ELECTRIC WATER HEATER BRANCH CIRCUITS
Branch-Circuit Rating:
NEC 422.13 applies to all fixed storage electric water heaters having a capacity of 120 gallons (454.2 L) or less. This would include most typical residential electric water heaters. An electric water heater is a continuous load per 422.13, which would require that the branch-circuit rating must not be less than 125% of the nameplate rating.
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Above: FGR. 13
Twenty-four-hour water heater installation. The total energy
demand for the water heater is fed through a separate watt-hour meter at
a lower kWh rate than the regular meter.
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Disconnecting Means:
For obvious safety reasons, NEC 422.30 requires that an appliance have a means of disconnecting it from the power source. In most instances, this is a separate disconnect switch.
NEC 422.31(B) tells us that for permanently connected appliances rated greater than 300 volt amperes or 1/8 horsepower, the disconnecting means is permitted to be the branch-circuit switch or circuit breaker for the appliance if the switch or breaker is within sight of the appliance or is capable of being locked in the "OFF" position.
The locking provision must be permanently installed on or at the switch or circuit breaker used as the disconnecting means. The locking provision must remain in place with or without the lock installed.
Workers have been seriously shocked when working on appliances that they thought were de energized, because after turning off the power, someone came along wondering why the switch was off and turned the power back on.
Listed lock-off devices are available from the various manufacturers of circuit breakers.
Disconnect switches have brackets with holes through which a padlock is installed. OSHA lock off requirements are discussed in Section 1.
NEC 422.35 requires that switches and circuit breakers used as the disconnecting means be of the indicating type, meaning they must clearly show that they are in the "ON" or "OFF" position.
Overcurrent Protection and Conductor Size
There are two separate issues that must be considered.
• The overcurrent protective device ampere rating must be calculated.
• The conductor size must be determined.
Do both calculations. Then compare the results of both calculations to make sure that both overcurrent device and conductors are Code compliant.
Branch-Circuit Overcurrent Protection
NEC 422.11(E) states that for single non-motor operated electrical appliances, the overcurrent protective device shall not exceed the protective device rating marked on the appliance. If the appliance has no such marking, then the overcurrent device is to be sized as follows:
• If the appliance does not exceed 13.3 amperes- 20 amperes
• If the appliance draws more than 13.3 amperes- 150% of the appliance rating
For a single non-motor-operated appliance, if the 150% sizing does not result in a standard size overcurrent device rating as listed in 240.6(A), then it’s permitted to go to the next standard size.
EXAMPLE:
A water heater nameplate indicates 4500 watts, 240 volts. What is the maximum size fuse permitted by the Code? Solution I = W / E = 4500 /240 =18.8 amperes
An electric water heater is a continuous load per 422.13. The branch-circuit rating must not be less than 125% of the water heater's nameplate rating.
The minimum branch-circuit rating is: 18.75 x 1.25 = 23.4 amperes
This would require a minimum 25-ampere fuse or circuit breaker, which is the next standard size larger than the calculated 23.4 amperes. See 240.6(A) for standard ampere ratings for fuses and circuit breakers.
The maximum overcurrent device for the water heater is 28.1 amperes.
This would be a 30-ampere fuse or circuit breaker, which is the next standard size larger than the calculated 28.1 amperes. But there is no reason to use a 30-ampere branch-circuit fuse or circuit breaker because we have already determined that a 25-ampere fuse or circuit breaker is suitable for the 4500-watt load.
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TBL. 4
Table showing typical small conductors, their allowable ampacities, and the maximum rating overcurrent devices permitted for these conductors.
Conductor | Size | Allowable Ampacity | Maximum Rating of Overcurrent | Device
14 AWG 15 amperes 15 amperes
12 AWG 20 amperes 20 amperes
10 AWG 30 amperes 30 amperes
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Conductor Size
The conductors supplying the 4500-watt water heater will be protected by the 25-ampere or breaker as determined above.
In Table 310.15(B)(16), we find the allow able ampacity of conductors in the 608C column as required by 110.14(C). In 240.4(D), we find the maximum overcurrent protection for small conductors. Putting this all together, we have the information shown in TBL. 4.
For the example, we selected a 25-ampere over current device. A 30-ampere OCD would have been acceptable. Next, we need to find a conductor that is properly protected by a 25- or 30-ampere OCD. The conductor would have to be a minimum 10 AWG Type THHN, which has an allowable ampacity of 30 amperes, more than adequate for the minimum branch-circuit rating of 23.4 amperes. A 10 AWG conductor is properly protected by a maximum 30-ampere overcurrent device.
If a 12 AWG Type THHN had been selected, it would have been suitable for the load but would not have been properly protected by the 25-ampere OCD. The maximum OCD for a 12 AWG is 20 amperes unless the branch circuit is for a special application such as for motors.
You might want to review Section 4 for a refresher as to why we use the 608C column of Table 310.15(B)(16) for selecting the conductor size.
The Water Heater for This Residence
The water heater for the residence is connected to Circuit A6-8. This is a 30-ampere, 2-pole circuit breaker located in the main panelboard in the work shop. This circuit is a straight 240 volts and does not require a neutral conductor.
Checking the Schedule of Special-Purpose Outlets, we find that the electric water heater has two heating elements: a 4500-watt element upper and a 4500-watt lower element. Thus it’s considered a quick-recovery unit. Because of the thermostats on the water heater, both elements cannot be energized at the same time. Therefore, the maximum load demand is: 18.75 or 18.8 amperes
For water heaters having two heating elements connected for nonsimultaneous operation, the name plate on the water heater will be marked with the largest element's wattage at rated voltage. For water heaters having two heating elements connected for simultaneous operation, the nameplate on the water heater will be marked with the total wattage of both elements at rated voltage.
Conductor Size and Raceway Size for the Water Heater in This Residence
The conductor and overcurrent device is required to be not smaller than 125 percent of the current.
18.8 amperes x 1.25 = 23.5 amperes This results in a 10 AWG copper conductor and a 30 ampere overcurrent device because the small conductor rule in 240.4(D)(4) generally requires a 12 AWG conductor to have overcurrent protection not greater than 20 amperes.
Table C1 in Annex C of the NEC indicates that trade size 1/2 EMT will be okay for two 10 THHN/ THWN conductors. For ease of installation, a short length of trade size ½ flexible metal conduit 18 to 24 in. (450 to 600 mm) long is attached to the EMT and is connected into the knockout provided for the purpose on the water heater. A 10 AWG equipment grounding conductor is required through the flexible metal conduit as required by the rules in NEC 250.118(5).
Cord/Plug Connections Not Permitted for Water Heaters
NEC 400.7(A) and 422.16 list the uses where flexible cords are permitted. A key Code requirement, often violated, is that flexible cords shall be used only where the fastening means and mechanical connections are specifically designed to permit ready removal for maintenance and repair, and the appliance is intended or identified for flexible cord connection. Certainly the plumbing does not allow for the "ready removal" of the water heater.
The conductors in flexible cords cannot handle the high temperatures encountered on the water heater terminals. Check the instruction manual for proper installation methods.
Flexible cords are not permitted to be:
• used as a permanent wiring method.
• run through holes in walls, ceilings, etc.
• run through doorways, windows, or similar openings.
• attached to building surfaces.
• concealed above ceilings, in walls, or under floors.
• installed in raceways.
• used where subject to physical damage.
Equipment Grounding
The electric water heater is grounded through the trade size ½ EMT and an internal 10 AWG equipment grounding conductor installed through the flexible metal conduit (FMC) or by the equipment grounding conductor contained within the Type NM cable if used as the wiring method.
Code references are 250.110, 250.118, 250.134, and 348.60. The subject of equipment grounding is covered in Section 4.
EFFECT OF VOLTAGE VARIATION ON RESISTIVE HEATING ELEMENTS
The heating elements in the water heater in this residence are rated 240 volts. A resistive heating element will only produce rated wattage at rated voltage. It will operate at a lower wattage with a reduction in voltage. If connected to voltages above their rating, heating elements will have a very short life.
Ohm's law and the wattage formula show how the wattage and current depend on the applied voltage. Always use rated voltage to calculate the resistance, current draw, and wattage of a resistive heating element. Check manufacturers' specifications.
Nichrome wire, commonly used to make heating elements, has a "hot" resistance approximately 10% higher than its "cold" resistance.
CAUTION: When actually testing a heating element, never take an ohms reading on an energized circuit. Remove any other connected wires from the heating element terminal block on a water heater to prevent erratic ohms readings.
Always check from the heating element leads to ground to confirm that there is no ground fault in the heating element.
Let's take a look at a 3000-watt, 240-volt heating element and calculate its resistance and current draw.
In resistive circuits, current is directly proportional to voltage and can be simply calculated using a ratio and proportion formula. For example, if a 240-volt heating element draws 12.7 amperes at rated voltage, the current draw can be determined at any other applied voltage, say 208 volts.
Also, in a resistive circuit, wattage varies as the square of the current. Therefore, when the voltage on a heating element is doubled, the current also doubles and the wattage increases four times. When the voltage is reduced to one-half, the current is halved and the wattage is reduced to one-fourth.
A 240-volt heating element connected to 208 volts will have approximately three-quarters of the wattage rating than at rated voltage.
EFFECT OF VOLTAGE VARIATION ON MOTORS
The above formulas work only for resistive circuits. Formulas for inductive circuits such as an electric motor are much more complex than this guide is intended to cover. For typical electric motors, the following information will suffice (see TBL. 5).
As previously mentioned, NEMA rated electric motors are designed to operate at 610% of their nameplate voltage.
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TBL. 5
Table showing the approximate change in full-load current and starting-current for typical electric motors when operated at under voltage (90%) and over voltage (110%) conditions.
Voltage Full-Load Starting Variation Current; Current 110% 7% decrease 10-12% increase 90% 11% increase 10-12% decrease
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HEAT PUMP WATER HEATERS
With so much attention given to energy savings, heat pump water heaters have entered the scene.
Residential heat pump water heaters can save 50% to 60% over the energy consumption of resistance type electric water heaters, depending on energy rates. The heat energy transferred to the water is three to four times greater than the electrical energy used to operate the compressor, fan, and pump.
Heat pump water heaters work in conjunction with the normal resistance-type electric water heater.
Located indoors near the electric water heater and properly interconnected, the heat pump water heater becomes the primary source for hot water. The resistance electric heating elements in the electric water heater are the backup source should it be needed.
A heat pump water heater removes low-grade heat from the surrounding air, but instead of transferring the heat outdoors as does a regular air-conditioning unit, it transfers the heat to the water. The unit shuts off when the water reaches 1308F (54.448C). Moisture is removed from the air at approximately 1 pint per hour, reducing the need to run a dehumidifier. The exhausted cool air from the heat pump water heater is about 108 to 158F (-12.228C to -9.448C) below room temperature. The cool air can be used for limited cooling purposes, or it can be ducted outdoors.
A typical residential heat pump water heater rating is 240 volts, 60 Hz, single-phase, 500 watts.
Heat pump water heaters operate with a hermetic refrigerant motor compressor. The electric hookup is similar to a typical 240-volt, single-phase, residential air-conditioning unit. Article 440 of the NEC applies. This is covered in Section 23.
Electric utilities and numerous Web sites are good sources to learn more about the economics of installing heat pump water heaters to save energy.
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