8.7 Neutral Load Calculations and Harmonics — 220.61
Key Takeaways
- NEC 220.61(A) sets the feeder or service neutral load as the maximum unbalance of the load, calculated as the maximum net load between the neutral and any one ungrounded conductor.
- NEC 220.61(B)(1) permits the neutral load for household electric ranges, wall-mounted ovens, counter-mounted cooking units, and electric dryers to be taken at 70 percent of the load on the ungrounded conductors.
- NEC 220.61(B)(2) permits a further 70 percent factor on that portion of the unbalanced load in excess of 200 amperes for a 3-wire DC or single-phase AC system, or a 4-wire three-phase, 3-wire two-phase, or 5-wire two-phase system.
- NEC 220.61(C) prohibits any reduction for the neutral of a 3-wire circuit consisting of two ungrounded conductors and the neutral of a 4-wire three-phase wye system, and for the portion consisting of nonlinear loads on a 4-wire wye system.
- NEC 250.24(C) requires the grounded service conductor to be not smaller than the size required by Table 250.102(C)(1), which can govern over the calculated unbalanced load.
Why the Neutral Is Different
On a 120/240-volt single-phase system, the two ungrounded conductors are 180 degrees apart. A load connected line-to-line uses no neutral at all. A load connected line-to-neutral on leg A is partly cancelled by a load on leg B. The neutral therefore carries only the imbalance — often far less than either ungrounded conductor.
The Code lets you take advantage of that, within limits.
The Basic Rule — 220.61(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 conductor and any one ungrounded conductor.
Exception: A service or feeder supplying line-to-line loads only shall be permitted to have the grounded conductor sized in accordance with 250.24(C) for services or 250.102(D) for feeders.
The first move in any neutral calculation is therefore to subtract every straight line-to-line load from the total. In a dwelling that typically removes:
- 240-volt electric water heaters with no neutral;
- 240-volt baseboard heat with no neutral;
- 240-volt air conditioning with no neutral;
- 240-volt well pumps and similar two-wire loads.
What remains — general lighting, small appliance, laundry, and the line-to-neutral portion of ranges and dryers — forms the neutral load.
The Permitted Reductions — 220.61(B)
220.61(B)(1) 300-Volt to Neutral Loads. 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, shall be permitted to have the neutral load calculated at 70 percent of the load on the ungrounded conductors.
220.61(B)(2) Further Reductions. There shall be permitted a further demand factor of 70 percent to 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;
- a 4-wire, 3-phase system;
- a 3-wire, 2-phase system; or
- a 5-wire, 2-phase system.
Worked Example — Range and Dryer Neutral
A dwelling has a 12 kW range (Table 220.55 demand 8,000 VA) and a 5 kW dryer.
Range neutral 8,000 x 0.70 = 5,600 VA
Dryer neutral 5,000 x 0.70 = 3,500 VA
Worked Example — the 200-Ampere Reduction
A 120/240-volt single-phase feeder has a calculated maximum unbalanced load of 280 amperes.
First 200 A at 100% ................... 200 A
Remaining 80 A at 70% ................. 56 A
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Net neutral demand .................... 256 A
Worked Example — Full Dwelling Neutral
Return to the 2,400-square-foot Anchorage dwelling from section 8.4, which calculated to 37,120 VA on the ungrounded conductors.
Net lighting group (line-to-neutral) 6,045 VA
Fixed appliances:
Water heater 4,500 VA is 240 V, no neutral -> OMIT
Dishwasher 1,200, disposal 900, compactor 1,500
Sum 3,600 x 0.75 = 2,700 VA
Dryer 5,000 x 0.70 = 3,500 VA
Range 8,000 x 0.70 = 5,600 VA
Electric heat 12,000 VA is 240 V, no neutral -> OMIT
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Neutral calculated load 17,845 VA
Neutral current = 17,845 / 240 = 74.4 A
Compare with the 154.7-ampere ungrounded conductor load. The neutral is less than half — but see the floor rule below.
The Prohibited Reductions — 220.61(C)
There shall be no reduction of the neutral or grounded conductor capacity applied to the amount in 220.61(B)(2) for:
- Any portion of a 3-wire circuit consisting of 2 ungrounded conductors and the neutral conductor of a 4-wire, 3-phase, wye-connected system;
- That portion consisting of nonlinear loads supplied from a 4-wire, wye-connected, 3-phase system.
Why Triplen Harmonics Block the Reduction
In a balanced three-phase wye system with linear loads, the three phase currents are 120 degrees apart and sum to zero in the neutral. That is the physical basis for treating the neutral as lightly loaded.
Nonlinear loads — switch-mode power supplies, LED drivers, variable-frequency drives, electronic ballasts, computer equipment — draw current in short pulses near the voltage peak rather than sinusoidally. That distorted waveform contains harmonics. The third harmonic and its odd multiples (the ninth, fifteenth, and so on) are called triplen harmonics, and they have a crucial property: on a three-phase system they are in phase with each other on all three phases.
Because they are in phase, they do not cancel in the neutral — they add arithmetically. In a heavily nonlinear panel, the neutral can carry more current than any phase conductor, sometimes approaching 1.73 times the phase current in extreme cases.
This is why:
- 220.61(C)(2) forbids neutral reduction for the nonlinear portion;
- 310.15(E)(3) counts the neutral as a current-carrying conductor for adjustment purposes where the major portion of the load is nonlinear;
- Designers frequently specify oversized neutrals or separate neutrals per phase in data centers and LED-heavy retrofits.
Item (C)(1) addresses a related geometry: two phases of a wye system plus the neutral. With only two of the three phases present, the currents are 120 degrees apart rather than 180, so the neutral carries roughly the same magnitude as each phase rather than the difference.
The Floor: 250.24(C) and Table 250.102(C)(1)
The calculated neutral load is a minimum based on current. A separate rule sets a minimum based on the size of the ungrounded conductors:
250.24(C) Grounded Conductor Brought to Service Equipment. Where an AC system operating at 1000 volts or less is grounded at any point, the grounded conductor shall be routed with the ungrounded conductors to each service disconnecting means and shall be connected to each disconnecting means grounded conductor terminal or bus. The grounded conductor shall not be smaller than the required grounding electrode conductor specified in Table 250.102(C)(1).
The reason is fault current, not load current. During a line-to-ground fault at the service, the grounded conductor is the return path back to the source. It must be large enough to carry that fault current long enough for the overcurrent device to operate.
250.24(C)(1) covers the sizing for single raceway or cable, and 250.24(C)(2) covers parallel conductors, requiring the grounded conductor in each raceway to be sized on the basis of the ungrounded conductors in that raceway, and not smaller than 1/0 AWG.
Worked Example — the Floor Governs
The Anchorage dwelling above calculated a 74.4-ampere neutral, which 4 AWG copper (85 A at 75 degrees C) would satisfy. The ungrounded service conductors are 2/0 AWG copper.
From Table 250.102(C)(1), service-entrance conductors of 2/0 copper require a minimum grounded conductor of 4 AWG copper. Here the calculated load and the table happen to converge on the same answer. Had the calculated neutral come out at 40 amperes — satisfied by 8 AWG — the table would still force 4 AWG.
Always run both checks and take the larger.
Feeders — 250.102(D)
For feeders, the analogous rule is 215.2(A)(2) for ampacity and the supply-side bonding jumper rules of 250.102(D) where applicable. A feeder grounded conductor used as the return for line-to-neutral loads is sized by 220.61; where a feeder supplies only line-to-line loads and the grounded conductor is present solely for bonding or system reference, the sizing follows the bonding-jumper rules rather than a load calculation.
Summary Procedure
1. Start from the Article 220 calculated load.
2. REMOVE every straight line-to-line load with no neutral.
3. Apply 70% to ranges, ovens, cooking units, and dryers [220.61(B)(1)]
4. Apply 70% to the portion above 200 A, if the system qualifies [220.61(B)(2)]
5. Do NOT reduce:
- two phases plus neutral of a 4-wire wye [220.61(C)(1)]
- the nonlinear portion on a 4-wire wye [220.61(C)(2)]
6. Convert to amperes and select a conductor.
7. Check the floor: Table 250.102(C)(1) via 250.24(C).
8. Use the LARGER of steps 6 and 7.
A 120/240-volt single-phase feeder has a maximum unbalanced load of 320 amperes. What is the net neutral load after applying NEC 220.61(B)(2)?
Why does NEC 220.61(C)(2) prohibit neutral load reduction for nonlinear loads on a 4-wire, three-phase wye system?
A dwelling service supplies a 240-volt electric water heater with no neutral connection. How is that load treated in the neutral calculation?
A calculated neutral load would permit an 8 AWG copper grounded conductor, but the ungrounded service-entrance conductors are 2/0 AWG copper. What size grounded conductor is required?