derating /counting neutral in 4 wire 208 3 phase

michael1960

Member
Location
idaho
im confused!! I know 310.15 [4] [c] "clarifies " but im still way confused! its counted when greater than 50% of load is unbalanced. Which i think would mean a two hot plus neutral sub panel coming from that 4 wire 3ph main main panel.
but moving upstream how do you calculate feed to main panel. say a 100 amp 3 ph 4 wire 75 degree panel , you hope to use MC armored 1-1-1-1-4 alum XHHW 90 degree is 115 amps, but the termination is 75 degree main panel breaker so we use the 75 column- still ok at 100 amps. but would it be considered a ciircuit of 100 amps or less of AWG 1 or less and need to be sized from the 60 degree column? now termination is its a residential main panel is that considered mostly unbalanced and trigger counting the neutral as current carrying for an additional .80 reduction. this feeder getting bigger than the panel. im no engineer but wouldn't the neutral for this portion of the system be fairly balanced i would think the neutral could even be undersized a bit. I guess I just find it hard to believe a #1 aluminum isnt enough for a 100 amp panel.
 

augie47

Moderator
Staff member
Location
Tennessee
Take a look at 310.15(B)(4) again.
I'm not sure where you are getting your 50% "rule"
On a 208Y/120 system the neutral would count if:
(a) It is one neutral for one phase conductor (not a MWBC)
(b) The neutral is part of a 3 wire circuit (Neutral-Phase-Phase) off of a 4 wire wye system (the 3rd phase not present)
(c) The major portion of the load consists of non-linear, harmonic currents.

You mentioned a residential panel so it is highly unlikely you would have a major non-linear load. You 1-1-1-1-4 Al should be fine
 

roger

Moderator
Staff member
Location
Fl
Occupation
Electrician
I think you are confusing unbalanced with non-linear.

Roger
 

michael1960

Member
Location
idaho
thanks all. i thought i needn't count the neutral in that portion of system unless it had atypical loads, and i admit i do have confusion even still, on unbalanced , non linear ,and harmonic distortion. i am aware of the idyosycracy of theses systems to pick up echos waves or harmonic distortions from the return current of transformers from so many of todays electronics aas well as things like flourescent lights and ironically these very devices are most sensetive to such noise. and im aware increased danger of shared neutrals on these systems particularly if they become unbalanced. but honestly i only vaguely understand. running neutrals for every circuit and using ganged breakers to deter inadvertant unbalancing seems straightforward and not very onerous. but im wondering is the trend more and more of these transformed loads creeping into home use it seems lighting is certainly going that way besides cfls id think leds also will be the same. and of course all the computer/media gizmos whats left a load of laundry or dishes? so i guess i get harmonic distortion and balancing the loads among poles, non linear is hazier. would 208 circuits of two hots fall into this category, say for a air window conditioner?
BTW a 100 amp main panel w #1 alum would not trigger the " circuit of 100 amp or less using awg #1 or less must use the 60 degree column" rule why , because it not a circuit per se?
im considering buying this building and at first thought i liked that iit had the 4 wire 3phase but want to know what id be buying into. i could move my shop into it and have a residential above which is how its been used but i think id want have ale the mep redone.
 

roger

Moderator
Staff member
Location
Fl
Occupation
Electrician
In short and generally speaking, the only balanced circuits you will ever have is two wire circuits, there is no problem with unbalanced loading of MWBC's (barring extreme harmonic distortion) and it is simply the nature of the beast.


Roger
 

infinity

Moderator
Staff member
Location
New Jersey
BTW a 100 amp main panel w #1 alum would not trigger the " circuit of 100 amp or less using awg #1 or less must use the 60 degree column" rule why , because it not a circuit per se?
im considering buying this building and at first thought i liked that iit had the 4 wire 3phase but want to know what id be buying into. i could move my shop into it and have a residential above which is how its been used but i think id want have ale the mep redone.
You're not including the exception to that 60? C rule which states that terminal ratings of 75? C will allow you to size the conductor at it's 75? C ampacity.
 

michael1960

Member
Location
idaho
thanks again can i ask a silly question now . why are the one pole breakers giving almost 120 but a two pole is 208 and it looks like they have been using 220 appliances and getting away with it i calculated it should only be about 75% power do they just run badly is it safe whats curious is the outlets test at almost 120 117
 
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