EGC Sizing AC vs DC

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csoc64

Senior Member
Location
northeast
Can anyone help me understand why EGC's for circuits on the DC side do not need to be re-sized proportionally for sizing adjustments in ungrounded conductors (690.45) while this does not apply to EGC's on the AC side, which fall under 250.122(B)? WE like to use SER for connections from the AC solar disco to the main panel instead of hard piping simply for ease of install. However, in some cases this limits us to #6 SER, as the EGC sizing is reduced in sizes #4 SER and larger. Or am I missing something?
 

Carultch

Senior Member
Location
Massachusetts
Can anyone help me understand why EGC's for circuits on the DC side do not need to be re-sized proportionally for sizing adjustments in ungrounded conductors (690.45) while this does not apply to EGC's on the AC side, which fall under 250.122(B)? WE like to use SER for connections from the AC solar disco to the main panel instead of hard piping simply for ease of install. However, in some cases this limits us to #6 SER, as the EGC sizing is reduced in sizes #4 SER and larger. Or am I missing something?

It probably has to do with the fault currents available in each circuit. In the DC circuit, this is limited to Isc at maximum irradiance. In the AC circuit, it is a function of what is available at the service point, which can be quite large, and measured in kilo-amperes. Remember, this is a PV specific rule, so it does not apply to DC circuits in general.

In SER and SEU wiring, it is common for a reduced ground. They typically size this according to the most likely application, with a safety factor in case you do have a distance upsizing. Keep in mind that for larger amp circuits, the ground wire kcmil is a lot smaller of a fraction than the main wire kcmil.

Also, just because a manufacturer's datasheet shows a product could exist, doesn't necessarily mean any distributor stocks it, or is able to supply it. I have a project where I'd like to use copper SER 6-6-6, but no distributors have been able to supply it in my quantity, even with a lead time. They can either provide SEU or 4-6's of SER. So before spec'ing cable in this family, make sure you have the conversation with a distributor, or expect a substitution.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
Can anyone help me understand why EGC's for circuits on the DC side do not need to be re-sized proportionally for sizing adjustments...

Some thoughts...

Fault current in PV circuits isn't so much a function of the resistance of the conductor, as it is in AC circuits that maintain voltage. The current is limited to the short-circuit current of the PV panels, and the conductors are required to be oversized based on that short circuit current. So increasing an EGC size for a PV circuit isn't going to result in increasing the fault current to where an overcurrent device will trip. Compare this to AC circuits where you rely on fault current tripping the breaker, and if the EGC isn't large enough to carry that fault current then you've defeated your safety device.

Going further...
PV ground fault currents are supposed to be detected by a GFDI instead of an overcurrent device. The GFDI trip currents are set much lower than the operating current, whereas AC trip currents are typically the overcurrent device, which is rated higher than the operating current. Typically grounded inverters use a 1-5A fuse where the currents are 10-hundreds of amps, and transformerless inverters can detect ground-faults at milliamp levels. If the devices can't work properly with a small EGC, they are unlikely to work much better with a larger one. Basically, they'd better work with any EGC.
 
On the same topic: Does anyone else find the "increased in size" requirement frequently skipped/forgotten for Inverter output circuits? I see it all the time in systems that were not designed by us. Is there some "loophole" like perhaps a creative use of "ambient temperature on the roof" or bundling? Right now we are doing a 400kw rooftop and using tray cable. The inverter output is about 35 amps and I cant see how they can use tray cable. I didnt run any numbers, but there are several different sizes so at least some of it sized for VD.
 

Carultch

Senior Member
Location
Massachusetts
On the same topic: Does anyone else find the "increased in size" requirement frequently skipped/forgotten for Inverter output circuits? I see it all the time in systems that were not designed by us. Is there some "loophole" like perhaps a creative use of "ambient temperature on the roof" or bundling? Right now we are doing a 400kw rooftop and using tray cable. The inverter output is about 35 amps and I cant see how they can use tray cable. I didnt run any numbers, but there are several different sizes so at least some of it sized for VD.

NEC2014 specifies that EGC is only upsized when you increase the size above "the minimum size that has sufficient ampacity for the intended installation". Previous editions of the NEC were unclear on this issue.

In otherwords, if temperature corrections or bundling ampacity adjustments are the reason you increase the size of the main wires, you therefore don't need to increase the EGC above the default size. There is a proposal for NEC2017, to undo this clarification, and still require it when you increase size because of derate factors.

The main intention of this is voltage drop, to make sure you have enough of an effective ground-fault current path, when resistance increases with distance.

There are other reasons you might increase the main wires, for which I don't think you should have to increase the EGC. However you do, according to the 2014 NEC.
1. Using larger wire left over from your previous job.

2. Standardizing on a minimum size, such as #10, and using it even where #14 could suffice. Perhaps because in your scope of work, you rarely work with anything smaller that 30A.

3. Having to use a larger size than normally necessary, because equipment terminals aren't capable of receiving the minimum size per the NEC ampacity calculations. I've seen equipment with terminals for #8 minimum, even though the device has a full load amperes of 14 A (20A OCPD, therefore #12 default).
 
NEC2014 specifies that EGC is only upsized when you increase the size above "the minimum size that has sufficient ampacity for the intended installation". Previous editions of the NEC were unclear on this issue.

In otherwords, if temperature corrections or bundling ampacity adjustments are the reason you increase the size of the main wires, you therefore don't need to increase the EGC above the default size.

Right so that is what I was thinking was someone could say "no we didnt increase the size for VD, we increased it because with all the heat up there and the bundling, that is the minimum ampacity, therefor we didnt have to increase the size of the EGC...."
 

Carultch

Senior Member
Location
Massachusetts
Right so that is what I was thinking was someone could say "no we didnt increase the size for VD, we increased it because with all the heat up there and the bundling, that is the minimum ampacity, therefor we didnt have to increase the size of the EGC...."


That's correct.


What are your thoughts on the other reasons for increasing size of main conductors that I mentioned? And whether you think EGC should need the increase in size?
 
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