3-phase 4 wire 100 amp subpanel feeders

[Confused at times]

Ok, so this one always catches my attention when I ask another electrician 'why' in regards to a code question. The typical response can vary but usually is ,"it's how we always did it!." Ok fair enough, we know green = ground, it's in the code book but who can remember where? (Not a real question)

Anyway now to the reason of this post. I was talking to another journeyman about sizing feeders for a 100amp 4wire subpanel that is about 60ft away(120/208). I was taught to use #2's with with a #8 ground. (The joys of using memory and not looking it up) Fair enough. But I had this moment of curiosity to look up some code. So memory tells me to go to table 310.16 for feeders and 250.66 for the gec. To be honest I'm not sure I've had the desire to look into a code book for years.

I've got an older code book (think I heard 310.16 is gone) and notice that #3's are rated at 100amps at 75c. I do a double take and say to myself 'odd'. Then I realize break times over and I gotta get back to work.

The next day I'm mulling it over and wonder why the hell we install #2's and not #3's. any code gurus have a reason behind this? Can we use #3's? I assume I'm just forgetting a fundamental, probably derating.

 

[roger]

Ok, so this one always catches my attention when I ask another electrician 'why' in regards to a code question. The typical response can vary but usually is ,"it's how we always did it!." Ok fair enough, we know green = ground, it's in the code book but who can remember where? (Not a real question)

If you are talking about the EGC 250.119

Anyway now to the reason of this post. I was talking to another journeyman about sizing feeders for a 100amp 4wire subpanel that is about 60ft away(120/208). I was taught to use #2's with with a #8 ground. (The joys of using memory and not looking it up) Fair enough. But I had this moment of curiosity to look up some code. So memory tells me to go to table 310.16 for feeders and 250.66 for the gec. To be honest I'm not sure I've had the desire to look into a code book for years.

You wouldn't need a GEC unless this sub panel is in a detached building.

The next day I'm mulling it over and wonder why the hell we install #2's and not #3's. any code gurus have a reason behind this? Can we use #3's? I assume I'm just forgetting a fundamental, probably derating.

Are you using copper or aluminum conductors? If you are using copper conductors with a 75 deg rating, #3 is fine and unless you do a VD calculation you wouldn't know if larger conductors are really necessary.

Roger

 

[MasterTheNEC]

Ok, so this one always catches my attention when I ask another electrician 'why' in regards to a code question. The typical response can vary but usually is ,"it's how we always did it!." Ok fair enough, we know green = ground, it's in the code book but who can remember where? (Not a real question)

Anyway now to the reason of this post. I was talking to another journeyman about sizing feeders for a 100amp 4wire subpanel that is about 60ft away(120/208). I was taught to use #2's with with a #8 ground. (The joys of using memory and not looking it up) Fair enough. But I had this moment of curiosity to look up some code. So memory tells me to go to table 310.16 for feeders and 250.66 for the gec. To be honest I'm not sure I've had the desire to look into a code book for years.

I've got an older code book (think I heard 310.16 is gone) and notice that #3's are rated at 100amps at 75c. I do a double take and say to myself 'odd'. Then I realize break times over and I gotta get back to work.

The next day I'm mulling it over and wonder why the hell we install #2's and not #3's. any code gurus have a reason behind this? Can we use #3's? I assume I'm just forgetting a fundamental, probably derating.

Firstly do not take this wrong when is say......I find it hard to "BE" an electrician and not have a NEC handy at all times but maybe thats just me, I have it on my phone, tablets, home computers, handbook (various copies), softbound copy, PDF....you name it and I have it handy.

Yes, Section 310.16 is now Section 310.15(B)(16) to better harmonize with the layout of 310.15.

Based on your question here is my assumptions...

1) You have done as many have done in the past - Selected 2 AWG AL which is at 90A and believe it is ok to protect it with a 100A OCPD. When in reality it should have a 90A OCPD [240.6(A)]. Again I am making an assumption (always bad to do on this forum)...but it's my prerogative at this moment:angel:. And most jurisdictions probably passed your 2 AWG AL on a 100A OCPD without blinking an eye.

So that brings me to the resolution of "can you use 3 AWG for your 100A remote distribution panel" and the answer is Yes, if you choose 3 AWG CU for your installation.

Now in terms of Voltage Drop, we know that this is not a NEC requirement (and we have no fire pumps involved in this question...lol) but it is a great design strategy to consider. But as Roger pointed out that may be the reason for the increase in size IF copper was being used...but at 120/208 I would not think it would matter that much for volume installers to spend on it as it would only be about a 2% - VD if CU is considered and the end load was 100A.

So it would not be something I would take into consideration (Voltage Drop that is).

Just my thoughts on it...

 

[Confused at times]

You wouldn't need a GEC unless this sub panel is in a detached building.



Are you using copper or aluminum conductors? If you are using copper conductors with a 75 deg rating, #3 is fine and unless you do a VD calculation you wouldn't know if larger conductors are really necessary.

Roger

Sry yeah note to self don't try posting on a cell phone at 5something in the morning on the train half awake.

not a gec, which 250.66 table is for, but equipment grounding conductor( auto correct keeps changing e g c to gec) which is 250.122 if memory serves me correctly. ( I Definetly need to get a current code book. And stop relying on shoddy memory)

Anyway was talking to a guy and he said we typically use #2's (cu) because the supply shops here don't usually have #3's in stock and these short runs the price difference is minimal.

 

[Confused at times]

Firstly do not take this wrong when is say......I find it hard to "BE" an electrician and not have a NEC handy at all times but maybe thats just me, I have it on my phone, tablets, home computers, handbook (various copies), softbound copy, PDF....you name it and I have it handy.

Just my thoughts on it...

You sir are 100% correct, and I do have access to the most current on my iphone here, but get annoyed trying to look stuff up on it. But hey I keep my uglys book handy for those tables if ever necessary My code book paperback is so old I'm not sure anything but the tables is unchanged. (Scratch that they got rid of 310.16 so I guess those have changed too) I do need to go get a new one and now that you kindly worded it that way I feel less like a man so I will go buy a current (thumbs up)!


yes the run is copper not aluminum. Called our shop guy and he said most others do order #2's instead since the supply houses typically don't have #3's on the shelf.


thanks for the info guys (and the kick in the arse to get up to date). Back to lighting and trimming! The joys of T.i.!

 

[jim dungar]

Anyway was talking to a guy and he said we typically use #2's (cu) because the supply shops here don't usually have #3's in stock and these short runs the price difference is minimal.

That is what most did in this area, also.
Until they learned that the supply houses started to carry #3 about 15 years ago.

 

[MasterTheNEC]

You sir are 100% correct, and I do have access to the most current on my iphone here, but get annoyed trying to look stuff up on it. But hey I keep my uglys book handy for those tables if ever necessary My code book paperback is so old I'm not sure anything but the tables is unchanged. (Scratch that they got rid of 310.16 so I guess those have changed too) I do need to go get a new one and now that you kindly worded it that way I feel less like a man so I will go buy a current (thumbs up)!


yes the run is copper not aluminum. Called our shop guy and he said most others do order #2's instead since the supply houses typically don't have #3's on the shelf.


thanks for the info guys (and the kick in the arse to get up to date). Back to lighting and trimming! The joys of T.i.!

Hey fella...I would NEVER kick a man when he is down.......lol....Plus the NFPA needs your ongoing support:angel:

 

[pete m.]

I was taught to use #2's with with a #8 ground.

250.122(B) will also come into play causing the EGC to be 6 awg cu. That is if the terminations for the conductors are rated at 75 degrees.

Pete

 

[augie47]

Wow! So many things come to mind....
First, and as others note, don't take this personally either, but the term "ground" can get you into so many problems. For the Code, "Ground" is Earth.
Beyond that point to really "talk Code" you need to use the more descriptive terms such as Grounded Conductor, Equipment Grounding Conductor, and Grounding Electrode Conductor.
The specific one you are discussing can lead you to different answers

It has been my experience that many 100 amp panels end up being wired with #2 Cu for exactly the reason your mentioned... a lot of supply housed don't stock #3..perhaps because many electricians do as you have and say #2 for 100 amp.. kinda a chick vs egg thing.

From there, if we are discussing and equipment grounding conductor, 250.66 is not the correct Table, 250.122 is and it will allow a #8 Cu for the EGC.

That will cover a vast majority of the installs. When such items as voltage drop, ambient temperature, conduit fill and harmonic currents come into play your #3 may not be the correct conductor and as pete notes if your #3s have to be increased, your #8 would need to be also.

 

[GoldDigger]

Wow! So many things come to mind....
First, and as others note, don't take this personally either, but the term "ground" can get you into so many problems. For the Code, "Ground" is Earth.
Beyond that point to really "talk Code" you need to use the more descriptive terms such as Grounded Conductor, Equipment Grounding Conductor, and Grounding Electrode Conductor.
The specific one you are discussing can lead you to different answers

It has been my experience that many 100 amp panels end up being wired with #2 Cu for exactly the reason your mentioned... a lot of supply housed don't stock #3..perhaps because many electricians do as you have and say #2 for 100 amp.. kinda a chick vs egg thing.

From there, if we are discussing and equipment grounding conductor, 250.66 is not the correct Table, 250.122 is and it will allow a #8 Cu for the EGC.

That will cover a vast majority of the installs. When such items as voltage drop, ambient temperature, conduit fill and harmonic currents come into play your #3 may not be the correct conductor and as pete notes if your #3s have to be increased, your #8 would need to be also.

Is the difference between the required #3 and the used #2 great enough to trigger an increase in the EGC size?
Does the code want to distinguish between upsizing conductors for VD and upsizing conductors because that was what was on the truck?

 

[Smart $]

Is the difference between the required #3 and the used #2 great enough to trigger an increase in the EGC size?
Does the code want to distinguish between upsizing conductors for VD and upsizing conductors because that was what was on the truck?

Depends. If #3 is sufficient after all requirements are applied (and #4 would not), then yes.

No on the latter question.

 

[eaw3rd@cox.net]

@\#2 vs #3 feeder

@\#2 vs #3 feeder

I would say that the #2 is preferred as it is more readily available

 

[roger]

I would say that the #2 is preferred as it is more readily available

I hear that all the time but all the supply houses I use have thousands of feet of #3 in stock.

Roger

 

[infinity]

Depends. If #3 is sufficient after all requirements are applied (and #4 would not), then yes.

No on the latter question.

I agree, even though in some cases it doesn't make any sense the reason for the increase in size is irrelevant when applying 250.122(B).

 

[curt swartz]

All my suppliers stock #3. For some reason the wire manufactures have decided to only offer black for #3. All other sizes from 14awg through 600kcmil are available in at least 9 colors. I sometimes use #2 for 100 amp feeders so I can use colored feeders instead of phase tape.

 

[GoldDigger]

All my suppliers stock #3. For some reason the wire manufactures have decided to only offer black for #3. All other sizes from 14awg through 600kcmil are available in at least 9 colors. I sometimes use #2 for 100 amp feeders so I can use colored feeders instead of phase tape.

One likely reason for black only is low volume.
They may sell enough to justify production runs and stocking for one color, but not for 1/9 that amount or less.

 

[infinity]

All my suppliers stock #3. For some reason the wire manufactures have decided to only offer black for #3. All other sizes from 14awg through 600kcmil are available in at least 9 colors. I sometimes use #2 for 100 amp feeders so I can use colored feeders instead of phase tape.

Just wondering, when using #2 AWG instead of #3 AWG do you use a larger EGC or are you using a metallic raceway as the EGC?

 

[Dennis Alwon]

If I were feeding a sub panel and had to use #2 at 75C I would use a 125 amp overcurrent protective device and be done with it. Of course the panel would have to be rated at least 125 amps.

 

[kwired]

If I were feeding a sub panel and had to use #2 at 75C I would use a 125 amp overcurrent protective device and be done with it. Of course the panel would have to be rated at least 125 amps.

That would depend on a few things before I would automatically do that. First one is what panel/series, or other equipment is involved? Fused switches? You need a 200 amp switch to hold a 125 amp fuse instead of a 100 amp switch - more $$$ Miniature series breakers - I am mostly familiar with QO series - there is a price jump from QO2100 to QO125. They don't make a three pole 125 in plug on series anyway. Feed originating in an I-line panel? Cost difference between a 100 and 125 amp breaker is little or nothing though.

 

[petersonra]

we don't generally use #1 or #3 because it is harder to get. a lot of times if I put those sizes on a drawing it ends up being the next size because that is what the shop has laying around.

the boss prefers to use what is available rather than ordering something most of the time.

It would be different probably if the quantities being used were a lot more but the cost difference between 50 ft of #1 and 50 feet of 1/0 is not enough to worry about.