need help figuring OCPD for circuit.

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69gp

Senior Member
Location
MA
Just trying to figure out what the max OCPD breaker I can use on the following setup.

1) DC circuit
2) using a 4 wire 250 MCM aluminum direct burial cable.
3) 2 wires from the cable are to be paralleled. hence 2 sets of 250 MCM per phase.
4) MC is buried for approximately 30' and run exposed under solar panels for anywhere from 50' to 400'.

5) not sure how this plays into it but where the cables are underground there are anywhere from 8 to 16 of these cables grouped together. Should these cables be considered when de-rating.
 

69gp

Senior Member
Location
MA
I don't see an exemption for underground so IMO, the de-rating should take place.

Just so I understand better the photo below shows the MC cables coming out of the ground each one of these cables has 4- 250 MCM aluminum conductors set up so that each cable uses 2 conductors in parallel to carry the load.

If I use table 310.15- 250 mcm THWN2 aluminum under the 90 degree column I have 230 amps. times 2 for the parallel conductors for a total of 460 amps x .80 for de-rating end up at 368 amps.
Now my question is since the cables are all grouped together in the ground do I need to figure all the cables that are in the ground that are grouped together to figure my de-rating?


IMG_20140726_180904478.jpg
 

hurk27

Senior Member
Not A PV person but I sure hope those panels will be held down by more than those half blocks on each leg????

A good wind will rip those solar panels right out off the ground, Thats allot of wind area that can be grabbed, the ones I have seen were always in poured concrete into the ground, never seen any just waited down like that??

I just remembered a statement my professor at MFT made a long time ago:

"Wouldn't be nice if we could just grow electrons in a farm field"

looks like his statement kind of came true:p
 
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JoeStillman

Senior Member
Location
West Chester, PA
I would say it's not necessary to derate a bundle of direct-buried cables. It isn't the same as cables in a raceway. As poor a thermal conductor as soil is, it is still better than the air inside a conduit.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Not A PV person but I sure hope those panels will be held down by more than those half blocks on each leg????

A good wind will rip those solar panels right out off the ground, Thats allot of wind area that can be grabbed, the ones I have seen were always in poured concrete into the ground, never seen any just waited down like that??

Ballasted arrays like this are common in the PV world. Usually the racking company determines the number and placement of the blocks for an installation in a particular wind zone.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
I would say it's not necessary to derate a bundle of direct-buried cables. It isn't the same as cables in a raceway. As poor a thermal conductor as soil is, it is still better than the air inside a conduit.

Table 310.15(B)(3)(a) shows derating for conductors "in a raceway or cable". I believe that this situation calls for a 0.8 derate because the runs are 4 CCC's in a cable.
 
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ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Just trying to figure out what the max OCPD breaker I can use on the following setup.

1) DC circuit
2) using a 4 wire 250 MCM aluminum direct burial cable.
3) 2 wires from the cable are to be paralleled. hence 2 sets of 250 MCM per phase.
4) MC is buried for approximately 30' and run exposed under solar panels for anywhere from 50' to 400'.

5) not sure how this plays into it but where the cables are underground there are anywhere from 8 to 16 of these cables grouped together. Should these cables be considered when de-rating.

What is your Isc for each of these circuits? OCPD should be the next standard size up from (Isc)(1.25)(1.25). Once you have that then you need to back calculate if your chosen conductors are protected by that OCPD.
 
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Smart $

Esteemed Member
Location
Ohio
I would say it's not necessary to derate a bundle of direct-buried cables. It isn't the same as cables in a raceway. As poor a thermal conductor as soil is, it is still better than the air inside a conduit.
The cables in the picture appear to have an overall outer jacket. That disqualifies them from the exemptions for AC/MC.
 
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