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Rating label
Devices that consume electricity (refrigerator, water heater, computer,
light bulb, microwave etc) each have a rating label.
The label shows volts and watts (power consumption).
The label sometimes shows amps.
Divide watts by volts to find amps.
For example 120 volt 100 watt light bulb uses .8 amps.
Add up total amp consumption or total load on the circuit.
Safe Maximum for a breaker is 80% rated value of breaker.
If circuit has 10 120 volt light bulbs @ .8 amp each, then total load
is 8 amps and any breaker will work fine.
Small
appliance branch circuit is a circuit that
supplies power to receptacles in kitchen, dining, and
laundry where small appliances are used. These circuits must be 20 amp,
and wired with 12 gauge wire. Code:
Kitchen counter outlets require 2 small appliance branch circuits.
Dedicated
120 volt circuit serves only 1 Load
By code, a
microwave oven requires
‘dedicated’ breaker ,,,, dedicated means the circuit
breaker supplies
power to no other device except the microwave.
The
mathematics shows why. The typical 120 volt 1500 watt microwave oven
uses 12.5 amps. (1500 watt divided by 120 volt = 12.5 amps)
The 80% safe maximum for 15 amp breaker is 12 amps. (15 amp x .80 = 12.5 amp)
The 15 amp breaker does not meet code requirement for 20 amp small
appliance circuit.
However if home has the 15 amp breaker, do not panic and replace
the 15 amp breaker with 20 amp unless the wire is 12 gauge.
The 15 amp breaker 'will work' for 12.5 amp microwave, because the load
is barely over safe maximum and not 'continuously
operating.'
Contrast with 1500 watt tea
kettle or space heater or window air conditioner or large refrigerator
that are likely continuously
operated, and code requires strict
adherence to the 80% safe maximum rule. (Breaker amp rating x .80 = safe maximum amps).
A
continuously operating load, particularly motors (refrigerator, pool
pump, window air conditioner) will cause wire to get warm and then cool
and then get warm again next time load turns on. This causes expansion
and contraction, wear and tear on breakers and connections.
By
following the 80% rule, heat stays within safe limits so electrical
installation doesn't deteriorate over the long run.
The
best choice for microwave, tea kettle, space heater is 20 amp breaker
and 12 gauge wire. The safe maximum for 20 amp breaker is 16 amps (20
amp breaker x .80 = 16 amps). The 20 amp breaker will easily support
the 12.5 amp microwave ... but will not support
additional loads like a refrigerator that consumes 7-8 amps.
1500
watt tea kettle or space heater will also consume 12.5 amps, and 20
circuit breaker can max out quickly if other loads are running at same
time. Choose 1200 watt or 1000 watt appliances if circuits are
overloaded.
Safety:
''• Only use one heat-producing appliance (such as a coffee maker,
toaster, wok, space heater, clothes iron etc.) plugged into a receptacle outlet at a time.
• Major appliances (refrigerators, dryers, washers, stoves, air
conditioners, water heater, microwave ovens, etc.) should be plugged
directly into a wall receptacle outlet. Extension cords plug strips
should not be used.'' Do not plug larger loads into a timer module,
home automation module etc. Wattage monitor can be added for a few
days, but loose plug-in connections risk spark and short that can
damage some appliance electronics. A loose connection causes more heat,
and will weaken eclectic outlets, and loosen the contact points,
leading to further heat and more deterioration (damage) over time.
Deteriorated electrical installations are one of the main causes for electrical problems, short circuit, fire etc.
Dedicated
240 volt circuits
-240 volt loads, like Clothes Dryer, water heater, oven, HVAC etc
are on a dedicated breaker that serves no other 240
volt load.
The reason is the voltage is very strong and the appliance is
usually draws near-maximum amperage from the
breaker.
-For example 240 volt 4500 watt residential water heater
consumes 18.75 amps, requiring a 30 amp breaker with safe maximum 24
amps.
-That leaves an extra 5.25 amps. (24 amps minus 18.75 = 5.25)
-Since
the circuit has extra 5.25 amp capacity, it is ok to use the 240
volt line going to water heater, HVAC etc to power a timer, control
circuit, counter, indicator light, voltage monitor, wifi device, hour
meter, Kwh meter, or other small-watt device etc that is hardwired into
circuit. (Hardwire means the device is not a plug-in module, but is
attached permanently into the wiring as part of the circuit)
Other
dedicated circuits
Instruction manuals for other household
devices might also recommend dedicated circuit breaker, for example
flue-dampered water heater. The power vent and flue dampered type water
heaters only consume a few amps,
but the operation of the electronic gas control valve is  susceptible
to
surge and line noise
that can come from other loads on same breaker. For example a motor
load or some other type of electrical load can cause voltage surge
events
that will affect electronics.
Power vent and flue dampered water heaters benefit from a surge outlet.
All electronics should be surge protected.
Multiple surge devices on one circuit are fine. The closer the
protection is located to electronics, the better it protects.
Correct polarity and grounding
are required for reliable surge function.
Good household grounding improves ability of Type 3 surge outlets and
surge protect strips to defect the surge by jumping the gap to ground.
Buy:
15 amp surge suppressor receptacle
Leviton J-box surge protection kit
15 amp duplex surge protector
20 amp commercial surge protector
Outlet
tester for polarity
Resource
Surge outlet manual
If
electronics continually reset or throw error codes:
install noise filter, and check polarity and grounding of outlets using
circuit analyzer.
Footnote: Power vented and flue dampered water heaters specifically say
no GFCI because it causes
error codes on the gas valve.
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