Sizing grounded service conductor for 3 phase 4 wire delta
- Thread starter rhovee
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- Placerville, CA, USA
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- Retired PV System Designer
If all of your loads are wired line to line, but the service is a grounded wye, the neutral conductor must be brought to the service disconnect but need not be carried any further.
For circuits where there are any line to neutral loads, possibly balanced, the NEC allows for the neutral to be reduced in size compared to the ungrounded conductors. The basic rule is that the neutral must be able to carry the calculated neutral current. I am still searching for the related articles.
In the case of line to neutral loads with high harmonic content (non-linear loads) the neutral may even have to be larger then the ungrounded circuit conductors.
Rhovee,
The neutral for a three-phase, four-wire delta must be sized for the maximum unbalance that can occur between the A- and C-phases (220.61) or 250.24(C) whichever is larger. Where paralleled No. 1/0 is the smallest conductor permitted. I have never seen a four-wire delta with a neutral the same size as the A- and C-phases. This owing to the fact that the A- and C-phases must be sized for the sum of three-phase load and single-phase load. The neutral is sized only for the unblanced load on the single-phase transformer.
The neutral for a three-phase, four-wire delta must be sized for the maximum unbalance that can occur between the A- and C-phases (220.61) or 250.24(C) whichever is larger. Where paralleled No. 1/0 is the smallest conductor permitted. I have never seen a four-wire delta with a neutral the same size as the A- and C-phases. This owing to the fact that the A- and C-phases must be sized for the sum of three-phase load and single-phase load. The neutral is sized only for the unblanced load on the single-phase transformer.
rlundsrud
Senior Member
- Location
- chicago, il, USA
I believe there is a rule for services 1200 amps and greater that allow the neutral to be smaller than the phase conductors.
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
yes there is a rule however there is also a rule that allows you to calculate the neutral load and then use a conductor sized accordingly. The only catch ist hat the grounded conductor must be sized at least as large as the gec. So I can have a 20amp service and have a #4 grounded conductor if that was sized for the load.
I don't think so. In most cases, the grounded/grounding electrode/equipment grounding conductor is never required to be larger than ungrounded supply conductors.
Perhaps you meant a 200A service with no line-to-neutral loads, the grounded service conductor must still be sized at #4 Cu.
mbrooke
Batteries Included
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- United States
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- Technician
I take it you typed 200amps but the computer was fed from a 20amp circuit, so it adjusted accordingly.
kwired
Electron manager
- Location
- NE Nebraska
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- EC
Service conductors(including the grounded conductor) are sized according to 230.42.
230.42(A)(1) says "The sum of the noncontinuous loads plus 125 percent of continuous loads"
The exception that follows says "Grounded conductors that are not connected to an overcurrent device shall be permitted to be sized at 100 percent of the continuous and noncontinuous load." So we typically only need to size the grounded conductor to 100 percent of load. The catch is there is a minimum size.....
230.42(C) says: "The grounded conductor shall not be smaller than the minimum size as required by 250.24(C)"
230.42(A)(1) says "The sum of the noncontinuous loads plus 125 percent of continuous loads"
The exception that follows says "Grounded conductors that are not connected to an overcurrent device shall be permitted to be sized at 100 percent of the continuous and noncontinuous load." So we typically only need to size the grounded conductor to 100 percent of load. The catch is there is a minimum size.....
230.42(C) says: "The grounded conductor shall not be smaller than the minimum size as required by 250.24(C)"
Also keep in mind that 100% of the load can be determined by 220.61...
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