Circuit Breakers & Semiconductors
What kind of circuit breakers are needed for commercial buildings?
I have a school project to do involving 120/208V 3 phase wiring.
What kind of circuit breakers would an AC and baseboard heater require?
They are both rated above 20 Kw. Would it be 3 pole? This is what is confusing me…Ive only done residential projects with single and double pole breakers.
3-pole – 120/208 is 3 phases – each 120volt, measuring 208V btwn any two phases.
In a 3 phase installation – the neutral is not used as part of the appliance operation – only to ground the appliance cabinet for safety.
The neutral is used for operating appliances using 120V – single phase.
120/208V single phase only uses the neutral for grounding
3 phase 208V – 20KW, (20,000 watts) would require a triple pole breaker and the neutral would be used as a ground, for the cabinet, for safety. In a commercial building the smallest breaker is 20 amp.
Residential homes are supplied with 120/240V, single phase service, due to the smaller size appliances. The smallest size breaker is 15 amp.
Many A/C units are 208V – 3 phase when the unit is 7-1/2 ton or larger. The 3 phase unit would, of course, be fused with a triple breaker.
Baseboard units are usually 120/208 -1 phase – due to the maximum wattage assigned to electric baseboard units. There can be some 3-phase, and it would require a 3 pole breaker but, most are 1 phase since it is necessary to distribute a number of baseboard units in an area to spread the heat rather than have 1 large unit in one place. Unless a large 3 phase unit is in the center of the area ,and fans are used for distribution, the area would be uncomfortable.
Wattage is divided by the voltage to select the breaker size.
1 – 20 KW unit at 120V would require a minimum of 1- 200 amp breaker and very large wire and not be efficient and not operate due to a severe imbalance in the Power Company transformer ..
1 – 20 KWunit at 120/1208V single phase would require 2 – 100 amp breakers and large wire and not be efficient.
1 – 20 KW unit at 208V three phase would require 3 – 60 amp breakers and smaller wire and be more efficient.
Bottom line:
120V operation – 1 pole breaker.
120/208V single phase operation – 2 pole breaker.
208V – 3 phase operation – 3 pole breaker.
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