Feeder Sizing

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cppoly

Senior Member
Location
New York
For sizing feeders, is electric heaters and air conditioners sized at 100% or 125%, or does the NEC leave this decision up to you?
 

cppoly

Senior Member
Location
New York
215.2(A) doesn?t mention specifically these loads. It just says for noncontinuous loads to use 100% and continuous loads to use 125%. So does this become the debate of whether these loads are truly
 

suemarkp

Senior Member
Location
Kent, WA
Occupation
Retired Engineer
And 424.3 for heaters (but you can argue this wording only applies to branch circuits and not feeders).

Even with this, the answer isn't obvious. If the heater or AC unit has an MCA value (new units should), I'd say use that in your calculations as it will include the 125% factor. If all you have is watts on a heater, I think you need to treat it as continuous so you convert to amps and treat as a continuous load or multiply by 1.25.

Also, which feeder calculation are you doing -- standard or optional?
 

m sleem

Senior Member
Location
Cairo
IMO & based on the definition of continuous load which working continuously for a min 3 hrs, the water heater is not a continuous load & for the air conditioning (all fans are continuous loads and compressors are non-continuous loads).
 

1793

Senior Member
Location
Louisville, Kentucky
Occupation
Inspector
IMO & based on the definition of continuous load which working continuously for a min 3 hrs, the water heater is not a continuous load & for the air conditioning (all fans are continuous loads and compressors are non-continuous loads).

When and where in this thread was water heater mentioned?

Water Heaters are considered continuous loads per 422.13, 2011.
 

m sleem

Senior Member
Location
Cairo
When and where in this thread was water heater mentioned?

Water Heaters are considered continuous loads per 422.13, 2011.
You'r right but don't you see a conflict with the definition of the continuous load?

Continuous Load. A load where the maximum current is
expected to continue for 3 hours or more.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
You'r right but don't you see a conflict with the definition of the continuous load?
Your family does not take three hour showers?

More seriously, if you look at the ratio of the power input of an instant (on-demand) water heater to that of a conventional water heater, you may be willing to consider that a conventional heater may have a low enough recovery rate that occasionally household use will result in the heater element staying on for three hours or more?
 

m sleem

Senior Member
Location
Cairo
Your family does not take three hour showers?

More seriously, if you look at the ratio of the power input of an instant (on-demand) water heater to that of a conventional water heater, you may be willing to consider that a conventional heater may have a low enough recovery rate that occasionally household use will result in the heater element staying on for three hours or more?
I'm sorry but where home was mentioned in OP? my proposal pertains to commercial applications, aside from that the water heater in homes are switching off automatically based on thermostat control even if the shower takes 10 hrs:D.
 

kwired

Electron manager
Location
NE Nebraska
You'r right but don't you see a conflict with the definition of the continuous load?

422.13 says: "A fixed storage-type water heater that has a capacity of 450 L (120 gal) or less shall be considered a continuous load for the purposes of sizing branch circuits."

Other equipment that may not meet the definition of continuous load may still have specific places in code that still says to treat that particular load as a continuous load or at least to treat it as continuous for the application of a specific section. I believe general purpose/general use motors are typically going to be required to be treated as continuous loads as well as most air conditioning and refrigeration equipment, at least for branch circuit calculations, feeder/service calculations maybe not.
 

MasterTheNEC

CEO and President of Electrical Code Academy, Inc.
Location
McKinney, Texas
Occupation
CEO
You'r right but don't you see a conflict with the definition of the continuous load?
Not really....

Fact is, if the ground water coming in to a water heater (recently drained by a teenager who likes to take a 40 minute shower) is very cold it may take the elements more than 3 hours to come up to 120 degrees. The rate of rise may be only 15-20 degrees an hour and if the water coming in is 50 degrees, it will take 3 or more hours to come up to the temperature settings. So to acknowledge this fact, Section 422.12 considers this a continuous load when sizing the branch circuit.

Makes perfect sense to me.
 

kwired

Electron manager
Location
NE Nebraska
Not really....

Fact is, if the ground water coming in to a water heater (recently drained by a teenager who likes to take a 40 minute shower) is very cold it may take the elements more than 3 hours to come up to 120 degrees. The rate of rise may be only 15-20 degrees an hour and if the water coming in is 50 degrees, it will take 3 or more hours to come up to the temperature settings. So to acknowledge this fact, Section 422.12 considers this a continuous load when sizing the branch circuit.

Makes perfect sense to me.
True, but what size elements do you have in your water heater? I have 50-55 deg incoming water temp and a 4500 watt 80 gallon water heater, I can have fairly warm water in about half an hour - it will not last for too long at this point for baths or showers but if no water is being used I'd think it likely is able to get to 120 degrees before 3 hours are up.

OK I just looked up how much heat is needed to raise the water a specific temp, it should take 12.69 kilowatt hours to raise 80 gallons of water 65 deg F(120-55). If a 4500 watt element ran for 3 hours it would use 13.5 kilowatt hours, so the 80 gallon tank should fully heat to 120 within three hours if incoming temp was 55, if no water was used during this time. If water is being used it gets more complicated as it will depend on the rate it is being used, but you could have periods of over three hour run time pretty easily I would guess.
 

MasterTheNEC

CEO and President of Electrical Code Academy, Inc.
Location
McKinney, Texas
Occupation
CEO
True, but what size elements do you have in your water heater? I have 50-55 deg incoming water temp and a 4500 watt 80 gallon water heater, I can have fairly warm water in about half an hour - it will not last for too long at this point for baths or showers but if no water is being used I'd think it likely is able to get to 120 degrees before 3 hours are up.

OK I just looked up how much heat is needed to raise the water a specific temp, it should take 12.69 kilowatt hours to raise 80 gallons of water 65 deg F(120-55). If a 4500 watt element ran for 3 hours it would use 13.5 kilowatt hours, so the 80 gallon tank should fully heat to 120 within three hours if incoming temp was 55, if no water was used during this time. If water is being used it gets more complicated as it will depend on the rate it is being used, but you could have periods of over three hour run time pretty easily I would guess.
Just say "Yes" you are correct MasterTheNEC and end it with that true statement...;)
 

kwired

Electron manager
Location
NE Nebraska
My example was for an 80 gallon tank, 40 and 50 gallon tanks are pretty common with same size heat elements so those wouldn't be as close to needing three hours to heat up - but usage rate again is still a big variable here.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
Just say "Yes" you are correct MasterTheNEC and end it with that true statement...;)
Yes you are correct.:happysad: An electric water heater will heat ground water to 120? in less than an hour with the standard 4500 watt elements. :D But why are we on water heaters?
 

kwired

Electron manager
Location
NE Nebraska
But why are we on water heaters?
We got slightly derailed from OP at determining what is a continuous load, plus OP is addressing feeders and not branch circuits - so maybe the question is can a load be continuous for branch circuit calculations but not for feeder calculations? I say sometimes it can be that way.
 

suemarkp

Senior Member
Location
Kent, WA
Occupation
Retired Engineer
For motor/compressor loads, I think you can take them at 100% and add 25% to the one largest motor and one largest compressor. How you would do this is problematic, as HVAC nameplate info contains MCA, and the RLA/FLA of motors and compressors. But if there are electric controls in it, that isn't covered in the nameplate except under MCA. You may be able to reverse it out to take the motor and compressor loads, plus 25%, and the missing amps between this and MCA is your controls. So you'd take the biggest MCA value you have and add the full amps of all the others (no 125% factor on those).

Heaters will be install specific. If you have a central heater with only one stage, it will probably be considered continuous. If you have a heater with multiples heating stages, perhaps not. If you have multiple separate heaters, also probably not continuous. But you need to know their function (e.g. if this is an industrial process heater and it basically runs full on all day long, it will be continuous; If it is space heating and the place never shuts down, it would be difficult to call it continuous since it only just maintains temperature).

If you only have one heater or AC unit on the feeder, I think there are rules that state the feeder can't be smaller than the branch circuit size. So multiple units increases the odds that the feeder won't be continuous and code seems to allow it. But if you think it would be common to have all things running at the same time all day long with minimal to no cycling (e.g. we keep the doors open all the time), I'd size the feeder to the MCA value of all items connected.
 
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