Neutral Cable Ampacity/Size on 100 A NGR 1

[ThePunisher]

We have a 200 A, 10 Seconds NGR applied into the 13.8/7.96 kV secondary wye transformer neutral.

The CEC 2018 rule 10-308.1).e says that it will "BE SIZED TO CONDUCT THE RATED CURRENT OF THE IMPEDANCE GRODUNING DEVICE..."

Does this mean that we would treat 200 A as continuous if the NGR is only rated for 10 seconds and our protection would trip it for less than 2 seconds?

Can I use the GF pickup time into consideration and calculate the size of conductor that can withstand 100 A for 2 seconds.

1. If I size like it was 200 A continuous, I would be using #1/0 AWG

2. If I use the cable withstand (XLPE, 8 kV , General Cable), using the attached short circuit withstand for 200 A at max 2 seconds, I can use #8 or #6 AWG which can withstand 200 A for 35 seconds and 89 seconds respectively. If I use #12 (minimum as per CEC), it can only withstand for 5.5 seconds, so I won't use it.

I appreciate everyone's experience and expertise.

Thank you very much

 

[ThePunisher]

Sorry about the title, its 200A and not 100A

 

[itsmoked]

Suggestion:
Hit Edit on the bottom right corner of your first post.
Then edit the title any way you want to.
Submit it again.

Then, again, in the bottom right of your SECOND post, hit delete.

Then I'll come along and DELETE this post and no one will ever know you screwed up the title.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Consider the consequences of a fault that draws only 190 Amps and your protection does not trip.
Your neutral conductor will become a fusible link that leaves your circuit ungrounded.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Parchie]

@punisher,
IDK much about CEC.

I²t can be used to determine the continuous capacity of the conductors in your NGR.
In your case, you have Amps = 200, and t = 10 seconds! Therefore, your I²t = 200² X 10s = 400,000 A²-sec.
If we consider a 3-hour period as "continuous", the required ampere rating of your NGR conductors will be I² = 400.000 A²-sec/(3*60*60) = 37 or I = 6.1 amps!

But, there are limitations as the the size of a GEC wrt to the un-grounded phase conductors (see NEC 250.66), so have to use a minimum of AWG 8 Cu or AWG #6 Al if your phase conductors are #2 or smaller or otherwise!
Hope this doesn't muddle the issue/s.

 

[waross]

BE SIZED TO CONDUCT THE RATED CURRENT OF THE IMPEDANCE GRODUNING DEVICE.

The rated current is 200 Amps.
The code does not mention continuous current in this rule.

IDK much about CEC

Yes

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[RRaghunath]

Yes, I think your cable ampere rating and duration shall at least match that of the NGR.

 

[dpc]

These are generally short runs and it doesn't seem like somewhere to try to economize. I'd size it for at least 200 A and then not worry about it.

 

[waross]

The law in Canada:

The CEC 2018 rule 10-308.1).e says that it will "BE SIZED TO CONDUCT THE RATED CURRENT OF THE IMPEDANCE GRODUNING DEVICE..."

That's the law.
No short time rating.
No speculation.
No shortcuts.

If I size like it was 200 A continuous, I would be using #1/0 AWG

Yes that is correct.
Please note the difference between continuous current which you want to use and rated current which the code rule specifies.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LionelHutz]

The code isn't that cut and dry or simple. Wire de-ratings for time are elsewhere in the code. If it's 200A with a 30s time rating then it'd be rather easy to prove to an inspector that you can start with a 1/0 size to match the current and then down-size the wire due to the time limit by using a percentage factor from a time table. I'm not going to bother opening the code to find any specifics. I've successfully done similar for capacitors that were only energized during motor starting even though the capacitor section of the code has nothing about being able to doe that.

 

[waross]

I'm not going to bother opening the code to find any specifics.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LionelHutz]

So? What's your point. I've got a funeral to take care of and don't have time to look up code. If you're as smart as you think you are you can figure it out.

 

[waross]

I am sorry for your loss.
As to your other question,
"What's your point?"
I HAVE looked at the code several times. I am familiar with short time ratings, intermittent ratings and rules allowing reduced conductor sizes.
The code does allow the rating of an NGR to be less than the rated current when it will be disconnected in a short time.
The code does not allow any such reduction in the ampacity of the conductors to and from an NGR.

But let's look at some reasonable conditions.
Why do we use an NGR?
To limit damage in the event of a ground fault.
Will a 200 Amp NGR allow more damage than a 5 Amp NGR?
Very much more.
Why don't we then use a 5 Amp NGR.
We have installations where cable charging currents are close to or more than 5 Amps.
A 10 or 20 Amp NGR may be needed to protect a long cable without being tripped by charging currents.
Is charging current the only factor in selecting an NGR?
No, triplen harmonic currents returning on the ground may be much worse than charging currents.
So how do we select a rating for an NGR?
The trip setting must be greater than any current normally returning on the neutral.
The NGR trip setting must be high enough to avoid nuisance tripping on normal currents.
The NGR rating must be greater than the trip setting.

With the information given, and in the absence of a stated trip setting I would take the worst case approach and anticipate that there may be a continuous current close to the trip current and consider this to be the normal accepted ampacity of the NGR conductors.
Erring on the side of safety I do not consider the code requirement to be unreasonable.
It is reasonable to require that the NGR conductors have the sane ampacity as the rating of the NGR.

If it's 200A with a 30s time rating then it'd be rather easy to prove to an inspector that you can start with a 1/0 size to match the current and then down-size the wire due to the time limit by using a percentage factor from a time table.

That will prove that the cable is adequate for any fault that trips the protection.
It does not address the maximum continuous current that may flow without causing a trip.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LionelHutz]

It does not address the maximum continuous current that may flow without causing a trip.

Give one good reason why the wire must handle the current continuously when the resistor will burn after 10 seconds.

The resistor is rated 200A for 10sec so why can't the wire be rated at 200A for 10sec?

And stop with stupid arguments like no trip setting being given. That's just a cop-out when the OP made the resistor ratings very clear which means he must be setting a short time limit to protect the resistor he chose.

 

[davidbeach]

The code is the code. You can't argue sense with the code. Long term serenity comes from recognizing that the code is what it is. It doesn't make sense, it doesn't hold up to a thorough engineering analysis. But that doesn't matter, the code is what the code is. It's just an irrational fact of life.

 

[Parchie]

@david,
I'm sure the code will change with reactions from the industry or similar concerned practitioners! If my memory serves me right, the provision on allowing series-ratings is one example.

 

[davidbeach]

The code will change if someone submits a proposal that is accepted by the code making panel for that section of the code and the survives the comment period. A strong engineering justification for the change does not mean that it will be approved. Back when my work was under the NEC I submitted a few proposals, had one accepted (and no, I don't recall what it was now), and had the rest rejected for reasons that made no sense. That's how it works.

 

[LionelHutz]

Sized to conduct the rated current can easily be argued as sized to conduct 200A for 10 seconds.