petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- Semi-retired engineer
I didn't even know they made two styles.
Service conductors for 1600A service at 480V
- Thread starter mshields
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I didn't even know they made two styles.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
3-phase VFDs are almost never, if ever, connected L-N, therefore they are not usually subject to 310.15(B)(5)(c).
1-phase VFDs are rarely connected L-N, except for some 120V devices, so typically they do not contribute any current on the neutral conductor.
Read that subsection carefully. It's based on a 4-wire, 3-phase wye circuit. In many to most cases that is what the feeder, service, or sds circuit is, even though the vfd's are not connected to the neutral... and it does not say the determination is based solely on the neutral-connected loads.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
You are right the circuit is the conductors feeding the load. How many times have you brought a neutral conductor (i.e. 4 wire wye) to a VFD?
But it is the system, the conductors from the source, that is 3 phase 4 wire wye .
Carultch
Senior Member
- Location
- Massachusetts
Few, if any, products of manufactured equipment (disconnects, panelboards, etc) will be listed as a system for you to use take credit for the 90C conductor ampacity values. And this is the case, even if you have lugs that are labeled AL9CU. The equipment as as system must be rated for 90C, not just the lug manufactured as part of the equipment.
It is only when you use a separately installed connector in the field on both sides, that is rated for 90C, that you can use the 90C values at terminations. Such connectors do exist. You need 90C insulation on both sides, and a connector that is listed for 90C. The segment of wire connecting to a piece of manufactured equipment needs 75C wire sizing.
I haven't seen an exact code reference clarifying whether or not this splicing needs to occur in a separate enclosure. For a conservative approach, it should occur in a separate enclosure.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
Nice diagram, thanks!
Service and feeder conductors are not circuit conductors??? I believe they are. Please cite a section which says they are not?
FWIW, that diagram is from the SqD bulletin on terminations:
http://www2.schneider-electric.com/...2/en_US/Wire Terminations 0110DB9901R2-02.pdf
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
In Article 100 the definition for both Branch and Feeder expressly use the word 'circuit', however the definitions for 'Service' do not.
I believe my conductor definition was in compliance with the intent of the NEC to acknowledge that systems may be different than circuits.
All the same, do we not determine size/ampacity per Article 310 for the lot?
By what you are saying, service conductors (and perhaps feeder conductors [not quite sure how to take your reply]) would be excluded from a 310.15(B)(5)(c) consideration... and I do not believe that is the intent of the NEC.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
I said that 3-phase VFDs and the majority of 1-phase VFDs do not contribute current (harmonic or otherwise) on the neutral conductor, therefore per 310.15(B)(5) the neutral conductor may not need to be considered.
My point is: just because VFDs (Line-Line connected) are becoming more popular, we should not expect neutral conductors to need to be sized drastically different than they have historically been done.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
+1
No matter how large the harmonic content, a non-linear load like a rectifier bridge or a luminaire with ballast which only connects to the three ungrounded conductors (three phase) or the two ungrounded conductors (single phase) cannot put any current on the neutral.
And I'm not saying line-to-line or 3Ø non-linear loads contribute to neutral current.
What I am saying is 310.15(B)(5)(c) states "where the major portion of the load consists of nonlinear loads", that majority load is with respect to the total [circuit] load, not just the neutral-connected [circuit] load. Probably not the intent of the Code, but that's the way it's worded. If you are saying it says only the neutral-connected load is considered, then you are reading more into it than is stated.
I checked and there are no PI's on this section or else I'd submit a Comment to see. Is there another way to submit a comment on an otherwise unrevised section? I'll have to investigate...
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Wow, the title of the NEC section is NEUTRAL CONDUCTOR, but you say that is not the conductor the code is discussing when it mentions the portion of loading.
I have had enough trolling.
Apply the rules of English sentence structure to this:
"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; ..."
- Location
- Massachusetts
If the neutral is not being used it is not part of the circuit is it?
To me that wording is ambiguous at best.
If a neutral is run but isn't being used (i.e. no load connected to it), it is not a 4-wire circuit.... which covers vfd branch circuits... but where you have a service or feeder with both vfd loads and neutral connected loads, it is a 4-wire circuit.
mivey
Senior Member
I have been using ten feet. Can't remember where it came from but the best I can recall it was based on some published language. I also recall something about how far you were from the cooler underground ambient when transitioning from underground but it escapes me at the moment.
I think the 10' came from someone(s) trying to parallel concepts for this and 310.15(A)(2) Exception... but I have no proof. :roll:
kingpb
Senior Member
- Location
- SE USA as far as you can go
- Occupation
- Engineer, Registered
Just sticking in my 0.02; but wouldn't the harmonics from a 3ph or L-L drive (without a neutral) still generate harmonics on the 3ph-4W system?
It would seem that harmonics could be present on the 3ph-4W neutral system (service); albeit possibly reduced due to taking into account the canceling effect from other loads/circuits on the same board; but still enough that the neutral would need to be considered current carrying.
It would seem that harmonics could be present on the 3ph-4W neutral system (service); albeit possibly reduced due to taking into account the canceling effect from other loads/circuits on the same board; but still enough that the neutral would need to be considered current carrying.
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