Neutral calculation
- Thread starter terille
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- Lockport, IL
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- Semi-Retired Electrical Engineer
Re: Neutral calculation
You're in the wrong section completely. 250 is about grounding, and 250.66 is about the grounding electrode conductor (i.e., from the panel to the ground rod). The size of the neutral is based on the load calculation, per article 220.
You're in the wrong section completely. 250 is about grounding, and 250.66 is about the grounding electrode conductor (i.e., from the panel to the ground rod). The size of the neutral is based on the load calculation, per article 220.
jwelectric
Senior Member
- Location
- North Carolina
Re: Neutral calculation
220.61 Feeder or Service Neutral Load.
(A) Basic Calculation. The feeder or service neutral load shall be the maximum unbalance of the load determined by this article. The maximum unbalanced load shall be the maximum net calculated load between the neutral and any one ungrounded conductor.
Exception: For 3-wire, 2-phase or 5-wire, 2-phase systems, the maximum unbalanced load shall be the maximum net calculated load between the neutral and any one ungrounded conductor multiplied by 140 percent.
(B) Permitted Reductions. A service or feeder supplying the following loads shall be permitted to have an additional demand factor of 70 percent applied to the amount in 220.61(B)(1) or portion of the amount in 220.61(B)(2) determined by the basic calculation:
(1) A feeder or service supplying household electric ranges, wall-mounted ovens, counter-mounted cooking units, and electric dryers, where the maximum unbalanced load has been determined in accordance with Table 220.55 for ranges and Table 220.54 for dryers
(2) That portion of the unbalanced load in excess of 200 amperes where the feeder or service is supplied from a 3-wire dc or single-phase ac system, or a 4-wire, 3-phase; 3-wire, 2-phase system, or a 5-wire, 2-phase system
(C) Prohibited Reductions. There shall be no reduction of the neutral or grounded conductor capacity applied to the amount in 220.61(C)(1), or portion of the amount in (C)(2), from that determined by the basic calculation:
(1) Any portion of a 3-wire circuit consisting of 2-phase wires and the neutral of a 4-wire, 3-phase, wye-connected system
(2) That portion consisting of nonlinear loads supplied from a 4-wire, wye-connected, 3-phase system
FPN No. 1: See Examples D1(A), D1(B), D2(B), D4(A), and D5(A) in Annex D.
FPN No. 2: A 3-phase, 4-wire, wye-connected power system used to supply power to nonlinear loads may necessitate that the power system design allow for the possibility of high harmonic neutral currents.
220.61 Feeder or Service Neutral Load.
(A) Basic Calculation. The feeder or service neutral load shall be the maximum unbalance of the load determined by this article. The maximum unbalanced load shall be the maximum net calculated load between the neutral and any one ungrounded conductor.
Exception: For 3-wire, 2-phase or 5-wire, 2-phase systems, the maximum unbalanced load shall be the maximum net calculated load between the neutral and any one ungrounded conductor multiplied by 140 percent.
(B) Permitted Reductions. A service or feeder supplying the following loads shall be permitted to have an additional demand factor of 70 percent applied to the amount in 220.61(B)(1) or portion of the amount in 220.61(B)(2) determined by the basic calculation:
(1) A feeder or service supplying household electric ranges, wall-mounted ovens, counter-mounted cooking units, and electric dryers, where the maximum unbalanced load has been determined in accordance with Table 220.55 for ranges and Table 220.54 for dryers
(2) That portion of the unbalanced load in excess of 200 amperes where the feeder or service is supplied from a 3-wire dc or single-phase ac system, or a 4-wire, 3-phase; 3-wire, 2-phase system, or a 5-wire, 2-phase system
(C) Prohibited Reductions. There shall be no reduction of the neutral or grounded conductor capacity applied to the amount in 220.61(C)(1), or portion of the amount in (C)(2), from that determined by the basic calculation:
(1) Any portion of a 3-wire circuit consisting of 2-phase wires and the neutral of a 4-wire, 3-phase, wye-connected system
(2) That portion consisting of nonlinear loads supplied from a 4-wire, wye-connected, 3-phase system
FPN No. 1: See Examples D1(A), D1(B), D2(B), D4(A), and D5(A) in Annex D.
FPN No. 2: A 3-phase, 4-wire, wye-connected power system used to supply power to nonlinear loads may necessitate that the power system design allow for the possibility of high harmonic neutral currents.
Re: Neutral calculation
Well 315-5 said it is permitted to be smaller if it complies with 215.2,220.61,230.42
215.2.1 said not smaller than required by 250.122
table 250.122 @300 amps is a #4 cu. table 310.16 @ 75deg thhn 85 amps
230.42.c saidnot lless than 250.24c
250.24c said comply with c.1, c.2, c.3
The only one that pertainent to this installation is c.1
250.24.c.c1 refers you to table 250.66 as not smaller than
table 250.66 shows feeders at 3/0 cu. to be a# 4
once again a #4 thhn @ 75deg is rated for 85 amps.
would you please clarify your answer for me.
Well 315-5 said it is permitted to be smaller if it complies with 215.2,220.61,230.42
215.2.1 said not smaller than required by 250.122
table 250.122 @300 amps is a #4 cu. table 310.16 @ 75deg thhn 85 amps
230.42.c saidnot lless than 250.24c
250.24c said comply with c.1, c.2, c.3
The only one that pertainent to this installation is c.1
250.24.c.c1 refers you to table 250.66 as not smaller than
table 250.66 shows feeders at 3/0 cu. to be a# 4
once again a #4 thhn @ 75deg is rated for 85 amps.
would you please clarify your answer for me.
jwelectric
Senior Member
- Location
- North Carolina
Re: Neutral calculation
What you are reading is saying that the grounded (neutral) can never be smaller than the grounding (bonding) conductor or the grounding electrode conductor.
The way to calculate (compute) this conductor is by 220. 61. I posted the section for you so you could read it. Look at the title of the section: ?Feeder or Service Neutral Load.?
In your original post you stated that this is for a dwelling unit. Let?s look at a SE cable sized for the service. When you go to the supply house and buy the SE cable the grounded (neutral) will always be two trade sizes smaller. This is a good clue to what size the grounded (neutral) will need to be.
What you are reading is saying that the grounded (neutral) can never be smaller than the grounding (bonding) conductor or the grounding electrode conductor.
The way to calculate (compute) this conductor is by 220. 61. I posted the section for you so you could read it. Look at the title of the section: ?Feeder or Service Neutral Load.?
In your original post you stated that this is for a dwelling unit. Let?s look at a SE cable sized for the service. When you go to the supply house and buy the SE cable the grounded (neutral) will always be two trade sizes smaller. This is a good clue to what size the grounded (neutral) will need to be.
john m. caloggero
Senior Member
Re: Neutral calculation
There are a couple of items that must be considered when determining the size of grounded conductor (neutral)in this situation. The grounded conductor size is based on the maximum unbalance that can occur between an ungrounded conductor and the grounded conductor (neutral), HOWEVER, it can never be smaller than the size of the grounded conductor required by Table 250.66 based on the size of the size of the ungrounded service entrance conductors.
There are a couple of items that must be considered when determining the size of grounded conductor (neutral)in this situation. The grounded conductor size is based on the maximum unbalance that can occur between an ungrounded conductor and the grounded conductor (neutral), HOWEVER, it can never be smaller than the size of the grounded conductor required by Table 250.66 based on the size of the size of the ungrounded service entrance conductors.
ryan_618
Senior Member
- Location
- Salt Lake City, Utah
Re: Neutral calculation
Guys, we need to remember that the grounded service conductor is the fault clearing conductor after the fault hits the main (or system) bonding jumper. For this reason, there must a minimum size to carry fault current...regardless of the nuetral load.
Read Mr. Calogerro's post again, which is right on.
Guys, we need to remember that the grounded service conductor is the fault clearing conductor after the fault hits the main (or system) bonding jumper. For this reason, there must a minimum size to carry fault current...regardless of the nuetral load.
Read Mr. Calogerro's post again, which is right on.
pierre
Senior Member
- Location
- Westchester County, New York
Re: Neutral calculation
Take a look at the examples in Annex D
D1(a) and D1(b).
Take a good look and it shows the calculations necessary, including 220.61.
As has been stated, 250.66 would be the minimum size possible without the calculations.
In example D1(a), the calculated neutral is a 6 AWG. If you use table 250.66 it would be an 8 AWG. That is the difference.
[ October 26, 2005, 01:00 AM: Message edited by: pierre ]
Take a look at the examples in Annex D
D1(a) and D1(b).
Take a good look and it shows the calculations necessary, including 220.61.
As has been stated, 250.66 would be the minimum size possible without the calculations.
In example D1(a), the calculated neutral is a 6 AWG. If you use table 250.66 it would be an 8 AWG. That is the difference.
[ October 26, 2005, 01:00 AM: Message edited by: pierre ]
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