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#42713 - 09/24/04 09:59 PM Here's the Derating Discussion!!! Yee-Haww  
Scott35  Offline

Broom Pusher and
Member
Joined: Oct 2000
Posts: 2,707
Anaheim, CA. USA
Carried over from the "Residential Homerun" Thread

Residential Homerun

OK, I'll get the Ball A-rollin'...

System is 208Y/120 VAC 3Ø 4 Wire.
Panelboard has 200 Amp Max. load.
Batman has been captured by the Penguin... oh wait, now it's the other way around... (must add humor).

We have a series of Modular Furniture, which is 40 Feet from the Panelboard (AKA Sub Panel).
The Mod. Furniture of concern has 12 Workstations total - split up into two sections (6 W/S "on the left", 6 W/S "on the right").

We will run two "Full Boats" ("Full Boat = 4 Wire Multiwire Circuit) from the Panelboard to the Mod. Furniture, via a common 1" Raceway (EMT in this case).
One "Full Boat" feeds the 6 W/S "on the left", and the other "Full Boat" feeds the 6 W/S "on the right".

Branch Circuitry per "Full Boat" will consist of 3 # 12 THHN cu for the Ungrounded Conductors, 1 # 10 THHN cu for the Grounded Conductor. To Bond the Raceway, an additional # 12 cu is included for the Equipment Grounding Conductor.

Circuit Breakers in the Panelboard for these Branch Circuits are 20/1 (20 Amp single pole).

Load Current per Line (L-C Loads) will be 12 Amps Max.

THD (Total Harmonic Distortion) is <10%

OK, let me know:

  • Is this NEC compliant?
  • Will it work?
  • Is this a Design Issue?
  • Does this example make sense?
  • Will the Penguin escape once again?


Feel free to present any information you wish.

Scott35


Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

Work Gear for Electricians and the Trades

#42714 - 09/25/04 02:46 AM Re: Here's the Derating Discussion!!! Yee-Haww  
e57  Offline
Member
Joined: May 2003
Posts: 2,876
S.F.,CA USA
Is this NEC compliant? YES

Will it work? YES

Is this a Design Issue? I don't get it? I might do it that way, load seems high though....

Does this example make sense? YES

Will the Penguin escape once again? Of course! It's in the script......

I like the over-sized center on the round-house, under the magic number nine! Distance isn't all that long.... And, if you wanted to get easy-sleazy 'bout it for a quicky slapper, I know where you can get several thousand foot rolls of 12/8 w/g MC! (no over-sized N's though)


Mark Heller
"Well - I oughta....." -Jackie Gleason

#42715 - 09/25/04 02:58 AM Re: Here's the Derating Discussion!!! Yee-Haww  
e57  Offline
Member
Joined: May 2003
Posts: 2,876
S.F.,CA USA
Oh... I went back to the other thread.... You wanted a real answer, with math and code refferances and stuff, right?


Mark Heller
"Well - I oughta....." -Jackie Gleason

#42716 - 09/25/04 06:56 AM Re: Here's the Derating Discussion!!! Yee-Haww  
Scott35  Offline

Broom Pusher and
Member
Joined: Oct 2000
Posts: 2,707
Anaheim, CA. USA
E, Thanks for the reply postings!!!

I'll explain the "Design Issue" thing after a few more replies - and as far as the Penguin is concerned, last I heard He was hanging out with the Joker. They may be planning another daring scheme!

Back to reality [Linked Image],
As mentioned in your 2nd posting, Math and Code references would be great things to include with reply postings, but not mandatory
(or in the words of the NEC:"Inclusion of reference data may be ommitted, if permitted by the Authority Having Jurisdiction.) The AHJ in this case, will be the Forum as a whole.

After a few more replies, then we will / Shall bring in the Code Section references + the methods to obtain such conclusions.

Sounds like more fun than a Barrel O' Monkeys!!!... but then again, just how much "Fun Value" does a Barrel Of Monkeys relate to?
[Linked Image]

Scott35

***Edited to fikks sphellieng airers***
[Linked Image]
S.E.T.

[This message has been edited by Scott35 (edited 09-25-2004).]


Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

#42717 - 09/25/04 11:36 AM Re: Here's the Derating Discussion!!! Yee-Haww  
iwire  Offline
Moderator
Joined: Jan 2003
Posts: 4,391
North Attleboro, MA USA
The manufactures of modular furniture that I see often specify 15 amp OCPDs.

However I find most engineers are unaware of this or choose to ignore it. [Linked Image]

OK I will give this a whirl

IMO you have 8 current carrying conductors.

Quote
310.15(B)(2)(c)On a 4-wire, 3-phase wye circuit where the major portion of the load consists of nonlinear loads, harmonic currents are present in the neutral conductor; the neutral shall therefore be considered a current-carrying conductor.


That is assuming the work stations have PCs, the pencil sharpeners, desk lamps etc. are just a small portion of the load.

I do not know of any NEC requirement to up size the neutral although it makes good sense considering the furniture whip will probably have a 10 AWG grounded conductor? (For extra credit is it a grounded conductor before we connect it to the branch circuit or is just the white wire? [Linked Image])

8 Current Carring Conductors, Table 310.15(B)(2)(a) shows a 70% for 7 - 9 CCCs

Table 310.16 shows 12 AWG Copper THHN rated at 30 amps.

30 x .7 = 21 amps

Table 310.16 shows 10 AWG Copper THHN rated at 40 amps.

40 x .7 = 28 amps

Seems both the 12 AWGs and the 10 AWGs are still rated high enough to use 20 amp breakers.

  • Is this NEC compliant? Yes
  • Will it work? Sure
  • Is this a Design Issue? The use of 10 AWG for the grounded conductor was a design choice
  • Does this example make sense? To me, yes, I deal with this exact situation often. [Linked Image]
  • Will the Penguin escape once again?
Probably......Batman is a major wuss [Linked Image]

Bob




[This message has been edited by iwire (edited 09-25-2004).]


Bob Badger
Construction & Maintenance Electrician
Massachusetts

#42718 - 09/25/04 12:48 PM Re: Here's the Derating Discussion!!! Yee-Haww  
SolarPowered  Offline
Member
Joined: Jul 2004
Posts: 625
Palo Alto, CA, USA
But would <10% THD be construed as, "where the major portion of the load consists of nonlinear loads"?


#42719 - 09/25/04 01:43 PM Re: Here's the Derating Discussion!!! Yee-Haww  
iwire  Offline
Moderator
Joined: Jan 2003
Posts: 4,391
North Attleboro, MA USA
The NEC does not use THD level as the determining factor.

The NEC requirement states "major portion of the load consists of nonlinear loads".

PCs are non-linear loads and in Scott's example I would say that the major portion of the load is PCs.

In my opinion that is enough to have to treat the neutral as a CCC.:0

However that is just my opinion, Bob [Linked Image]


Bob Badger
Construction & Maintenance Electrician
Massachusetts

#42720 - 09/25/04 02:18 PM Re: Here's the Derating Discussion!!! Yee-Haww  
SolarPowered  Offline
Member
Joined: Jul 2004
Posts: 625
Palo Alto, CA, USA
I'll throw in another curve ball. While the older PCs are nonlinear, the many of the newer ones with "wide-mouth" power supplies (100-240V) are highly linear.


#42721 - 09/25/04 02:49 PM Re: Here's the Derating Discussion!!! Yee-Haww  
NJ Wireman  Offline
Member
Joined: Aug 2004
Posts: 186
New Jersey (South Jersey)
Just a thought but why not use furniture feed, i use to do ALOT of work for a large bank firm here on the east coast (Commer***) im sure you can figure it out we did this type of work on a daily basis. Check with your supplier. jb above the ceiling, fish the wall or use power pole your choice and your in like flin.


#42722 - 09/26/04 07:57 AM Re: Here's the Derating Discussion!!! Yee-Haww  
electure  Offline


Member
Joined: Dec 2000
Posts: 4,259
Fullerton, CA USA
Isn't the use of a 1" raceway a design issue?


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