1/C with 3/C in parallel?

G

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
electrofelon said:
Can you expand on that roger, I'm not seeing where that is necessarily a violation. The way I look at it, if the inners of the cable assembly match all the conditions and the other run you can sort of ignore that it's a cable assembly.
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The thermal characteristics are not the same. And resistance does change with temperature, even though the change is small.
 
G

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Consulting Electrical Engineer - Photovoltaic Systems
If the original set of 1/0 conductors are fed by, say, a 150A OCPD, then the EGC needs to be #6. For two parallel sets of 1/0 protected by the same OCPD the EGC needs to be a minimum of (2)(13.3 mm^2) = 26.6 mm^2, or #3, and that #3 EGC needs to be in both conduits. How are you addressing that?
 
D

Dale001289

Senior Member
Location
Georgia
ggunn said:
If the original set of 1/0 conductors are fed by, say, a 150A OCPD, then the EGC needs to be #6. For two parallel sets of 1/0 protected by the same OCPD the EGC needs to be a minimum of (2)(13.3 mm^2) = 26.6 mm^2, or #3, and that #3 EGC needs to be in both conduits. How are you addressing that?
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I’ll just tie the rigid galvanized steel conduit into the undersized #6 EGC of the 3/C.
This will provide than enough cm to compensate for the required fault path. Problem solved.


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D

Dale001289

Senior Member
Location
Georgia
electrofelon said:
Hey I hate the NEC and have very little respect for that document, I am just telling you what the code says. If you already have in mind what you're going to do and you're not getting this inspected then why are you asking?
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I just like to cover all the basis… you have to admit, there are a lot of very smart people here on the forum


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G

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Consulting Electrical Engineer - Photovoltaic Systems
electrofelon said:
Hey I hate the NEC and have very little respect for that document, I am just telling you what the code says. If you already have in mind what you're going to do and you're not getting this inspected then why are you asking?
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That's not to mention that if the inspector is following the letter of the NEC and you fail an inspection, you fail, and you may not have a leg to stand on just by saying that what you have done is good enough in your opinion.

One thing I have wondered, though: if you were to collapse, say, a 2" steel EMT conduit to a wire with that cross sectional area and consider the conductivity for steel, to what copper wire gauge would it be equivalent?

FWIW, I am agnostic on the veracity of the NEC. The rules are the rules, and the NEC is codified by law in most jurisdictions, so I do my best to comply with it.
 
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W

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
ggunn said:
One thing I have wondered, though: if you were to collapse, say, a 2" steel EMT conduit to a wire with that cross sectional area and consider the conductivity for steel, to what copper wire gauge would it be equivalent?
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I was surprised a few minutes of googling did not turn up a definitive reference for the resistivity of the type of steel used in EMT. A passing reference by @gar in an old thread here indicated it was a factor of 10 less copper, and included a link to a thread on his measurement of the resistance of a 10' stick of 1/2" EMT, but unfortunately the link was broken (went to the wrong thread).

Anyway, 2" EMT has an OD of 55.8 mm and a wall thickness of 1.65 mm. So its cross-sectional area is pi/4 * (55.8^2 - 52.5^2) = 280.7 mm^2. If the factor of 10 is correct, the equivalent copper conductor area would be 28.07 mm^2. Which is between a #3 AWG and #2 AWG in area.

Cheers, Wayne
 
electrofelon

electrofelon

Senior Member
Location
Cherry Valley NY, Seattle, WA
wwhitney said:
I was surprised a few minutes of googling did not turn up a definitive reference for the resistivity of the type of steel used in EMT. A passing reference by @gar in an old thread here indicated it was a factor of 10 less copper, and included a link to a thread on his measurement of the resistance of a 10' stick of 1/2" EMT, but unfortunately the link was broken (went to the wrong thread).

Anyway, 2" EMT has an OD of 55.8 mm and a wall thickness of 1.65 mm. So its cross-sectional area is pi/4 * (55.8^2 - 52.5^2) = 280.7 mm^2. If the factor of 10 is correct, the equivalent copper conductor area would be 28.07 mm^2. Which is between a #3 AWG and #2 AWG in area.

Cheers, Wayne
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Even not knowing the exact steel alloy used for emt, isn't the resistivity of steel alloys all pretty close? Or not?
 
electrofelon

electrofelon

Senior Member
Location
Cherry Valley NY, Seattle, WA
kingpb said:
Perhaps I missed it in previous posts, but where did it say you are using MV.
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I'm not sure why Dale is so hesitant to supply details, but I don't think it is MV. RHH/RHW conductors are available up to 2kv, but unless it's a PV system, it would certainly be an oddball system to be above 600 volts. It seems the question is can one set be 600 volt RHW and the other set be 2KV RHW and be an acceptable parallel combination (assuming he does not actually have a greater than 600 V system).
 
kingpb

kingpb

Senior Member
Location
SE USA as far as you can go
Occupation
Engineer, Registered
electrofelon said:
I'm not sure why Dale is so hesitant to supply details, but I don't think it is MV. RHH/RHW conductors are available up to 2kv, but unless it's a PV system, it would certainly be an oddball system to be above 600 volts. It seems the question is can one set be 600 volt RHW and the other set be 2KV RHW and be an acceptable parallel combination (assuming he does not actually have a greater than 600 V system).
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I agree, so much secrecy. Looks like something may have already been installed, it got red tagged, and now trying to justify an argument. As a PE, I would not allow this installation. It may technically work, but it does not meet my standards of quality and professional workmanship.
 
G

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Consulting Electrical Engineer - Photovoltaic Systems
kingpb said:
Perhaps I missed it in previous posts, but where did it say you are using MV.
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Post #33:
"Okonite, 1/0, 3/Cw/ground Okoseal ? Copper
90degree C unshielded 2kV rated
I don’t recall if it’s TC rated"

It does not explicitly say MV.
 
D

Dale001289

Senior Member
Location
Georgia
wwhitney said:
I was surprised a few minutes of googling did not turn up a definitive reference for the resistivity of the type of steel used in EMT. A passing reference by @gar in an old thread here indicated it was a factor of 10 less copper, and included a link to a thread on his measurement of the resistance of a 10' stick of 1/2" EMT, but unfortunately the link was broken (went to the wrong thread).

Anyway, 2" EMT has an OD of 55.8 mm and a wall thickness of 1.65 mm. So its cross-sectional area is pi/4 * (55.8^2 - 52.5^2) = 280.7 mm^2. If the factor of 10 is correct, the equivalent copper conductor area would be 28.07 mm^2. Which is between a #3 AWG and #2 AWG in area.

Cheers, Wayne
Click to expand...

Wayne. Nice work, conceptually it’s the same thing as using CLX as your EGC


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W

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
Dale001289 said:
It doesn’t say the “same materials”.
To me it means “same capability”
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The code doesn't say 'same materials'. It also doesn't say 'same capability'. It says 'same characteristics'.

The conventional interpretation of 'same characteristics' is same material, same insulation, same conductor arrangement, same type of conduit, etc.

This is based on having current divide evenly between the paralleled conductors. Small differences in impedance between the two conductors will result in uneven current sharing.

If you picked two random conductors, both of 150A ampacity (say 1/0 Cu with a 75C rating of 150A in a steel conduit and a 1/0 Cu with a 75C rating as part of an Aluminum MC cable, and put them in parallel, then they would have _different_ characteristics and you would expect uneven current sharing.

-Jon
 
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