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Here's the Derating Discussion!!! Yee-Haww #42713 09/24/04 08:59 PM
Joined: Oct 2000
Posts: 2,721
Scott35 Offline OP
Broom Pusher and
Member
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
Re: Here's the Derating Discussion!!! Yee-Haww #42714 09/25/04 01:46 AM
Joined: May 2003
Posts: 2,876
E
e57 Offline
Member
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
Re: Here's the Derating Discussion!!! Yee-Haww #42715 09/25/04 01:58 AM
Joined: May 2003
Posts: 2,876
E
e57 Offline
Member
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
Re: Here's the Derating Discussion!!! Yee-Haww #42716 09/25/04 05:56 AM
Joined: Oct 2000
Posts: 2,721
Scott35 Offline OP
Broom Pusher and
Member
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!
Re: Here's the Derating Discussion!!! Yee-Haww #42717 09/25/04 10:36 AM
Joined: Jan 2003
Posts: 4,391
I
iwire Offline
Moderator
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
Re: Here's the Derating Discussion!!! Yee-Haww #42718 09/25/04 11:48 AM
Joined: Jul 2004
Posts: 625
S
SolarPowered Offline
Member
But would <10% THD be construed as, "where the major portion of the load consists of nonlinear loads"?

Re: Here's the Derating Discussion!!! Yee-Haww #42719 09/25/04 12:43 PM
Joined: Jan 2003
Posts: 4,391
I
iwire Offline
Moderator
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
Re: Here's the Derating Discussion!!! Yee-Haww #42720 09/25/04 01:18 PM
Joined: Jul 2004
Posts: 625
S
SolarPowered Offline
Member
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.

Re: Here's the Derating Discussion!!! Yee-Haww #42721 09/25/04 01:49 PM
Joined: Aug 2004
Posts: 186
N
NJ Wireman Offline
Member
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.

Re: Here's the Derating Discussion!!! Yee-Haww #42722 09/26/04 06:57 AM
Joined: Dec 2000
Posts: 4,283
electure Offline
Member
Isn't the use of a 1" raceway a design issue?

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