....solar panels - plural???

For a long time I gave in to the common usage, but relatively recently (a few months ago) I decided to start purging the word 'panel' from all plans and communications where it actually refered to solar modules. And you know what? So far no one has tripped over it.

Using USE-2 today in any case is a cheap move. But this last one is just IMHO, not CODE.:roll:

FWIW, in the 2017 NEC USE-2 will be allowable again, if I read the revisions correctly.
 

Zee

Senior Member
Location
CA
Hmmm, good to know about coming USE-2 acceptance.

I agree, using the term "panels" to refer to modules is totally fine by me...I don't want to be some stuck up solar pro guy. :thumbsup:
Kind of like tossing around the acronym "PV" with customers...most lay people don't know what the hell that is.

My only issue is with the singular form, "panel". On a plan-set, if you say "from panel to inverter" it genuinely can be really confusing. Matter of fact, I think it was what confused the OP to think the 42 AMP circuit was #10!

At least make it plural! "from panels to inverter." And if you feel really generous, add the adjective, "solar".:p
 
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