Size of EGC in parallel installation

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coop3339

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Hi

I have a question on sizing a EGC for a parallel feeder. It is for a solar field 900' away from the buildings service. Its full loads current is 800A and the OCPD is set at 1000A without GFP. (I don't think I need GFP because solar in not considered a service)

I am using 6-sets (3-600AL, 1-3/OAL N and 1-?AL G)
According to 250.122 the EGC for 1000A would be 4/O AL
The increased size for voltage drop is 2/O to 600 for the 133A feeders
250.122(B) requires the EGC be increased proportionally to the ungrounded conductors.
The 600 is 4.5 times larger than the 2/O per CM
so I would need a min of 4.5*211,592(the CM of 4/O) = 954,033CM.
250.122(F) requires a full EGC per set.
Does this mean I need to add (2) 500kcml AL conductors to each set for the EGC or can it be reduced to (1) 600kcml AL to match the largest ungrounded conductor in the set per 250.122(A)?

Is there an exception when using an OCPD with GFP?
I found one for cables but it does not seem to fit here.
Also if there is an exception, I would probibly need GFP on the OCPD coming from the solar field also?


Seems unnecessary for the ground to be larger then the feeder.

Thanks
 
coop3339 said:
Hi

I have a question on sizing a EGC for a parallel feeder. It is for a solar field 900' away from the buildings service. Its full loads current is 800A and the OCPD is set at 1000A without GFP. (I don't think I need GFP because solar in not considered a service)

I am using 6-sets (3-600AL, 1-3/OAL N and 1-?AL G)
According to 250.122 the EGC for 1000A would be 4/O AL
The increased size for voltage drop is 2/O to 600 for the 133A feeders
250.122(B) requires the EGC be increased proportionally to the ungrounded conductors.
The 600 is 4.5 times larger than the 2/O per CM
so I would need a min of 4.5*211,592(the CM of 4/O) = 954,033CM.
250.122(F) requires a full EGC per set.
Does this mean I need to add (2) 500kcml AL conductors to each set for the EGC or can it be reduced to (1) 600kcml AL to match the largest ungrounded conductor in the set per 250.122(A)?

Is there an exception when using an OCPD with GFP?
I found one for cables but it does not seem to fit here.
Also if there is an exception, I would probibly need GFP on the OCPD coming from the solar field also?


Seems unnecessary for the ground to be larger then the feeder.

Thanks
Click to expand...


I agree it seems unreasonable for the EGC to be larger than the main conductors. But that is what the code specifically requires in this case. I don't know of a way around it, or if the CMP who set up the algorithm realized that it could happen.

I will also mention that this also affects your neutral size. Because 705.95(B) prescribes the neutral to be equal to or larger than the EGC.

As for paralleling up two 500's to make the equivalent of a 1000 kcmil EGC, I am not aware that the NEC allows this. Please correct me if I am wrong, because I can see that in extreme examples, the calculations might prescribe a size that doesn't exist. From as nearly as I can tell, if calculations indicate you'd need a 1000 kcmil EGC, you need a 1000 kcmil EGC.

Is there any possibility to splitting the 800A feed into two 400A feeds? And not combine them until much closer to the point of interconnection?
 
I stand corrected:

250.122 Size of Equipment Grounding Conductors.
(A) General. Copper, aluminum, or copper-clad aluminum equipment grounding conductors of the wire type shall not be smaller than shown in Table 250.122 but shall not be required to be larger than the circuit conductors supplying the equipment.
Click to expand...
 
Carultch said:
I will also mention that this also affects your neutral size. Because 705.95(B) prescribes the neutral to be equal to or larger than the EGC.
Click to expand...

I don't think this applies because the neut is sized per 250.102. Usually 705.95(B) would allow for a neutral smaller then the required neutral per 250.102.
 
coop3339 said:
Hi

I have a question on sizing a EGC for a parallel feeder. It is for a solar field 900' away from the buildings service. Its full loads current is 800A and the OCPD is set at 1000A without GFP. (I don't think I need GFP because solar in not considered a service)

I am using 6-sets (3-600AL, 1-3/OAL N and 1-?AL G)
According to 250.122 the EGC for 1000A would be 4/O AL
The increased size for voltage drop is 2/O to 600 for the 133A feeders
250.122(B) requires the EGC be increased proportionally to the ungrounded conductors.
The 600 is 4.5 times larger than the 2/O per CM
so I would need a min of 4.5*211,592(the CM of 4/O) = 954,033CM.
250.122(F) requires a full EGC per set.
Click to expand...

Your starting conductor size should be 4/0 AL, not 2/0 AL... Six 2/0 AL is only good for 810A, not properly protected by the 1000A OCPD.

That would make the increased EGC 600mcm, the same size as the ungrounded conductors.
 
coop3339 said:
I am using 6-sets (3-600AL, 1-3/OAL N and 1-?AL G)
According to 250.122 the EGC for 1000A would be 4/O AL
The increased size for voltage drop is 2/O to 600 for the 133A feeders
250.122(B) requires the EGC be increased proportionally to the ungrounded conductors.
The 600 is 4.5 times larger than the 2/O per CM
so I would need a min of 4.5*211,592(the CM of 4/O) = 954,033CM.
250.122(F) requires a full EGC per set.
Click to expand...

As David noted you started with the incorrect conductor size because this number was incorrect in your calculation. It should have been 1000/6=166.67.
 
coop3339 said:
I don't think this applies because the neut is sized per 250.102. Usually 705.95(B) would allow for a neutral smaller then the required neutral per 250.102.
Click to expand...
You didn't provide the pertinent information to know whether 705.95(A) or (B) applies. In any case, I believe the smallest would be under (B), and that would be not smaller than an EGC sized per 250.122, as EGC sizing is not stipulated anywhere in Article 705.

705.95 Ampacity of Neutral Conductor. The ampacity of
the neutral conductors shall comply with either (A) or (B).

(A) Neutral Conductor for Single Phase, 2-Wire Inverter
Output. If a single-phase, 2-wire inverter output is
connected to the neutral and one ungrounded conductor
(only) of a 3-wire system or of a 3-phase, 4-wire, wyeconnected
system, the maximum load connected between
the neutral and any one ungrounded conductor plus the
inverter output rating shall not exceed the ampacity of the
neutral conductor.

(B) Neutral Conductor for Instrumentation, Voltage
Detection or Phase Detection. A conductor used solely for
instrumentation, voltage detection, or phase detection and
connected to a single-phase or 3-phase utility-interactive
inverter, shall be permitted to be sized at less than the
ampacity of the other current-carrying conductors and shall
be sized equal to or larger than the equipment grounding
conductor.
Click to expand...
 
Smart $ said:
You didn't provide the pertinent information to know whether 705.95(A) or (B) applies. In any case, I believe the smallest would be under (B), and that would be not smaller than an EGC sized per 250.122, as EGC sizing is not stipulated anywhere in Article 705.
Click to expand...

It would be 705.95(B) but I don't think that was the intention of the code. It would not be logical to require a neutral used for instrumentation and metering to be larger then the normally required neutral. I do agree it could be interrupted that way though.
 
david luchini said:
Your starting conductor size should be 4/0 AL, not 2/0 AL... Six 2/0 AL is only good for 810A, not properly protected by the 1000A OCPD.

That would make the increased EGC 600mcm, the same size as the ungrounded conductors.
Click to expand...

Thanks for the correction, one math mistake in a calculation can have dire consequences.
 
coop3339 said:
It would be 705.95(B) but I don't think that was the intention of the code. It would not be logical to require a neutral used for instrumentation and metering to be larger then the normally required neutral. I do agree it could be interrupted that way though.
Click to expand...


Agreed.

So ultimately the two questions we have are as follows:
1. Is there any application where the EGC is required to be larger than the ungrounded current-carrying conductors? Parallel sets of conductors may seem to prompt this, when the ungrounded conductors are divided much more than ordinarily expected.
2. In such an application, does the neutral also have to be at least as large as the EGC, even when the EGC is larger than the ungrounded current-carrying conductors?
 
coop3339 said:
It would be 705.95(B) but I don't think that was the intention of the code. It would not be logical to require a neutral used for instrumentation and metering to be larger then the normally required neutral. I do agree it could be interrupted that way though.
Click to expand...
In that scenario, not less than the minimum required EGC size is the normally required neutral size.
 
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