conductor sizing and OCPD

Status
Not open for further replies.

ocoee

Member
Location
Golden Co
I know this might seem basic. I do need some clarification I would also like if at all possible to keep this simple.

Assume no Derates here, As an example PV source circuit 6.46 ISC x 22 strings in parallel 142.12 ISC x 125% = 177.65 (690.8 A1) conductors must be able to carry this current. So this would imply 3/0 Cu at 75d for 200 amps.

Next OCPD 690-8b1 require OCPD to be 125% of of the above 177.65 so 177.65 x125%= 222.0 OCPD must be rated to carry this number ...therefore a 225 amp breaker or fuse.

So do I now increase the size of the 3/0 conductor to 4/0 copper to protect the conductor from the possibility of current flow exceeding the value of the OCPD

Or do I simply have wire sized at 125% of ISC and A breaker sized 125% of of this calculated value?

Thank you
 

ocoee

Member
Location
Golden Co
Clarification

Clarification

Sorry, the questions pertains to OCPD for PV Output circuits and their conductor sizing and not that of the PV source circuits.
 

PWDickerson

Senior Member
Location
Clinton, WA
Occupation
Solar Contractor
"Assume no Derates here, As an example PV source circuit 6.46 ISC x 22 strings in parallel 142.12 ISC x 125% = 177.65 (690.8 A1) conductors must be able to carry this current. So this would imply 3/0 Cu at 75d for 200 amps."

This is incorrect. The DC conductors of a PV system need to be sized at 1.56 * Isc [1.25 for 690.8(A)(1) and 1.25 again for 690.8(B)(1)] unless conditions of use corrections require a larger size. This is a minimum of 222A for the circuits you described. Assuming 75C terminals, you would need 4/0 CU conductors and a 225A OCPD.
 

ocoee

Member
Location
Golden Co
ARRRRG!

ARRRRG!

Talking about source circuits only here
Ok so.. 690.8A1 pv source circuits "the maximum current shall be the sum of parallel module rated short circuit currents multiplied by 125%"

Maximum current therefore = 6.46 x 1.25 = 8.07 amps

Now 690.8B1a refers to OCPD only correct? The OCPD must be sized "to carry not less than 125% of the maximum currents calculated in 690.8A"

So now I have maximum current of 8.07 as calculated in 690.8a and a OCPD required to carry 10.09A as calculated in 690.8b1 and the next standard size up 15 A... correct?

So right now thus far my understanding is we have 690.8a1 that refers to maximum current and 690.8b1 which refers to OCPD.

I am now trying to make the leap for conductor sizing at 1.56 as referenced in the (informational note)

690.8b In bold Ampacity and over current device ratings "photovoltaic system currents shall be considered to be continuous"
Does this mean to say that the maximum current as calculated by 690.8a needs to be again multiplied by 125% so really my maximum current is 6.46 x 1.25 x 1.25 for a I max of 10.09? Is this what the informational note is telling me? "where the requirements of both are applied" If they are referring to the above 690.8A Maximum current calculation and not additionally the OCPD in what instance are they not both applied?

Now, additionally if this is correct my than 690.8b1 requires my OCPD calculation to be 10.09 x 125% = 12.61 Amps
and a 15 amp OCPD.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Talking about source circuits only here
Ok so.. 690.8A1 pv source circuits "the maximum current shall be the sum of parallel module rated short circuit currents multiplied by 125%"

Maximum current therefore = 6.46 x 1.25 = 8.07 amps

Now 690.8B1a refers to OCPD only correct? The OCPD must be sized "to carry not less than 125% of the maximum currents calculated in 690.8A"

So now I have maximum current of 8.07 as calculated in 690.8a and a OCPD required to carry 10.09A as calculated in 690.8b1 and the next standard size up 15 A... correct?

So right now thus far my understanding is we have 690.8a1 that refers to maximum current and 690.8b1 which refers to OCPD.

I am now trying to make the leap for conductor sizing at 1.56 as referenced in the (informational note)

690.8b In bold Ampacity and over current device ratings "photovoltaic system currents shall be considered to be continuous"
Does this mean to say that the maximum current as calculated by 690.8a needs to be again multiplied by 125% so really my maximum current is 6.46 x 1.25 x 1.25 for a I max of 10.09? Is this what the informational note is telling me? "where the requirements of both are applied" If they are referring to the above 690.8A Maximum current calculation and not additionally the OCPD in what instance are they not both applied?

Now, additionally if this is correct my than 690.8b1 requires my OCPD calculation to be 10.09 x 125% = 12.61 Amps
and a 15 amp OCPD.

Your DC conductors must be sized to carry 1.25 X 1.25 X Isc. The first 1.25X is to accommodate insolation of more than 1000W/m2 (it happens frequently here in Texas) and the second 1.25X is for continuous operation. Your DC OCPD is the next standard size up from that number irrespective of wire size (which can vary because of temperature and conduit fill derates and voltage drop considerations). The calculation is the same whether you are talking about individual strings (source circuits) or combined strings (PV output circuits).
 
Last edited:

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
Let's also note that it matters what code cycle you're on. 2008 and before is simply greater than 1.56 of nameplate Isc. 2011 adds the more complicated requirement of either 1.56 or 1.25 after conditions of use. The latter is for conductors and NOT OCPD, which is still 1.56.

In 2011, 690.8(B)(1)(d) references 240.4(B), which allows the OCPD to be the next standard size up higher than the conductor rating, when below 800A.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Let's also note that it matters what code cycle you're on. 2008 and before is simply greater than 1.56 of nameplate Isc. 2011 adds the more complicated requirement of either (1.56) or (1.25 after conditions of use).
Whichever is the larger size wire, of course. Parentheses added for clarity.
 

BillK-AZ

Senior Member
Location
Mesa Arizona
Points not mentioned in this thread:

1. The PV module series OCPD, when required (due to parallel strings of series modules), can not exceed the "Series fuse" listed on the module label.

2. The conductor size is determined by the OCPD when a OCPD is required, and must protect the cable.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Points not mentioned in this thread:


2. The conductor size is determined by the OCPD when a OCPD is required, and must protect the cable.

I'm pretty sure that the OCPD is allowed to be the next size up from the ampacity of the conductor if it's less than 800A.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
2. The conductor size is determined by the OCPD when a OCPD is required, and must protect the cable.


I believe you're intending (in part) to paraphrase 690.8(B)(2)(c). My personal opinion is that that section is so vague as to be essentially meaningless; I've looked at it a few times and found it baffling. If it was intended to mean that the conductor ampacity must exceed the OCPD rating, then it should have said exactly that. Otherwise it adds nothing substantive; the rest of 690.8(B) sufficiently lays out the requirements. Perhaps it is simply meant as a reminder to look at the rules for OCPDs (namely Art 240). In that case it is rather perfunctory, and not strictly necessary, or else it could have been an informational note instead.

I do not agree that the conductor size is determined by the OCPD under the 2011 code. The conductor size may be required to be higher if the conditions of use derating is less than 0.8. But this requirement does not extend to the OCPD, which is simply required to be greater than 1.56xIsc.

I also agree with ggunn's reply.
 

Smart $

Esteemed Member
Location
Ohio
I believe you're intending (in part) to paraphrase 690.8(B)(2)(c). My personal opinion is that that section is so vague as to be essentially meaningless; I've looked at it a few times and found it baffling. If it was intended to mean that the conductor ampacity must exceed the OCPD rating, then it should have said exactly that. Otherwise it adds nothing substantive; the rest of 690.8(B) sufficiently lays out the requirements. Perhaps it is simply meant as a reminder to look at the rules for OCPDs (namely Art 240). In that case it is rather perfunctory, and not strictly necessary, or else it could have been an informational note instead.

...
690.8(B)(2)(c) applies in conjunction with 690.8(B)(1)(d). For OCPD's of 800A and less rating, the minimum conductor ampacity considered protected is equivalent to next lower standard OCPD rating + 1A, which is 240.4(B) permission applied in reverse.
 

Andrew445

Inactive, Email Never Verified
690.8(B)(2)(c) applies in conjunction with 690.8(B)(1)(d). For OCPD's of 800A and less rating, the minimum conductor ampacity considered protected is equivalent to next lower standard OCPD rating + 1A, which is 240.4(B) permission applied in reverse.

I think I'm understanding this part correctly. My method has typically been to first determine the conductor size for the larger of (A) or (B). And then after that, apply the derates to the ampacity of that conductor size and ensure that the next higher standard OCPD device rating is at LEAST that which was determined earlier with the 1.56*Isc. If the derated ampacity is, for example, 288A and the OCPD is sized at 350A, I increase the cable size until the derated ampacity is greater than 300A in order to ensure that it is still protected by the required OCPD size.


Assuming this is all correct, I have a question in regards to temperature ratings. The cable being used is 90? but the inverter terminals are 75?C. Does this require me to use the 75? columns of data in all steps? For example, in looking at 690.8(B)(2)(b) can I use the 90? column of the ambient temperature derate factor?

My second question is the following. It seems to me that a conduit fill derate should apply to the cable, which is rated at 90?C, but should not apply to the terminal. Does it make sense to calculate everything assuming 90?C, and then go back and do a check with the terminals for only ambient temperature derate using 75? columns?
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
The cable being used is 90? but the inverter terminals are 75?C. Does this require me to use the 75? columns of data in all steps?

No. The 75C rating of the terminals is not relevant to the size of the OCPD. 110.14(C) lays out requirements for conductors based on their size and what terminals they are connected to. Article 240 makes lays out no such requirement for the sizing of OCPDs to conductors.

Put another way, you size the conductor based on either size/terminals or calculated derated ampacity (whichever requires the larger size). But you can size the OCPD to the conductor based on calculated derated ampacity without regard to terminals.

For example, in looking at 690.8(B)(2)(b) can I use the 90? column of the ambient temperature derate factor?

Since you are talking about the conductor ampacity in this section, you have to look at both the conductors rating at 75C, and at the conductors calculated derated ampacity based on its 90C rating. Then you use the larger size required.

Does it make sense to calculate everything assuming 90?C, and then go back and do a check with the terminals for only ambient temperature derate using 75? columns?

That's almost right. When you check it against the 75C column, you don't need to include any other derating.
 
Last edited:

Andrew445

Inactive, Email Never Verified
Thank you for the excellent reply!

Since you are talking about the conductor ampacity in this section, you have to look at both the conductors rating at 75C, and at the conductors calculated derated ampacity based on its 90C rating. Then you use the larger size required.

I assume that these two conductor sizes you are comparing are from 690.8(B)(2)(a) and (b). This is interesting because my understanding is that this section is asking you to compare current values, not conductor sizes. You would only proceed with conductor selection after determining which current value is larger. I also wonder why you would use the 75C column for (a) instead of the 90C column. Are you saying that using the 90C column is a "correction factor for conditions of use?"

For a quick example, let's say Isc = 8.6A, 24-strings, ambient temp. 38C, maximum 300kcmil parallel Aluminum conductors in conduit.
OCPD = 1.56*8.6*24 = 322 --> 350A

690.8(B)(2)(a) 1.25*1.25*8.6*24 = 322A
690.8(B)(2)(b) 1.25*8.6*24/(0.91*0.80) = 354A

354A being the larger of the 2 values, I come up with parallel #4/0 from the 90?C column.

Checking 690.8(B)(2)(c) 205*2*0.91*0.80 = 298.5. This is not protected by the 350A ocpd, and therefore must be upsized to parallel 250 kcmil.

Terminal check with 75C column: 205*2*0.88 = 360.8 > (1.25*8.6*24 =)258A Okay.

So the final size is parallel 250 kcmil. It is my interpretation that the terminals exist in the ambient temperature and therefore must be derated. Because what else is the point of using an ampacity value from the table that is based on a lower ambient temperature (30C)?
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
Thank you for the excellent reply!

I assume that these two conductor sizes you are comparing are from 690.8(B)(2)(a) and (b).

No, I was talking about 110.14C and 310.15. The two code sections you refer to provide an additional layer of complication (which, I should admit, I wasn't considering).

This is interesting because my understanding is that this section is asking you to compare current values, not conductor sizes.You would only proceed with conductor selection after determining which current value is larger.

:huh: I think I can agree with that. But 110.14(C) still needs to be applied to the conductor chosen.

I also wonder why you would use the 75C column for (a) instead of the 90C column. Are you saying that using the 90C column is a "correction factor for conditions of use?"

110.14(C) says you have to go by the 75C column, unless the terminals are listed for use with higher rated wire, which they usually aren't. That's the reasoning.

For a quick example, let's say Isc = 8.6A, 24-strings, ambient temp. 38C, maximum 300kcmil parallel Aluminum conductors in conduit.
OCPD = 1.56*8.6*24 = 322 --> 350A

690.8(B)(2)(a) 1.25*1.25*8.6*24 = 322A
690.8(B)(2)(b) 1.25*8.6*24/(0.91*0.80) = 354A

354A being the larger of the 2 values, I come up with parallel #4/0 from the 90?C column.

All good as far as I can see.

Checking 690.8(B)(2)(c) 205*2*0.91*0.80 = 298.5. This is not protected by the 350A ocpd, and therefore must be upsized to parallel 250 kcmil.

That sucks. To be clear, if that calc came out to 301A then 240.4(B) allows you consider it protected by a 350A fuse.

Terminal check with 75C column: 205*2*0.88 = 360.8 > (1.25*8.6*24 =)258A Okay.

So the final size is parallel 250 kcmil. It is my interpretation that the terminals exist in the ambient temperature and therefore must be derated. Because what else is the point of using an ampacity value from the table that is based on a lower ambient temperature (30C)?

I could be wrong, but I'm pretty sure most of the moderators on this forum believe that 110.14(C) does not require the incorporation of any additional derations, even ambient temp. Thus simply 205*2 = 410 and definitely good.
 

Smart $

Esteemed Member
Location
Ohio
Thank you for the excellent reply!



I assume that these two conductor sizes you are comparing are from 690.8(B)(2)(a) and (b). This is interesting because my understanding is that this section is asking you to compare current values, not conductor sizes. You would only proceed with conductor selection after determining which current value is larger. I also wonder why you would use the 75C column for (a) instead of the 90C column. Are you saying that using the 90C column is a "correction factor for conditions of use?"

For a quick example, let's say Isc = 8.6A, 24-strings, ambient temp. 38C, maximum 300kcmil parallel Aluminum conductors in conduit.
OCPD = 1.56*8.6*24 = 322 --> 350A

690.8(B)(2)(a) 1.25*1.25*8.6*24 = 322A
690.8(B)(2)(b) 1.25*8.6*24/(0.91*0.80) = 354A

354A being the larger of the 2 values, I come up with parallel #4/0 from the 90?C column.

Checking 690.8(B)(2)(c) 205*2*0.91*0.80 = 298.5. This is not protected by the 350A ocpd, and therefore must be upsized to parallel 250 kcmil.

Terminal check with 75C column: 205*2*0.88 = 360.8 > (1.25*8.6*24 =)258A Okay.

So the final size is parallel 250 kcmil. It is my interpretation that the terminals exist in the ambient temperature and therefore must be derated. Because what else is the point of using an ampacity value from the table that is based on a lower ambient temperature (30C)?
IMO, it's easier to understand if you don't mix 690.8(A) with 690.8(B)... i.e. calculate them separately. I know many teach Isc * 1.56, but this way is less confusing when you get to 690.8(B) determinations, and the method will be same regardless of which side of the inverter you are on. Let's call the result of 690.8(A) just "A"...


A = 8.6A ? 24 ? 125% = 258A​


A' = 258A ? 125% = 323A

690.8(B)(1)(a) minimum OCPD rating = A'. Does not correspond to standard OCPD rating so use next larger standard rating at 350A.

690.8(B)(2)(a) AND 690.8(B)(1)(b) to comply with 110.14(C) for 75?C also use A'. Smallest 75?C conductor with an ampacity of 323A or greater is larger than 300kcmil restriction, so two 2/0 (175A each). This is the minimum size whether you use 75- or 90?C conductors.​

Now that you have both OCPD rating and minimum size conductor (?2), you factor for conductor type, conditions of use(i.e. derating), AND verify the conductor is protected by the OCPD. Type and condition are two 2/0 minimum 90?C Cu conductors times two for a total of four in the same conduit (80% adjustment) at 38?C ambient (0.91 correction). For determination we use the larger of "A" (258A) or minimum ampacity protected by OCPD (301A).


301A ? 80% ? 0.91 = 413A

Using parallel conductors, each must have an allowable ampacity (which means table value) of not less than one half of 413A, or 207A each. The smallest 90?C Cu conductor having an allowable ampacity of 207A or greater is 3/0.

Two 3/0 in parallel and 350A OCPD satisfy all the conditions of 690.8(B).
 

Smart $

Esteemed Member
Location
Ohio
I should add that a 690.8(B)(1)(c) condition is not included in the preceding example. If the OCPD is operated in an ambient temperature greater than 40 degrees, the manufacturer's correction may lower the OCPD rating, but the minimum ampacity protected would still be the same under the NEC.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
I should add that a 690.8(B)(1)(c) condition is not included in the preceding example. If the OCPD is operated in an ambient temperature greater than 40 degrees, the manufacturer's correction may lower the OCPD rating, but the minimum ampacity protected would still be the same under the NEC.
Subtle, but a good point!
The protection would still have to be effective if the ambient changed. :)
I guess one consequence of this is that if you have to install a larger nominal-sized OCPD to get proper load operation at high ambient, you also have to upsize the circuit conductors to match the nominal rating of the OCPD even if the ambient derating of wire alone did not end up requiring it.
Could this also lead to a situation where the Maximum OCPD specified for a motor load might not even be attainable if providing the MCA is inconsistent with the OCPD rating at high ambient?
Would that be a case where the only solution is to locate the OCPD in a cooler area or use electronic trip only rather than mag/thermal?
Truly an interesting rat hole you have opened. :thumbsup:
 
Last edited:
Status
Not open for further replies.
Top