The Guide - MEM Technical Support

Motor Circuit Protection; Selectivity
Circuit Breakers

Circuit Breaker Selection Charts for Selectivity with Direct-On-Line & Star-Delta Starters

Direct-On-Line:
Typically direct-on-line (d.o.l.) starting will create a start-up current inrush of 6 - 8 x full load current. In addition this inrush can take several seconds to begin to fall to full load current (f.l.c.).

For selectivity with circuit breakers this start-up characteristic must not 'trip' the circuit breaker (see below).

Circuit breaker characteristic curve MDH310
Typical d.o.l. motor circuit characteristic 2.2kW 3ph.

Star-Delta

Star-delta starting circuits exhibit lower starting currents than d.o.l., typically 3 - 4 x full load current. However, a transient peak is normally associated with the changeover from star to delta. Hence MEM recommends that the same selection tables are used for both d.o.l. and star-delta starting circuits.

Three Phase Motors @ 415V
kW	F.L.C. (Ie)	MCB Rating Type C (A)	MCB Rating Type D (A)	MCCB Rating G Frame (A)	MCCB Rating F Frame (A)		MCCB Rating J Frame (A)	MCCB Rating K Frame (A)	MCCB Rating L Frame (A)	Fuselinks D.O.L. STD Fuse (A)	Motor Rated Fuse
					STD Fixed Trip	Adj. Trip
0.37	1.3	4	2	-	-	-	-	-	-	6	-
0.56	1.6	4	2	-	-	-	-	-	-	6	-
0.75	1.8	4	2	-	-	-	-	-	-	10	-
1.1	2.6	6	4	-	-	-	-	-	-	10	-
1.5	3.4	6	6	-	-	-	-	-	-	10	-
2.2	5	10	10	-	-	-	-	-	-	16	-
3	6.5	13	13	-	-	-	-	-	-	16	-
3.7	8	16	13	-	-	-	-	-	-	20	-
4	8	16	13	16	16	-	-	-	-	20	20M25
5.5	11	20	20	20	20	-	-	-	-	25	20M32
7.5	15	32	25	32	32	-	-	-	-	40	32M40
9.3	18	32	32	32	32	-	-	-	-	40	32M40
10	20	40	32	40	40	-	-	-	-	50	32M50
11	22	40	40	40	40	-	-	-	-	50	32M50
15	28	50	50	50	50	50	-	-	-	63	32M63
18.5	36	63	63	63	63	63	-	-	-	80	63M80
22	39	63	63	80	80	63	-	-	-	80	63M80
30	52	-	-	100	100	80	-	-	-	100	63M100
37	69	-	-	125	125	100	-	-	-	160	100M160
45	79	-	-	-	160	125	-	-	-	160	100M160
55	96	-	-	-	200	160	-	-	-	200	100M200
75	125	-	-	-	-	-	200	-	-	200	200M250
90	156	-	-	-	-	-	250	-	-	250	200M250
110	189	-	-	-	-	-	250	320	-	315	200M315
132	224	-	-	-	-	-	-	400	-	355	315M400
150	255	-	-	-	-	-	-	400	-	400	315M400
160	275	-	-	-	-	-	-	400	630	450	-
185	318	-	-	-	-	-	-	-	800	500	-
200	339	-	-	-	-	-	-	-	800	500	-

Single Phase Motors @ 240V
kW	F.L.C. (Ie)	MCB Rating Type C (A)	MCB Rating Type D (A)	MCCB Rating G Frame (A)	MCCB Rating F Frame (A)		Fuselinks D.O.L.
					STD Fixed Trip	Adj.* Trip
0.25	2.6	6	6	-	-	-	10
0.37	3.6	10	10	-	-	-	10
0.56	5	16	13	-	-	-	16
0.75	6.7	20	16	16	16	-	20
1.1	9	25	20	16	32	-	25
1.5	12	32	25	32	32	-	32
2.2	17	50	40	32	40	-	40
3	22	63	50	40	40	-	50
3.7	25	63	63	50	50	50	63
4	27	63	63	50	50	50	63
5.5	38	-	-	63	80	63	100
7.5	50	-	-	80	100	80	100
* Using 3 pole MCCB

The Guide - Circuit Breakers - Main Menu
The Guide - Main Menu