DC photovoltaic positive and negative in separate raceways

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shputnik

Senior Member
Location
Utah
Occupation
Expert wirenut installer
Is there an issue with positive (black) conductors being in one raceway and the negative (white) conductors being in the other? Array to combiner box raceways.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Is there an issue with positive (black) conductors being in one raceway and the negative (white) conductors being in the other? Array to combiner box raceways.
Why would you want to do that? BTW, if the system is ungrounded (most are these days) none of your conductors can be white.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
It's not dangerous in the same way that AC circuits could start a fire with inductive heating, but it's still a violation of 300.6(I) in the NEC.

As for why you'd want to do it, I know sometimes it's tempting to jump just one conductor between arrays if you're circuit loops back the other way, but by and large if you're keeping things organized it should never happen.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
When working with bipolar arrays it is common to keep the three conductors (or sets of parallel conductors) separated in different raceways.
It minimizes the line to line voltage.
 

shputnik

Senior Member
Location
Utah
Occupation
Expert wirenut installer
Why would you want to do that? BTW, if the system is ungrounded (most are these days) none of your conductors can be white.

Was told to run white and black instead of red and black. So it must be the other system.
The reason is negative is in one side and positive is on the other. Was told to have all negatives on one side through sleeves and to combiner box. Then run positives across under array from combiner the through sleeves on other end of array.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Was told to run white and black instead of red and black. So it must be the other system.
Whoever is issuing your instructions may not be aware of this requirement, and it could (should, actually) cause you to fail your inspection if done incorrectly. If it were I doing the installation, I would verify that that the DC is negatively grounded at the inverter before proceeding. Doing it over is almost always much harder than doing it right the first time.

Free advice and worth every penny. :D

PS: If you post the make and model of the inverter(s) you are installing, we can most likely tell you what you are dealing with.
 

Carultch

Senior Member
Location
Massachusetts
It's not dangerous in the same way that AC circuits could start a fire with inductive heating, but it's still a violation of 300.6(I) in the NEC.

As for why you'd want to do it, I know sometimes it's tempting to jump just one conductor between arrays if you're circuit loops back the other way, but by and large if you're keeping things organized it should never happen.


The main issue is that you will magnetize the conduits if you do this. Unlike AC, where the inductive heating is dangerous, with DC, it will only statically magnetize the conduits. That is, it will create a steady magnetic field in the conduits and surroundings.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
Is there anything in 690 that changes the requirements of 300.3(B)?

I guess not. My mistake. But it's never arisen as an issue for me. Considering that one could use the free-in-air table, and that one will have already sized wire-gauge in the raceway for any derate, you'd pretty much never have a higher derate on the array.

The main issue is that you will magnetize the conduits if you do this. Unlike AC, where the inductive heating is dangerous, with DC, it will only statically magnetize the conduits. That is, it will create a steady magnetic field in the conduits and surroundings.

Yeah. I guess that could be interesting and potentially unsafe in a freak situation. How strong a magnetic field are we talking about at, say, 300V and 7 amps?
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
Was told to run white and black instead of red and black. So it must be the other system.
The reason is negative is in one side and positive is on the other. Was told to have all negatives on one side through sleeves and to combiner box. Then run positives across under array from combiner the through sleeves on other end of array.

Well, I don't know what your jobsite looks like, but this just sounds a bit weird. I agree with everything ggunn said. Anything run through sleeves should have the positive run with the negative for that given string or circuit.
 

shputnik

Senior Member
Location
Utah
Occupation
Expert wirenut installer
Well, I don't know what your jobsite looks like, but this just sounds a bit weird. I agree with everything ggunn said. Anything run through sleeves should have the positive run with the negative for that given string or circuit.

I attached a picture to show what I am questioning. As you can see thirteen moduels are across with negative on one side and positive on the other.
 

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Carultch

Senior Member
Location
Massachusetts
How strong a magnetic field are we talking about at, say, 300V and 7 amps?

Here's how to calculate it.
http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/magcur.html#c2

Let's take 0.1 meters [4"] away as our distance r, and 7A as our current. For non-ferrous conduit, the mu-value is mu0 of free space.

In non-ferrous conduit, we are talking about 0.14 Gauss of magnetic fields.
The Earth's magnetic field ranges from 0.25 Gauss to 0.65 Gauss.

So it is enough for your compass to measure, but not enough to cause any serious problems. That is, for just the single source circuit.
 
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Carultch

Senior Member
Location
Massachusetts
I attached a picture to show what I am questioning. As you can see thirteen moduels are across with negative on one side and positive on the other.

Obviously you will have to separate the polarities at some point, when the wires reach out to the two ends of the circuit. The best strategy is to keep them together for as much distance as possible, and then diverge individual circuit wiring when required. Find a decent thoroughfare and wire management structure for all the circuits to be routed with both polarities together, once you leave the vicinity of each source circuit. Use it to travel throughout the array as the "main road" to the combiner box. You want a two way street of currents, rather than one way streets on opposite sides of town.

This is in contrast to diverging all the positives to one side, all the negatives to the other side, and then distributing off to each circuit. This is what you should avoid doing, although there is no code requirement to prevent you from doing this.

Once you leave the vicinity of the array completely, you now are required to route both polarities together.
 
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jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
I attached a picture to show what I am questioning. As you can see thirteen moduels are across with negative on one side and positive on the other.

I guess I see nothing wrong with that in principle, leaving aside details that may not be in the diagram.

I don't understand why the sleeves are being used if everything is under the modules.
 
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