Thomson TS870 ATS — 100-1200A Commercial/Industrial Transfer Switch Guide

Overview#

The Thomson TS870 is the workhorse of Thomson's commercial and industrial ATS lineup, covering 100A to 1200A with mechanically interlocked switching units. Available in 2, 3, and 4-pole configurations across voltage classes from 120/240V through 347/600V, it handles the full range of commercial backup and industrial standby applications. Open-transition transfer is standard; closed-transition, bypass-isolation, and service-entrance configurations are available as options.

The TS870 uses the TSC 7320 controller as standard (2.3-inch LCD, RS485 Modbus, programmable I/O) with the TSC 900 premium controller (7-inch color touchscreen) available as an upgrade. All models are UL 1008 and CSA C22.2 No. 178 certified for emergency power system applications, with seismic certification per IBC 2012.

Withstand current ratings#

The TS870 has two distinct withstand tiers — the 1000-1200A frames offer significantly higher ratings than the 100-800A range:

100-800A frames (3-cycle):

Frame	@240V	@480V	@600V
100-150A	65,000A	25,000A	18,000A
200A	65,000A	N/A (240V max)	N/A
250A (3P)	65,000A	65,000A	35,000A
400-800A	65,000A	50,000A	35,000A

1000-1200A frames (3-cycle):

Frame	@240V	@480V	@600V
1000-1200A	150,000A	100,000A	65,000A

All frames carry a 6x overload rating for 10 cycles. With optional overcurrent trip units, interrupting ratings match withstand at 240V and 480V.

Enclosure dimensions (NEMA 1, ATS only)#

Frame	Poles	H x W x D (in)	Weight
100-200A	2, 3, 4	31.1 x 22.3 x 14	143 lbs
250A	2, 3, 4	35.1 x 27.3 x 14	172 lbs
400A	2, 3	43.1 x 34.3 x 13	227 lbs
400A	4	48.1 x 37.8 x 14.5	256 lbs
600A	2, 3	46.1 x 36.3 x 14.5	248 lbs
600A	4	48.1 x 37.8 x 14.5	256 lbs
800A	2, 3	48.1 x 37.8 x 14.5	309 lbs
800A	4	63.1 x 40.8 x 14.5	367 lbs
1000-1200A	2, 3, 4	70 x 34.3 x 14	550 lbs

Bypass-isolation units are significantly larger: 400-800A units are 80" tall, and 1000-1200A bypass units weigh 1,415-1,474 lbs.

Closed transition: how it works#

For 1000-1200A, the CTTS uses stored-energy motor operators with electrically interlocked molded case power switching units. The TSC 7320 controller includes a built-in sync check relay that monitors voltage differential and slip frequency between utility and generator. When both sources are within threshold, the controller initiates a closed-transition transfer lasting less than 100ms. If the paralleling timer exceeds its limit, extended paralleling protection opens the other source within 500ms.

For 400-800A closed transition, the TS870 uses the TSC 900 controller instead of the TSC 7320. The CTTS enclosure measures 64 x 30 x 13 inches (400 lbs).

Closed-transition mode can be bypassed to open-transition mode via the controller for situations where paralleling is not appropriate or utility coordination is not available.

TS870 vs TS970: which to specify#

Both cover 100-1200A. The key differences:

Factor	TS870 (Mech-held)	TS970 (Contactor)
Transfer speed	Slower (~300-500ms)	Faster (~100ms)
Closed transition	Available (400-1200A)	Not available
Bypass-isolation	Available	Not available
Service entrance	Available	Not available
Idle tolerance	Holds indefinitely	Coils may age
Cost	Higher	Lower
Max voltage	600V	480V

For the majority of commercial backup with open-transition only, the TS970 is the value choice. Reserve the TS870 for applications needing closed transition, bypass-isolation, service entrance, 600V capability, or environments where the switch sits idle for extended periods.

Our service experience#

We service Thomson TS870 units across commercial buildings and light-industrial facilities in the Bay Area and Central Coast. The mechanically-held mechanism is extremely reliable — we rarely see mechanism failures before year 15. The most common service items are lug retorquing (annual, especially on 600A+ frames where thermal cycling is significant) and controller firmware updates.

One practical advantage over contactor-based alternatives: in facilities that only lose utility power once or twice per year, the mechanical latch holds position indefinitely without drawing coil current. Contactors in similar low-use environments can develop issues from coil aging and contact oxidation. The trade-off is slower transfer speed, but for most commercial backup applications, the 200-500ms difference is immaterial.

The 200A frame's 240V limitation catches some specifiers off guard — for a 200A load at 480V, you must step up to the 250A frame. We flag this during design review for any Thomson 480V specification in the 200A range.

Frequently Asked Questions

What amperage range does the Thomson TS870 cover?

The TS870 covers 100A through 1200A. It is available in 2, 3, and 4-pole configurations across voltage classes from 120/240V through 347/600V at 50/60 Hz. Note: the 200A frame is limited to 240V maximum and is not available at 480V or 600V.

Does the Thomson TS870 support closed-transition transfer?

Yes. Closed transition is available from 400A through 1200A. For 400-800A, the CTTS uses the TSC 900 controller with a 7-inch color touchscreen. For 1000-1200A, the CTTS uses stored-energy motor operators with the TSC 7320 controller, which includes a built-in sync check relay. The paralleling time is less than 100ms with extended paralleling protection that isolates within 500ms if the window is exceeded.

What controllers are available for the TS870?

Two options: the TSC 7320 (standard) with a 2.3-inch backlit LCD display, RS485 Modbus RTU, and programmable I/O; or the TSC 900 (premium) with a 7-inch color touchscreen, RS232 Modbus via GHC, optional Ethernet Modbus TCP, and USB ports. The TSC 900 is required for 400-800A closed-transition configurations.

Does the TS870 offer bypass-isolation?

Yes. The TS870 bypass-isolation configuration provides a break-before-make bypass mechanism in a separate barriered compartment. It allows maintenance personnel to isolate and bypass the ATS without interrupting power to the load. For 1000-1200A units, the bypass uses electrically interlocked draw-out molded case power switching devices.

What are the TS870's withstand ratings?

Withstand varies by frame size. For 100-800A: 65,000A at 240V, 25,000-50,000A at 480V, and 18,000-35,000A at 600V. For 1000-1200A: 150,000A at 240V, 100,000A at 480V, and 65,000A at 600V. With optional overcurrent trip units, standard interrupting ratings match the withstand ratings at 240V and 480V.

What are the physical dimensions?

Dimensions vary by frame and pole count (NEMA 1 enclosure): 100-200A is 31.1 x 22.3 x 14 inches (143 lbs); 250A is 35.1 x 27.3 x 14 inches (172 lbs); 400A 3-pole is 43.1 x 34.3 x 13 inches (227 lbs); 600A 3-pole is 46.1 x 36.3 x 14.5 inches (248 lbs); 800A 3-pole is 48.1 x 37.8 x 14.5 inches (309 lbs); 1000-1200A is 70 x 34.3 x 14 inches (550 lbs). 4-pole frames are larger.

How does the TS870 compare to the ASCO 7000 Series?

Both are commercial/industrial ATS units with open and closed transition. The ASCO 7000 covers a wider amperage range (30-4000A vs 100-1200A) and uses contactor-based switching. The TS870 uses mechanically-held switching, which holds position indefinitely without coil power — an advantage in low-use environments. For loads above 1200A, step up to the Thomson TS880 (800-4000A). ASCO has deeper distributor penetration; Thomson typically offers more competitive pricing and lead times.

What seismic certifications does the TS870 have?

The TS870 is seismic certified per IBC 2012 Section 13, Occupancy Category IV, and ASCE7-05 Region 3 (minimum Ss=200%). This qualifies it for installation in essential facilities including hospitals, fire stations, and emergency response centers in high seismic zones.

Is the 200A frame limited in any way?

Yes. The 200A frame has a maximum voltage rating of 240V and withstand data is provided at 240V only (65,000A). It is not available at 480V or 600V. For 200A applications requiring 480V, specify the 250A frame instead, which is rated to 600V with withstand of 65,000A at 480V.

Service Specifications

Service	Frequency	Notes
Exercise under load	Monthly (NFPA 110)	30+ minutes of runtime with load transfer; TSC 7320 has built-in exerciser with up to 16 schedules
Connection torque inspection	Annually	Retorque all power lugs; thermal cycling loosens connections, especially on 600A+ frames
Contact inspection	Every 5 years or 1,000 operations	Inspect silver alloy contacts for pitting or erosion; replace as a set
Gear-motor mechanism	Every 5 years	Inspect brushless gear-motor and operating linkage; verify full travel on both sources
Controller firmware	As released	TSC 7320 updates via USB; TSC 900 updates via USB or Ethernet
Thermographic scan	Annually (600A+ frames)	IR scan of all bus connections and contacts under load; catch incipient hot connections

Key Terms

Closed-transition transfer

Sub-100ms overlap where both utility and generator are briefly paralleled during transfer, eliminating load interruption. Extended paralleling protection isolates within 500ms if the 100ms window is exceeded.

Mechanically-held switching

Switch positions maintained by a mechanical latch driven by a brushless gear-motor mechanism. Provides positive position indication and holds position indefinitely without drawing power.

Bypass-isolation

Configuration allowing maintenance personnel to manually isolate and bypass the ATS for servicing without interrupting power to the load. Break-before-make design prevents utility/generator paralleling.

TSC 7320 controller

Thomson's standard ATS controller with 2.3-inch backlit LCD display, 3-phase voltage monitoring, 10 programmable outputs, RS485 Modbus RTU, and built-in exercise timer with up to 16 schedules.

TSC 900 controller

Thomson's premium ATS controller with 7-inch color touchscreen, 8 programmable output contacts, RS232 Modbus via GHC, optional Ethernet Modbus TCP, and USB communication ports.

Service-entrance rated (SE)

ATS incorporating an isolating mechanism and overcurrent protection on the utility supply, eliminating the need for a separate upstream circuit breaker. For U.S. market only.

Thomson TS870 Automatic Transfer Switch