A.O. Smith NW 37-670 Calculating Amperage/Overcurrent Protection, Boiler Circuits, Power Circuit

Calculating Amperage/Overcurrent Protection

The boiler is factory wired for connection to three wire single-phase or three and four wire three-phase branch circuits. In addition, a ground conductor may be required.

A diagram of the wiring is furnished with the boiler for the electrician’s use. An amperage table is on page 4 of this manual. The boiler model and rating plate provides full load amperage data.

The rating of the overcurrent protection should be computed on the basis of 125 percent of the total connected load amperage. Where the standard ratings and settings do not correspond with this computation, the next higher standard rating or setting should be selected.

Boiler Circuits

The boiler’s electrical components are pictured and identified on page

5.The model and rating plate illustration on page 4 identifies the electrical characteristics. Basically, there are two electrical circuits:

•The control circuit, where the temperature control directly or indirectly operates the contactor coils.

•The power circuit, which is operated by the control circuit, carries the electrical load of the heating elements.

The following describes the circuits and includes typical wiring diagrams. All circuits are designed for 60 or 50 Hertz alternating current.

Refer to ELECTRICAL CONFIGURATION TABLE, below, and wiring diagram provided with your boiler before completing connections to electrical supply.

NOTE: Wiring diagrams in this manual are typical examples. The specific wiring diagram for your boiler is the one supplied with the boiler.

Control Circuit

All control circuits are operated on single-phase 120V. A transformer is used in the control circuit.

Control circuit wiring is 14 Awg, THHN or THWN type, rated 600 volts, 105°C.

Seperate instructional literature is provided with the boiler for the modulating solid state step control.

Portion of Table 310-16		Portion of Table 310-16
Allowable Ampacities of Insulated		Allowable Ampacities of
Copper Conductors		Insulated Aluminum and Copper-
Not more than three conductors in		Clad AluminumConductors
		Not more than three conductors
raceway or cable or direct burial
(based on ambient temperature of		in raceway or cable or direct
	30°C, 86°F.)	burial (based on ambient
		temperature of 30°C, 86°F.)
	Temperature Rating of	Temperature Rating of
	Conductor. See Table		Conductor. See Table
Size	310-13 in Code	Size	310-13 in Code
AWG	75°c	AWG	75°C
MCM	(167°F)	MCM	(167°F)
	TYPES		TYPES
	RH, RHW, RUH		RH, RHW, RUH
	(14-2),		(12-2),
	THW, THWN,		THW, THWN,
	XHHW, USE		XHHW, USE
18	- - - - -	12	15
16	- - - - -	10	25
14	15	8	40
12	20	6	50
10	30	4	65
8	45	3	75
6	65	2	90
4	85	1	100
3	100	1/0	120
2	115	2/0	135
1	130	3/0	155
1/0	150	4/0	180
2/0	175	250	205
3/0	200	300	230
4/0	230	350	250
250	255	400	270
300	285	500	310
350	310	600	340
400	335	700	375
500	380	750	385
600	420	800	395
700	460	900	425
750	475	1000	445
800	490	1250	485
900	520	1500	520
1000	545	1750	545
1250	590	2000	560
1500	625	These capacities relate only to conductors
1750	650	described in Table 310-13 in Code.
2000	665	For ambient temperatures over 30°C, see
		Correction Factors, Note 13 in Code

Power Circuit

The boiler is equipped with one of the power circuit configurations shown in the table.

Power circuit wiring is type THHN, rated 600 volts, 105°C, sized as necessary.

7