Motor Control: Starters, VFDs, and Contactors
Sizing motor starters (NEMA vs IEC), spec'ing VFDs (Schneider Altivar, Eaton PowerXL, Siemens SINAMICS), and picking the right contactor family — Square D 8536, Eaton Freedom, Siemens SIRIUS. HP tables, enclosure ratings, and application decision trees.
Motor control is where residential-scale electrical work ends and industrial spec-out starts. A single 50 HP motor on a chilled-water pump involves at minimum: a fused disconnect at the branch circuit, a motor starter (NEMA size 3 or IEC equivalent), overload protection sized to the motor's full-load amps and service factor, and — increasingly — a variable-frequency drive (VFD) upstream of the motor for soft-start and speed control. All three of the majors (Square D / Schneider, Eaton, Siemens) sell full stacks; PES Distribution stocks the working inventory of all three.
NEMA starter sizes are set by NEMA ICS 2. Each size has a defined maximum horsepower at each standard motor voltage (200 V, 230 V, 460 V, 575 V). Sizing rule: pick the smallest starter whose HP rating at your motor's voltage equals or exceeds the motor nameplate HP. Do not undersize — a starter that clicks in and out on inrush current will weld its contacts within months.
| NEMA size | Coil-contact current (A) | Max HP @ 200 V 3P | Max HP @ 230 V 3P | Max HP @ 460 V 3P | Max HP @ 575 V 3P |
|---|---|---|---|---|---|
| 00 | 9 A | 1.5 HP | 1.5 HP | 2 HP | 2 HP |
| 0 | 18 A | 3 HP | 3 HP | 5 HP | 5 HP |
| 1 | 27 A | 7.5 HP | 7.5 HP | 10 HP | 10 HP |
| 2 | 45 A | 10 HP | 15 HP | 25 HP | 25 HP |
| 3 | 90 A | 25 HP | 30 HP | 50 HP | 50 HP |
| 4 | 135 A | 40 HP | 50 HP | 100 HP | 100 HP |
| 5 | 270 A | 75 HP | 100 HP | 200 HP | 200 HP |
| 6 | 540 A | 150 HP | 200 HP | 400 HP | 400 HP |
| 7 | 810 A | — | 300 HP | 600 HP | 600 HP |
⚠ NEMA starters are oversized on purpose — do not swap to IEC without checking service factor.
IEC starters (Schneider TeSys, Eaton XT, Siemens SIRIUS 3RT) are sized much closer to the motor FLA. They cost less and take less panel space, but they have a much narrower duty tolerance — a NEMA size 1 will survive years of hard-start abuse that will cook an IEC starter rated for the same motor. Use NEMA on process-plant duty (frequent starts, high inrush, dusty enclosures); use IEC on light-duty and OEM machine builds where the coordination and duty cycle are engineered.
| Starter family | NEMA or IEC | Typical brand + part prefix | Panel-space (cubic in per starter) |
|---|---|---|---|
| Square D 8536 | NEMA | 8536SCG3V02H30 (size 1, 460 V, NEMA 3R) | ~180 in³ |
| Square D TeSys D | IEC | LC1D18BD (18 A, 24 VDC coil) | ~50 in³ |
| Eaton Freedom Series | NEMA | AN16BN0AC (size 1, 3-pole, non-reversing) | ~180 in³ |
| Eaton XT (IEC) | IEC | XTCE032C10T (32 A, 24 VAC coil) | ~50 in³ |
| Siemens SIRIUS 3RT (IEC) | IEC | 3RT2026-1AC20 (25 A, 24 VAC coil) | ~50 in³ |
| Siemens Class 14 (NEMA) | NEMA | 14CUB320B (size 1, 460 V, non-reversing) | ~180 in³ |
| Allen-Bradley 100-C (IEC) | IEC | 100-C43UEJ00 (43 A, 24 VDC coil) | ~55 in³ |
Every motor starter needs overload protection sized to the motor's full-load amps and service factor per NEC 430.32. Three technologies are in the field today: bimetallic thermal (the legacy heater-element approach), solid-state electronic (adjustable set point), and communication-networked electronic (EtherNet/IP or PROFIBUS reporting to a PLC).
| Overload type | Adjustment | Reset | Trip class | Typical brand |
|---|---|---|---|---|
| Bimetallic thermal (heater) | Fixed by heater size | Manual or auto | Class 10 or 20 (fixed) | Square D B-series heaters |
| Solid-state adjustable | Dial (0.4× – 1.2× FLA) | Manual or auto | Class 10 / 20 / 30 selectable | Eaton C440, Siemens 3RB2, Square D LR9D |
| Networked electronic | Software (via EIP/PB) | Manual, auto, or remote | Class 5 – 40 field-programmable | Allen-Bradley E300, Siemens SIMOCODE, Schneider TeSys T |
Choosing trip class — what actually matters
- Class 10 — trips at 6× FLA in 10 seconds. Standard for centrifugal loads (fans, pumps).
- Class 20 — trips at 6× FLA in 20 seconds. Standard for higher-inertia loads (compressors, conveyors).
- Class 30 — trips at 6× FLA in 30 seconds. High-inertia loads (large fans, mills, hoists) that take a long time to accelerate.
- Wrong class = nuisance trips (class too short) or motor damage (class too long). Cross-check with the motor manufacturer's start-time curve.
A VFD replaces the fixed-speed motor starter with a solid-state drive that varies the voltage and frequency to the motor. Benefits: soft-start (eliminates inrush), speed control (matches motor RPM to load demand — huge energy savings on centrifugal loads), and torque control (precise process control). Three VFD families dominate the industrial market:
| VFD family | HP range | Voltage | Typical application | Comms |
|---|---|---|---|---|
| Schneider Altivar 320 | 0.5 – 30 HP | 200 / 240 / 480 V | OEM machine building, HVAC pumps and fans | Modbus TCP, CANopen, PROFIBUS |
| Schneider Altivar 630 / 930 | 1 – 900 HP | 380 – 690 V | Process industry — pumps, compressors, extruders | Full industrial fieldbus |
| Eaton PowerXL DG1 | 0.5 – 250 HP | 230 / 480 / 600 V | General-purpose industrial | Modbus RTU/TCP, BACnet, PROFINET, EIP |
| Eaton PowerXL DH1 | 5 – 700 HP | 480 / 600 V | HVAC + heavy-duty pump/fan | BACnet MS/TP + IP, Modbus, EIP |
| Siemens SINAMICS G120 | 0.5 – 400 HP | 200 – 690 V | General-purpose + regeneration | PROFINET, EtherNet/IP, PROFIBUS DP |
| Siemens SINAMICS S210 | 0.7 – 100 HP | 400 V | Servo-motor precise motion control | PROFINET IRT |
| Allen-Bradley PowerFlex 525 | 0.5 – 30 HP | 200 – 600 V | Micro-drive OEM | EtherNet/IP, DSI |
| Allen-Bradley PowerFlex 755 | 0.75 – 1500 HP | 230 – 6600 V | Heavy industrial (mills, extruders) | Full CIP + full Rockwell integration |
⚠ VFDs generate harmonics — over 5% THD you need a line reactor or a harmonic filter.
IEEE 519 limits current THD at the point of common coupling to 5 % on typical industrial services. A standard 6-pulse VFD produces 30–40 % THD on its input. Solutions: 3 % or 5 % line reactor (cheapest, drops THD to ~30 %), 5 % DC-link choke (built into most modern drives, similar effect), 12-pulse or 18-pulse rectifier (drops THD to 10 % and 5 % respectively — expensive), or an active harmonic filter. Every VFD spec should include a harmonic study on services larger than 500 kVA.
A contactor is a solenoid-actuated 3-pole switch — the switching element behind a starter and behind a PLC-driven relay bank. Contactors are sized by their utility-category rating (AC-1 for resistive, AC-3 for standard motor duty, AC-4 for plugging / jogging) and their coil voltage (24 VDC and 120 VAC dominate industrial installs; 230 / 480 VAC used where a control transformer isn't available).
| Contactor family | AC-3 rating | Poles | Coil voltage options | Aux contacts |
|---|---|---|---|---|
| Square D 8910 (definite-purpose) | 20 – 90 A | 1 / 2 / 3 / 4 | 24, 120, 208, 240, 480 VAC | Fixed |
| Square D LC1D (TeSys D) | 9 – 150 A | 3 | 24, 120, 230 VAC + 24 VDC | 1NO + 1NC standard, up to 4-block |
| Eaton XT (IEC) | 7 – 175 A | 3 / 4 | 24 – 480 VAC / 24 – 250 VDC | 1NO + 1NC standard, up to 4 |
| Siemens SIRIUS 3RT | 7 – 250 A | 3 / 4 | 24, 120, 230, 400 VAC + 24 – 220 VDC | 1NO + 1NC standard, expandable |
| Allen-Bradley 100-C | 9 – 97 A | 3 | 24 – 480 VAC / 12 – 250 VDC | 1NO + 1NC, expandable |
| Allen-Bradley 100-D | 115 – 860 A | 3 | 24 – 500 VAC / 24 – 220 VDC | Fully modular |
Confirm motor nameplate — FLA, service factor, insulation class
A 50 HP 480 V 3P TEFC pump motor typically shows FLA = 65 A, SF = 1.15, Class F insulation, NEMA design B. Everything downstream sizes off FLA and SF.
Size the branch-circuit overcurrent protection per NEC 430.52
Inverse-time breaker: 250 % × FLA = 163 A → next standard size = 175 A. Fused disconnect: 175 % × FLA = 114 A → 125 A dual-element time-delay fuses. Both are code-compliant.
Size the branch-circuit conductors per NEC 430.22
125 % × FLA = 81 A. From NEC 310.16, #4 Cu THWN-2 (85 A) or #2 Al XHHW-2 (90 A) at 75 °C. Apply ambient / conduit-fill corrections.
Pick the motor starter — NEMA size 3 or IEC equivalent
50 HP at 460 V is exactly NEMA size 3 (Square D 8536SDG3, Eaton AN36BN0AB, Siemens 14GUC320B). IEC equivalent — Schneider TeSys D LC1D80, Eaton XTCE080, Siemens 3RT2038.
Overload protection — Class 10 or 20
Centrifugal pump = Class 10 usually adequate. Set to 115–125 % of FLA (matches motor SF). Square D LR9D5567, Eaton C440GN, Siemens 3RB3036.
VFD option — soft start + variable speed
If load is variable (variable pumping, water tower, cooling tower), spec a VFD: Schneider Altivar 630 ATV630D45N4 (60 HP class), Eaton PowerXL DG1-35065N (65 A), Siemens SINAMICS G120 CU240E-2 + PM240-2 65 A. Add a 3 % line reactor for THD control.
Enclosure and pilot devices
NEMA 12 for indoor pump room, NEMA 4 for outdoor or wet environment. Add HOA selector (hand / off / auto), local start / stop / reset pushbuttons, running lamp, and — if PLC-controlled — a control-power transformer with primary fuses.
NEMA vs IEC — which one should I spec?
NEMA for heavy-duty applications with frequent starts, high inrush, and dusty enclosures — cement plants, mining, saw mills, hoists. IEC for OEM machine builds, light-duty industrial, and any install where panel space is tight and the duty cycle is engineered. NEMA lasts longer under abuse; IEC is smaller and cheaper per HP.
Do PES starters come pre-assembled in an enclosure?
Both — we stock open-chassis starters (for panel-shop assembly) and pre-assembled combo starters (fused disconnect + starter + pilot devices in a NEMA 3R or 12 enclosure). Combo starters ship same-day for popular sizes; custom pilot-device layouts run 3–5 days.
How do I pick between Schneider Altivar and Eaton PowerXL for an HVAC pump?
Both are excellent HVAC VFDs. Altivar 630 has slightly better BACnet integration out-of-the-box and stronger fan/pump macros. PowerXL DH1 has better long-cable performance and a wider ambient range. On a Louisville job with 200 ft of shielded cable from the drive to the motor, we usually recommend PowerXL DH1 for its dv/dt output filter option.
Do you carry Allen-Bradley (Rockwell) contactors and drives?
Yes. 100-C, 100-D, and 100-E contactors; PowerFlex 525, 755, and 6000 drives. Allen-Bradley is the incumbent spec on any plant running Rockwell PLCs (Logix5000 / ControlLogix). PES is an authorized AB stocking distributor.
How do you handle harmonic mitigation on a large VFD?
We publish a harmonic study with every VFD quote over 100 HP. The default recommendation is a 5 % line reactor plus the drive's built-in DC-link choke, which usually gets THD under IEEE 519 limits on services 500 kVA and larger. Above 300 HP on a small service, we spec 18-pulse rectification or an active harmonic filter. Talk to the sales desk for a project-specific study.
What's the difference between a soft starter and a VFD?
A soft starter (Schneider ATS22, Eaton S811+, Siemens SIRIUS 3RW) ramps the voltage up over 5–30 seconds to reduce inrush, then bypasses to full-voltage running. It does not control speed. A VFD controls both voltage and frequency continuously — soft start plus variable speed. Soft starter is cheaper per HP; VFD is worth the premium anywhere load is variable.
NEMA size 4 vs size 5 for a 100 HP motor at 460 V?
Right at 100 HP / 460 V is the boundary — NEMA size 4 is rated 100 HP maximum, NEMA size 5 is rated 200 HP. For a 100 HP application with continuous full-load duty, we recommend size 5 for duty margin. For intermittent or lightly-loaded 100 HP, size 4 is fine.
Registered contractors — free calculators on the axis.pesdistribution.com portal
- Voltage Drop Calculator — NEC 210.19(A) informational note voltage-drop math for branch and feeder circuits, any conductor size.
- Load Balance Calculator — per-phase load balancing and neutral current on 3-phase 4-wire services.
- Conduit Fill Calculator — NEC Chapter 9 Table 1 fill percentages for EMT, RMC, PVC, and FMC in any trade size.
- Short Circuit / AIC Calculator — point-to-point available fault current at every panel and breaker AIC verification.
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Square D 8536 and TeSys, Eaton Freedom and XT, Siemens SIRIUS, Schneider Altivar, Eaton PowerXL, Siemens SINAMICS, Allen-Bradley 100-C / PowerFlex. Full-stack authorized-channel inventory.
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