ABB molded-case breakers: A2A series vs XT family (XT5N–XT7S)

The ABB A2A series covers molded-case circuit breakers from 125 to 250 A with a fixed thermal-magnetic trip unit, and is the choice for feeders and medium-sized distribution switchboards. The XT family (XT5N, XT6N and XT7S) takes over from 400 A up to 1600 A, with adjustable thermal-magnetic or electronic Ekip trip units. The choice between the two depends on the feeder current, the short-circuit level and whether the application requires adjusting the protection or communicating with it.

What is a molded-case circuit breaker?

A molded-case circuit breaker (MCCB) is an automatic circuit breaker whose mechanism, contacts, arc chutes and trip unit are integrated into a molded insulating housing. Unlike the plug-in breaker of a residential panelboard, it is bolted to the switchboard, handles currents from 100 A to more than 1000 A and offers high interrupting capacities. It is the breaker of power switchboards, motor control centers and transformer outputs.

A2A series: up to 250 A with fixed thermal-magnetic trip

The A2A series belongs to ABB's Formula range, designed to UL standards. In our catalog we carry the 250 A frame in 2 and 3 poles with a TMF (fixed thermal-magnetic) trip unit, in rated currents of 125, 150, 175, 200, 225 and 250 A. "Fixed" means the thermal and magnetic thresholds are factory-calibrated for that current and are not adjustable in the field.

• Range: 125 – 250 A, 2 and 3 poles.
• Trip unit: TMF, non-adjustable.
• Terminals: fixed front terminals (F F); CuAl mechanical lug kits for copper or aluminum cable, available in 2- and 6-piece sets.
• Applications: subpanel feeders, protection of small transformers, medium-current outgoing circuits in CCB switchboards, main breaker in commercial panelboards.

It is the right choice when the current is defined, the short-circuit level is moderate and there is no need for fine protection coordination with other breakers downstream.

XT family: from 400 to 1600 A with adjustment and electronics

The Tmax XT family is ABB's current generation of industrial molded-case circuit breakers. Each frame size covers a current range and accepts different trip units. The part numbers we carry are:

Model	Current	Trip unit	Poles
XT5N 400	400 A	TMA (adjustable thermal-magnetic)	3
XT5N 630	630 A	TMA	3
XT6N 800	800 A	TMA	3
XT7S 1250	1250 A	Ekip Dip LS/I (electronic)	3
XT7S 1600	1600 A	Ekip Dip LS/I (electronic)	3

TMA: adjustable thermal-magnetic

On the XT5N and XT6N, the TMA unit allows the thermal (overload) and magnetic (short-circuit) thresholds to be adjusted within a range, using selectors on the front of the breaker. This makes it possible to match the protection to the actual conductor and to coordinate with downstream breakers without changing the equipment.

Ekip Dip: electronic protection

On the XT7S, the Ekip Dip LS/I unit replaces the bimetal and the coil with an electronic relay and current sensors. The L (long-time overload), S (short-time short circuit) and I (instantaneous short circuit) functions are set with dip-type selectors. Advantages over thermal-magnetic units:

• Independent current and time settings for each function, which simplifies selectivity.
• Lower dispersion and greater trip repeatability.
• No drift with ambient temperature, unlike the bimetal.
• The option of adding metering and communication in the higher versions of the Ekip family.

When to use A2A and when to move to XT

1. Feeder current. Up to 250 A, A2A. From 400 A upward, XT5N; at 800 A, XT6N; from 1250 to 1600 A, XT7S.
2. Need for adjustment. If the conductor is oversized or the load may grow, the TMA unit on the XT5N/XT6N allows the protection to be adapted. The A2A does not.
3. Selectivity. In a chain of breakers in series (main, subpanel, branch), the short-time S function of the Ekip unit lets the downstream breaker trip before the main. With thermal-magnetic units, selectivity is more limited.
4. Loads with high inrush. Large motors and transformers require adjustment of the magnetic threshold; TMA or Ekip is recommended.
5. Short-circuit level. Compare the interrupting capacity of the model with the short-circuit study at the point of installation. If the value is high, review the higher-capacity versions within the same family.

Terminals and mounting

All the part numbers listed are fixed-mount versions with front terminals (F F). Conductors are connected with CuAl mechanical lug kits sized for each frame: for the XT6, up to two 120 – 240 mm² conductors per pole; for the XT7, up to four 240 mm² (500 kcmil) conductors per pole. When specifying the breaker, the lug kit must also be specified, since it is supplied separately. Mounting is bolt-on, onto a plate or onto the switchboard bus kit, with the orientation and clearances indicated by the manufacturer.

Information required to quote a molded-case circuit breaker

• Rated current of the feeder and conductor gauge.
• Number of poles and service voltage.
• Short-circuit level at the point of installation.
• Type of trip unit required (fixed, adjustable or electronic).
• Accessories: shunt trip, auxiliary contacts, motor operator.

The complete part numbers are in the industrial circuit breakers section. If you have the single-line diagram or the part number of the breaker to be replaced, send it on WhatsApp and we will confirm the model, the trip unit and the corresponding lug kit.

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