Continuous or Intermittent Duty?

Status
Not open for further replies.
For the the purposes of Art. 210.20 (A), is a 62 KW, 208 volt, 3 phase, THERMOSTATICALLY controlled oven considered Continuous Duty or Intermittent Duty (Art. 90)? This is one of 4 such ovens in use constantly for 18 hours a day.
 

iwire

Moderator
Staff member
Location
Massachusetts
The NEC will allow you to call them non-continuous loads, see Article 100 definition of continuous load.

That said, that may not be the best design.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
I agree. The thermostat will cycle the ovens on and off therefore they should not be continuous. I am not sure why the NEC considers a Water Heater to be continuous even though it has thermostats.

I guess it is possible-- if there are enough people at the house taking showers at the same time that the unit may run for more than 3 hours. Seems like that would be a very unusual circumstance but I think of xmas time around here when I have 13 family members staying at my house for a few days. I bet that heater doesn't get much relief then. :)
 

iwire

Moderator
Staff member
Location
Massachusetts
I am not sure why the NEC considers a Water Heater to be continuous even though it has thermostats.

My guess is that is to handle the times the water heater has to cold start, I bet it will run more then 3 hrs without a break if it has just been started with a tank of cold water.

But that is just a guess.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
My guess is that is to handle the times the water heater has to cold start, I bet it will run more then 3 hrs without a break if it has just been started with a tank of cold water.

But that is just a guess.

Actually it takes less than an hour, if memory serves me correctly.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
I found this but who knows how accurate the info is.

This depends on your method of heating the water. Here in America, we generally use water heater tanks to heat our water. If you look on your heater tank it will tell you the capacity in gallons and the amount of energy it uses to heat in british thermal units (BTU's) or in watts/kilowatts (W/kW).

To use an equation to find out how long it will take you have to know some descriptions and conversions.

1. 1 BTU is the amount of energy it takes to increase the temperature of one pound of water one degree Fahrenheit (F).

2. 1 W of energy equals about 3.4BTU/h.

**For example, lets say your heater says it uses 4000 W (or 4kW) then you would multiply 4000 X 3.4 to get 13,600 BTU's.**

3. 1 U.S. Gallon (gal) of water equals 8.3 pounds(lbs).

4. On average, water enters your home between 45 and 55 degrees F. This is used to establish change in temperature or Delta T.

**For example, guessing your water coming in is 50 degrees F and your heater is set at 110 degrees F, your change in temperature (Delta T) would be 60 (110-50=60)

The equation is the number of gallons in your tank times 8.3 pounds (the weight of a gallon) times the change in temperature, divided by the energy your heater uses (sometimes in watts but the equation must be in btu's). [gal X 8.3 X (delta T)/btu]

So let's say your heater is brand new and it can hold 40 Gal, is rated at 30,000 BTU, and is set to 120 degrees (with 50 degree water coming in), the equation would be:

40 X 8.3 X 70 / 30,000 = .77466666e

Just round that to .775 and multiply 60 X .775 to get 46.5 minutes. Thats how long it will take to heat the water in this situation.

Remember to find your numbers needed on your heater and if you want to be really accurate you can try to test the temp of your water coming from your cold water tap. As time goes by, your heater becomes less efficient and will not necessarily use that max BTU rating.
 

e57

Senior Member
I agree. The thermostat will cycle the ovens on and off therefore they should not be continuous. I am not sure why the NEC considers a Water Heater to be continuous even though it has thermostats.

I guess it is possible-- if there are enough people at the house taking showers at the same time that the unit may run for more than 3 hours. Seems like that would be a very unusual circumstance but I think of xmas time around here when I have 13 family members staying at my house for a few days. I bet that heater doesn't get much relief then. :)
Doing a building right now that has a water-heater as a boiler for hydronic - (temp set-up to help dry the rock) it has been running 24/7 for two weeks now - the building is all closed up.... The plumber figured it would take 5 days or so on max to even get warm... :roll:

If say you have a recirculation loop with a pump on a water heater it would have to heat the entire length of pipe before it went off - sometimes that takes a day or so at start-up.
 

jumper

Senior Member
I found this but who knows how accurate the info is.

OOH. OOH. I do. I do. In general your example is correct.

I doubt that the recovery is that quick.

In my experience, resi WH recovery is about 45 min to 1 hour depending on size and age.


One of engineers here could figure out how long it would take 4500 watts to raise 60 gallons from say 50F to 120F.

Could not find an engineer, would you settle for a nuclear physist.:cool: He said that the formula that Dennis posted was good enough for a reasonably close estimate.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
My guess is that is to handle the times the water heater has to cold start, I bet it will run more then 3 hrs without a break if it has just been started with a tank of cold water.
The same certainly applies to an HVAC system started during any temperature extreme.
 
Status
Not open for further replies.
Top