I am currently doing a job that requires approx. 25 20A 120V circuits to be used to feed individual computer work stations. Panel is 3 phase 120/208V 225A. I would love to only have to use one neutral(grounded circuit conductor) per 3 "hots", but since there is noway of knowing which work stations will be used and when, I feel that the neutrals will have to be counted as current carrying conductors. Am I correct?
multiwire branch circuits
- Thread starter metfan
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websparky
Senior Member
- Location
- Cleveland, Ohio
Re: multiwire branch circuits
Most definitely!
I would also suggest you increase the neutral size to 10AWG.
[ August 30, 2003, 05:11 PM: Message edited by: websparky ]
Most definitely!
Computers are nonlinear loads and will have harmonic currents.
I would also suggest you increase the neutral size to 10AWG.
[ August 30, 2003, 05:11 PM: Message edited by: websparky ]
Ed MacLaren
Senior Member
Re: multiwire branch circuits
I believe that you are correct.
Actually, computer work station loads are often high in harmonic content, and can require oversized neutral conductors.
The so-called "Super Neutral" MC cables are made for just such an installation.
Ed
I believe that you are correct.
Actually, computer work station loads are often high in harmonic content, and can require oversized neutral conductors.
The so-called "Super Neutral" MC cables are made for just such an installation.
Ed
websparky
Senior Member
- Location
- Cleveland, Ohio
Re: multiwire branch circuits
Absolutely Ed!
Absolutely Ed!
Re: multiwire branch circuits
Thanks for the replies. Another "problem" I have with making this installation NEC conforming is that these circuits are to be installed in an under floor raceway system and portions of the raceway will have more than 20 current carrying conductors. Assuming these must be 20A circuits is there anyway I can still use #10 wire?
Thanks for the replies. Another "problem" I have with making this installation NEC conforming is that these circuits are to be installed in an under floor raceway system and portions of the raceway will have more than 20 current carrying conductors. Assuming these must be 20A circuits is there anyway I can still use #10 wire?
websparky
Senior Member
- Location
- Cleveland, Ohio
Re: multiwire branch circuits
What product is the raceway? Is it a type of Wiremold? Is it an in-the-floor raceway?
Usually, you can not exceed 40 percent of the interior cross-sectional area of the raceway.
Don't forget, even if your conductors will fit, the existing conductor sizes would also have to be considered for the derating factors!
What product is the raceway? Is it a type of Wiremold? Is it an in-the-floor raceway?
Usually, you can not exceed 40 percent of the interior cross-sectional area of the raceway.
Don't forget, even if your conductors will fit, the existing conductor sizes would also have to be considered for the derating factors!
websparky
Senior Member
- Location
- Cleveland, Ohio
Re: multiwire branch circuits
OK,
3 phase 4 wire panel.
24 ungrounded conductors plus 8 grounded (neutral) conductors equal 32 current carrying conductors.
10AWG THHN is rated at 40A.
31-40 current carrying conductors at 40% capacity equals 16A which equals 15A OCD's.
However, most 20A OCD's are rated at 80% of marking which is 16A.
You make the call!
OK,
3 phase 4 wire panel.
24 ungrounded conductors plus 8 grounded (neutral) conductors equal 32 current carrying conductors.
10AWG THHN is rated at 40A.
31-40 current carrying conductors at 40% capacity equals 16A which equals 15A OCD's.
However, most 20A OCD's are rated at 80% of marking which is 16A.
You make the call!
Re: multiwire branch circuits
"24 ungrounded plus 8 neutrals = 32 current carrying conductors (32 ccc)"
Just a few suggestions earlier it is suggested a neutral with each circuit. If so, the BC (branch circuit) neutral need not be larger than the ungrounded conductor.
24 ungrounded plus 24 grounded conductors = 48 ccc, = 35% adjustment factor.
48 12AWG @ 90? = 30A * 35% = 10.5A requiring a 15A OCPD.
48 10AWG @ 90? = 40A * 35% = 14A requires a 15A OCPD.
If 24 circuits required, suggest using the 10AWG and keep the load(s) below the 14A FLA, unless the load will be continuous.
"24 ungrounded plus 8 neutrals = 32 current carrying conductors (32 ccc)"
Just a few suggestions earlier it is suggested a neutral with each circuit. If so, the BC (branch circuit) neutral need not be larger than the ungrounded conductor.
24 ungrounded plus 24 grounded conductors = 48 ccc, = 35% adjustment factor.
48 12AWG @ 90? = 30A * 35% = 10.5A requiring a 15A OCPD.
48 10AWG @ 90? = 40A * 35% = 14A requires a 15A OCPD.
If 24 circuits required, suggest using the 10AWG and keep the load(s) below the 14A FLA, unless the load will be continuous.
joe tedesco
Senior Member
- Location
- Boston, Massachusetts
Re: multiwire branch circuits
See also 2002 NEC, ARTICLE 374 Cellular Metal Floor Raceways
Disclaimers: www.joetedesco.com
See also 2002 NEC, ARTICLE 374 Cellular Metal Floor Raceways
Joe tedesco
Disclaimers: www.joetedesco.com
- Location
- Illinois
- Occupation
- retired electrician
Re: multiwire branch circuits
gwz2,
Remember if these are multioutlet circuits for cord and plug connected equipment that you cannot use the next higher standard size OCPD. See 240.4(B).
gwz2,
Remember if these are multioutlet circuits for cord and plug connected equipment that you cannot use the next higher standard size OCPD. See 240.4(B).
Re: multiwire branch circuits
Don, oversight on my part again.
About a year ago, we had the same discussion issue of receptacle outlets on branch circuits where the ampacity [ derating by ambient temperature and raceway fill ] of 10 AWG conductors were down to [ as I remember ] 12.5 ampacity and some of the loads were receptacle loads.
The actual NEC Section would be 210.19(A)(2) for branch circuit(s) for multiple receptacles.
'Conductors of branch circuits supplying more than one receptcale for cord-and-plug-connected portable loads shall have an ampacity of not less than the rating of the branch circuit'. Thus, the branch circuit conductors [ for multiple receptacles ] connected a 15A OCPD must have an ampacity of 15 amperes.
For a single receptacle on a branch circuit, 210.20 Overcurrent Protection. 210.20(D) ' Outlet Devices ( plural ). The rating or setting shall not exceed that specified in 210.21 for outlet devices ( plural ).' Then 210.21(B)(1) Single Receptacles., tells that the single receptacle must must have a rating not less than the branch circuit.
Kinda a catch-22. Branch circuit over-current device [ for receptacle ] can not be greater than conductor ampacity [ in this example, less than 15A, 210.19(A)(1) ] and the receptacle load and the receptacle rating can not be less than the OCPD.
Don, oversight on my part again.
About a year ago, we had the same discussion issue of receptacle outlets on branch circuits where the ampacity [ derating by ambient temperature and raceway fill ] of 10 AWG conductors were down to [ as I remember ] 12.5 ampacity and some of the loads were receptacle loads.
The actual NEC Section would be 210.19(A)(2) for branch circuit(s) for multiple receptacles.
'Conductors of branch circuits supplying more than one receptcale for cord-and-plug-connected portable loads shall have an ampacity of not less than the rating of the branch circuit'. Thus, the branch circuit conductors [ for multiple receptacles ] connected a 15A OCPD must have an ampacity of 15 amperes.
For a single receptacle on a branch circuit, 210.20 Overcurrent Protection. 210.20(D) ' Outlet Devices ( plural ). The rating or setting shall not exceed that specified in 210.21 for outlet devices ( plural ).' Then 210.21(B)(1) Single Receptacles., tells that the single receptacle must must have a rating not less than the branch circuit.
Kinda a catch-22. Branch circuit over-current device [ for receptacle ] can not be greater than conductor ampacity [ in this example, less than 15A, 210.19(A)(1) ] and the receptacle load and the receptacle rating can not be less than the OCPD.
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