A 180T NEMA general purpose AC motor and a 112 IEC metric motor sit within 4 mm of each other on shaft height, yet the bolt circles, shaft diameters, conduit-box position, and grease chemistry do not line up, so a nameplate match is not a mounting match [S3][S5].
NEMA MG 1 assigns frame numbers from enclosure, horsepower and rpm at 60 Hz; the IEC 60034 family assigns frame numbers from shaft height in millimetres only, with no power or speed embedded in the code [S1][S5]. For a process engineer cross-specifying a general purpose AC motor on a North American line that is also shipped to a European plant, that single design choice ripples through the BOM, the coupling selection, and the spare-parts bin.
What each code actually means
NEMA MG 1 frame numbers encode four variables in one string: enclosure family, shaft height in inches multiplied by 16, a letter for overall length (S, T, U, …), and a suffix for mounting (C-face, D-flange, etc.) [S1][S5]. IEC frame numbers carry only one variable, the shaft height in millimetres, written directly (80, 90, 100, 112, 132, 160, 180, 200, 225, 250, 280, 315, 355, 400), and the mounting style is given by a separate code (B3 foot, B5 flange, B14 face, B35 foot+flange) [S4][S5].
The practical consequence is a coding mismatch the moment a motor crosses an ocean. A 4-pole, 10 hp (7.5 kW) unit is a 215T in NEMA and a 132M in IEC, but the IEC designation carries no speed, no enclosure, and no length class, so the equivalent shaft height of 5.197 in (132 mm) sits roughly 3 mm below the NEMA 215T at 5.32 in (135 mm) [S5][S7]. That gap is the source of most field complaints when a US-built machine is shipped with IEC motors or vice versa.
Shaft-height comparison: where the 3-4 mm trap lives
The eMotors Direct side-by-side table shows the divergence grows with frame size: NEMA 56 / 140T sits at 3.5 in (88.9 mm) versus IEC 90 at 3.543 in (90 mm); NEMA 180T at 4.5 in (114.3 mm) versus IEC 112 at 4.409 in (112 mm); NEMA 320T at 8.0 in (203.2 mm) versus IEC 200 at 7.874 in (200 mm); NEMA 5000 at 12.5 in (317.5 mm) versus IEC 315 at 12.402 in (315 mm) [S5]. At the high end, NEMA 6800 at 17.0 in (431.8 mm) lines up against IEC 450 at 17.717 in (450 mm), a reversal of the pattern because IEC keeps a denser size series above 355 mm [S5].
For belt-driven loads the 3-4 mm delta can be absorbed by re-pulleying; for direct-coupled loads the only clean fix is a conversion flange or a full motor swap, because shaft diameter, key size, and foot-mount bolt pattern are not part of the frame code on either side and they differ independently of shaft height [S5][S7]. WEG's standard-dimensions poster drives the same point home: NEMA and IEC foot drilling (D, E, F, BA dimensions), shaft extension (U, N-W), and conduit-box keep-out zones are not aligned even when the two letters look numerically close [S7].
Mounting flanges and face patterns: B5/B14 vs C-face/D-flange
NEMA motors are compatible with a C-face kit and a D-flange; IEC metric frames use the B14 face and the B5 flange [S4]. The C-face / D-flange pattern has a smaller pilot and a coarser bolt circle than B5 / B14, and the two patterns will not bolt to the same casting without an adapter plate, so any retrofit of a VFD-driven motor package across standards must spec the adapter as a line item rather than assume a shared footprint [S4].
Conduit-box location is the other quiet gotcha: most NEMA general purpose AC motors ship with the box in the F1 (side) position wired with lead wires, while most IEC motors ship with the box at 12 o'clock (F3) on a terminal block, which forces a different routing path for shielded VFD output cable and a different gland plate when the unit is dropped onto a North American skid [S3].
Voltage, frequency, and service factor: electrical design differences that follow the frame
Frame size does not fix the electrical design, but it is the language in which the electrical differences are communicated. NEMA nameplates target 60 Hz at 230, 460 or 575 V with a ±10% voltage variation; IEC nameplates target 50 Hz at 220, 380, 400 or 690 V, and the standard splits allowable variation into Zone A (±5% voltage, ±2% frequency) and Zone B (±10% voltage, +3%/-5% frequency) [S1][S2]. The same physical frame is therefore not always wired the same way, and a NEMA 460 V motor ordered for a 400 V European plant needs reconnection or a transformer tap change, not just a bolt-up.
NEMA recognises a service factor on the nameplate, typically 1.15 for general purpose TEFC designs, so a 10 hp 1.15 SF motor can be loaded to 11.5 hp continuously with an accepted temperature rise; IEC does not use a service factor at all and instead uses duty types S1 through S10 (S1 continuous, S2 short-time, S3 intermittent periodic, S4 intermittent periodic with starting, S5 intermittent periodic with electric braking, S6 continuous operating periodic, S7 continuous operating periodic with electric braking, S8 continuous operating with related load/speed changes, S9 non-periodic load and speed variations, S10 discrete constant loads and speeds) [S1]. Insulation class is closer but not identical: NEMA defines A, B, F and H at 105, 130, 155 and 180 degC; IEC defines Y, A, E, B, F, H, N, R and 250 at 90, 105, 120, 130, 155, 180, 200, 220 and 250 degC respectively [S1].
Grease, shaft seals, and enclosure: the maintenance-side differences
Mixing populations in one plant is the practical problem. NEMA general purpose motors are usually greased with a polyurea-based grease; most IEC motors use a lithium-based grease, and the two chemistries are not compatible, so a maintenance tech who uses the same tube on both populations will reduce bearing life on one side or the other [S3]. NEMA offers a wider enclosure range, including open drip-proof (ODP) and TEFC variants common on fans and compressors; IEC tends to standardise on TEFC and severe-duty enclosures, with ODP much less common outside North America [S3].
Shaft sealing also diverges: NEMA general purpose motors often ship without a shaft seal except for severe-duty / IEEE-841 builds, which use a non-contact rotating seal; IEC motors typically include a lip seal or a labyrinth seal as standard, so an IEC motor swap onto a NEMA footplate may need an additional seal retrofit if the application is outdoor or washdown [S3]. These are exactly the kind of small-bore differences that drive a specifier toward buying the same motor accessory kit for both populations rather than mixing inventory.
Selection matrix: when to pick NEMA, when to pick IEC
Use this 4-criterion decision matrix when the frame-size question is open: (1) End-market geography: NEMA for US, Canada, Mexico, parts of South America and Saudi Arabia; IEC for the rest of the world, where roughly 70% of industrial motors sold globally are IEC metric [S3]. (2) Voltage and frequency at the site: 60 Hz at 230/460/575 V → NEMA; 50 Hz at 380/400/690 V → IEC; mixed sites usually split the BOM by skid [S1][S2]. (3) Mounting and coupling: belt drives tolerate the 3-4 mm shaft-height delta with a pulley change; direct-coupled loads need a conversion flange or a same-standard motor, and the cost of the adapter often wipes out any savings from a cross-border purchase [S5]. (4) Spare-parts and grease policy: fleets that already stock polyurea grease and side-conduit habit favour NEMA; fleets that already stock lithium grease and top-conduit habit favour IEC; mixed fleets should standardise regreasing procedure by frame family, not by nameplate brand [S3].
The recommendation falls out cleanly. For a single-region plant, buy to the regional standard and stop trying to unify the BOM. For an OEM shipping the same skid to both regions, build two motor variants from the same drive motor platform and accept the cost of the extra SKU; the savings from a forced cross-standard retrofit are usually consumed by the adapter flange, the re-greasing procedure rewrite, and the spare-parts doubling. For an engineering, procurement and construction (EPC) firm that has to hand over a working plant, the right call is to lock the frame standard to the end-user country's grid, not to the OEM's home country, and to call out the grease type and conduit-box position on the data sheet rather than leave them as defaults.
Frame-to-frame reference table (shaft height, inches / mm)
The condensed mapping below is taken from the eMotors Direct comparison [S5] and is the minimum set a specifier needs to recognise on a nameplate. It is not a drop-in replacement list; always cross-check shaft diameter (U), key size, and foot drilling (D, E, F, BA) before approving a substitution [S7]. NEMA 42 at 2.625 in (66.675 mm) ↔ IEC 71 at 2.795 in (71 mm); NEMA 48 at 3.0 in (76.2 mm) ↔ IEC 80 at 3.15 in (80 mm); NEMA 56 / 140T at 3.5 in (88.9 mm) ↔ IEC 90 at 3.543 in (90 mm); NEMA 180T at 4.5 in (114.3 mm) ↔ IEC 112 at 4.409 in (112 mm); NEMA 210T at 5.3 in (133.35 mm) ↔ IEC 132 at 5.197 in (132 mm); NEMA 250T at 6.3 in (158.75 mm) ↔ IEC 160 at 6.299 in (160 mm); NEMA 280T at 7.0 in (177.8 mm) ↔ IEC 180 at 7.087 in (180 mm); NEMA 320T at 8.0 in (203.2 mm) ↔ IEC 200 at 7.874 in (200 mm); NEMA 360T at 9.0 in (228.6 mm) ↔ IEC 225 at 8.858 in (225 mm); NEMA 400T at 10.0 in (254 mm) ↔ IEC 250 at 9.843 in (250 mm); NEMA 440T at 11.0 in (279.4 mm) ↔ IEC 280 at 11.024 in (280 mm); NEMA 5000 at 12.5 in (317.5 mm) ↔ IEC 315 at 12.402 in (315 mm); NEMA 5800 at 14.5 in (368.3 mm) ↔ IEC 355 at 13.976 in (355 mm); NEMA 6800 at 17.0 in (431.8 mm) ↔ IEC 450 at 17.717 in (450 mm) [S5].
Two signals to track over the next quarter. First, EASA's IEC-to-NEMA motor-ratings resource continues to add kW-to-hp cross-reference tables for ratings above 200 hp, which matters for process plants that are now standardising on 60 Hz IEC designs supplied from US-assembled inventory [S8]. Second, Baldor's IEC quick-reference chart (now under ABB) flags that the NEMA AA face rabbet dimension is a minimum-only value, with typical IEC-frame suppliers like Baldor shipping AA dimensions that meet or exceed NEMA even when the IEC face code is nominally smaller, which is useful to remember when a C-face / B5 adapter is being sourced [S9]. Both are good reasons to keep the frame-size decision on the data sheet rather than burying it in a vendor's standard product line.
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