power design for receptacles

[jcd]

i haven't designed power for duplex outlets in a few years. in an NEC class i learned that you can't design outlets using triplets (a triplet being three hots sharing one neutral and one ground all run together in a conduit). I learned that you must run seperate circuits, i.e., one hot with one neutral and one ground for ea circuit. here is my question: what do you do when you have system's furniture with a 5 wire harness (which requires circuits, 3hots, 1N, and 1gnd) or an 8 wire harness (4 circuits)?

 

[roger]

Re: power design for receptacles

Jcd,

learned that you can't design outlets using triplets (a triplet being three hots sharing one neutral and one ground all run together in a conduit).

whoever told you this is wrong, in otherwords, yes you can.

BTW, correct terminology would be "multi wire branch circuit".

Roger

[ October 10, 2003, 02:11 PM: Message edited by: roger ]

 

[jcd]

Re: power design for receptacles

Originally posted by jcd:

Originally posted by roger:
Jcd,

learned that you can't design outlets using triplets (a triplet being three hots sharing one neutral and one ground all run together in a conduit).

whoever told you this is wrong, in otherwords, yes you can.

BTW, correct terminology would be "multi wire branch circuit".

Roger

thanks! i thought that instructor was goofy. i was just reading about it in 210.4. for dwelling units, if you have a multiwire branch cct, you need to add a deal to the circuitbreakers to make sure the ccts are simultaniously opened or closed.

 

[bob]

Re: power design for receptacles

JCD
It sounds like you are installing circuits for a commercial building. If so, you can run in to problems if the voltage is 120/208 or 277/480 volt. The commercial load will normally consist of computers, copiers, UPS loads and others that will generate 3rd harmonic loads that can overload your neutral. If you are installing 3 phase conductors and one neutral you should consider doubling the neutral size or the other option is to install a separate neutral with each phase. This may be what the instructor was trying to get across to you.

 

[jcd]

Re: power design for receptacles

Originally posted by bob:
JCD
It sounds like you are installing circuits for a commercial building. If so, you can run in to problems if the voltage is 120/208 or 277/480 volt. The commercial load will normally consist of computers, copiers, UPS loads and others that will generate 3rd harmonic loads that can overload your neutral. If you are installing 3 phase conductors and one neutral you should consider doubling the neutral size or the other option is to install a separate neutral with each phase. This may be what the instructor was trying to get across to you.

yes, i am designing for an office space, with computers. thanks for the advice! i will design it using seperate neutrals with each cct, but, i did not know how this woudl mesh with the five wire harness of the systems furniture (which utiliazes is prewired with one neutral, one gnd and 3 phase wires.)

 

[nosparks]

Re: power design for receptacles

JCD the "triplet circuit" that you refer to is a multiwire branch circuit. Multiwire branch circuits are allowed with some consideration to the type of system by code. See 210.4, and in your case 240.20(B)(1).
My opinion: from a design stand point it can very well save you money on pipe and wire or cable for that matter, but if you lose that neutral conductor, you may well impose unwanted voltages to some very expensive office equipment.

Draw your circuit out, then remove the neutral conductor. You should be able to see that the only available path for current to flow is from receptacle to receptacle through the equipment. Therefore, you are applying line to line voltage across equipment that was only intended for line to neutral voltage. I hope this helps.

 

[Guest]

Re: power design for receptacles

Originally posted by jcd:
[<snip>you need to add a deal to the circuitbreakers to make sure the ccts are simultaniously opened or closed.

I believe the proper terminology for "a deal" is "a thingy". Just kidding: It's called a "a Tie-bar".

 

[hurk27]

Re: power design for receptacles

I have installed circuits to many of these office cubicles and most use a over-sized neutral. the last one I did had a # 10 but I have seen ones that had a #6 to a little neutral bar and then just #12 to each outlets. as far as ground wires go there is no need for a separate grounding wire for each circuit. it is there only for fault current and is not considered a current carrying conductor. but remember if the neutral is over sized then you must run the same sized neutral wire back to the main panel. This will be part of the manufactures listing spec's

 

[iwire]

Re: power design for receptacles

Office furniture feeds I deal with on a regular basis and as Wayne mentioned there are many different wiring configurations that the furniture comes in.

Your best bet IMO and how I deal with it as an installer is to find out exactly what the furniture whip (harness) will have for your installation and match it in colors and match or exceed the wire size to compensate for voltage drop.

In my area the most common wiring method is MC and we can get the MC to match any style furniture wiring.

Separate neutrals for each hot or common neutrals.

A very common configuration for us is a furniture whip with six 12 awg hots, two 10 awg neutrals, one 12 awg "dirty" ground and one 12 awg isolated ground.

My main point is whatever the furniture company is providing match it for the easiest installation.

If they are going the common neutral route you should to, if there are going the super neutral route do that.

Here are some examples of the cables available.

 

[caj1962]

Re: power design for receptacles

iwire
I to get involved with the furniture whips you described and have often wondered how they get all those conductors in a 1/2 inch sealtight. I often have asked the furniture people and of course they are just sales or installers and have no clue.

Have a great day.

 

[iwire]

Re: power design for receptacles

Originally posted by caj1962:
I to get involved with the furniture whips you described and have often wondered how they get all those conductors in a 1/2 inch sealtight......Is it because it is a listed assembly and is not covered by the NEC?

I do not know for sure, but I believe you are correct, UL listed assembly not covered by the NEC.

For those of you that do not see this equipment, the factory will put eight 12 awgs (6 hots 2 grounds) and two 10 awgs (2 Neutrals) in 1/2" LFMC.

Read the tags on this stuff when you see it arrive, if it needs 15 amp breakers you should get paid for them if 20 amp breakers were on the print.

[ October 14, 2003, 05:59 AM: Message edited by: iwire ]

 

[Guest]

Re: power design for receptacles

I guess the mfg could also argue that the Greenfield has more cooling surface area than smooth/sealed conduit and Greenfield has some air flow so the Greenfield sinks the heat away from the packed in conductors

If it was Sealtite the extra cooling surface would be locked in with the watertight thermoplastic.

It's interesting if you step back that the beginning and ending of the electrical journey falls under more liberal guidelines. The wo/man in the middle (the electrician) is held to a higher standard than the supplier (PoCo) and the fixture mfg. We might have #2 overhead on the service drop; 3/0 at the service entrance; #18 at the fixture.

 

[iwire]

Re: power design for receptacles

Originally posted by awwt:
If it was Sealtite the extra cooling surface would be locked in with the watertight thermoplastic.

LFMC = ARTICLE 350 Liquidtight Flexible Metal Conduit

I do not believe the manufacturer has to argue about conductor fill, if you work on factory assembled machinery you will find J-Boxes that you need a hammer to close.