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Author Information Topic:   Definition of Continuous and Non-Continuous Loads
Member

   
Name: Dennis Holm
Email: dholm@chronomite.com
Location: California
Title: Manufacturer
In Trade Since: 2000
Registered: Aug 2001
Total Posts: 1

posted August 23, 2001 at 05:06 PM       Edit/Delete Message
We are a manufacturer of Electric Tankless Water Heaters and I am often asked how the breaker or switches should be sized.

Our tankless water heaters are not water storage vessels. The heaters are only activated when the hot water is being used, typically for washing hands at a lavatory sink. Power to heat the water is therefore only being used when the hot water is turned on. A pressure switch activates the power.

I have looked through the NEC 1999 and think I understand how to calculate the load. Add 125% of the continuous load to 100% the non-continuous load.

The majority of our heaters are installed on dedicated lines, so the only amperage being drawn is from the heater.

A typical heater is 6KW, 240V, 25 Amps. Can this heater be installed on a 30 amp breaker/switch?

The other question is; What is the definition of Continuous and Non-Continuous load and can please tell where I can find it in the NEC 1999?

Thank you
Dennis L. Holm

IP: 165.121.117.133

unregistered posted August 23, 2001 at 06:11 PM           Edit/Delete Message
Yes, it should be on a dedicated 30 A double-pole breaker. How the conductors should be sized is slightly more involved. Since the water may be left running at the tap for days, I would treat it as a continuous load.

IP: 162.119.240.100

Member

   
Name: bob byrnes
Email: bobelectric@home.com
Location: New_Jersey
Title: Instructor
In Trade Since: 1963
Registered: Jul 2001
Total Posts: 34

posted August 23, 2001 at 08:24 PM       Edit/Delete Message
I agree with sparks with the wire continuous load is 3 hrs or longer artical 100 . I don't have a code book with me
quote:
Originally posted by DSpark@repairman.com:
Yes, it should be on a dedicated 30 A double-pole breaker. How the conductors should be sized is slightly more involved. Since the water may be left running at the tap for days, I would treat it as a continuous load.

IP: 24.191.125.188

unregistered posted August 23, 2001 at 08:48 PM           Edit/Delete Message
80% of 30amps is 24amps. Can a 25amp load be put on a 30amp c/b? Just curious.

IP: 205.188.199.158

Member

   
Name: Steve Mancuso
Email: electric@vermontel.net
Location: Vermont
Title: Contractor
In Trade Since: 1985
Registered: Oct 2000
Total Posts: 1676

posted August 23, 2001 at 10:03 PM       Edit/Delete Message
Continous Load. A load where the maximum current is expected to continue for 3 hours or more

art 100 definitions, pg 70-20 top...

220-3(b)(1) pg 70-48, branch, feeder & service calc...
Specific Applinace loads An outlet for a specific appliance or other load not covered in (2) throough (11) shall be computed based on the amperage rating of the appliance or load served.

240-6 pg 70-72, overcurrent protection
25A x 1.25%= 31.25 ( same math as Cal's)
next size up =35A ( good luck finding that one!)

IP: 205.240.83.181

Member

   
Name:
Email: electinsp@home.com
Location: Michigan
Title: Inspector
In Trade Since: 1962
Registered: Sep 2000
Total Posts: 258

posted August 24, 2001 at 06:14 AM       Edit/Delete Message
In response to Cals question, yes a 25 amp load can be placed on a 30 amp breaker. The load of 25 amps cannot be a continuous load unlesss the device is listed for use at 100% of its rating. Remember the load on the circuit is calculated first at 100% of non-continuous loads and 125% of continuous loads, the overcurrent protective device is selected for this load and if it contains continuous loads the device has to be selected using the same calculation or be rated for use at 100% of its rating. If there are no continuous loads in the calculation then the overcurrent device is selected to meet the non-continuous load as calculated.

In response to the original post, if the heater has no storage tank you can calculate the circuit from the name plate rating. If there is a storage tank then section 422-13 would require that the nameplate load be used at 125%. As stated above the definition of continuous load is a load that can be expected to be in operation for three hours or more.
DS brings up a good point about the water left running but this I would think is not a normal operation of the appliance, but something that may need to be considered depending on the installation location.

[This message has been edited by electinsp@home.com (edited August 24, 2001).]

IP: 24.4.252.150

Member

   
Name: Robert h Keis
Email: rkeis@dca.net
Location: Delaware
Title: Consultant
In Trade Since: 1960
Registered: Feb 2003
Total Posts: 9

posted August 25, 2001 at 03:10 PM       Edit/Delete Message
This water heater is not intended to run for days. How can you assume the water may be left running for days as you stated?

"Since the water may be left running at the tap for days, I would treat it as a continuous load." How do you justify this statement? The code don't cover such things as "may" be left running. We
"Continuous Load. A load where the maximum current is expected to continue for 3 hours or more….."
The key word in the definition is "expected". Not may. And so what is it does run for days? The chance of a 30 amp breaker tripping a 25 amp load is very, very remote. Is this going to start a fire? I bet not.
Someone ask if a 25 amp load can be put on a 30 amp breaker. The answer is yes. Again, the definition of "continuous load" must be addressed. You can put a "noncontinuous load" on a 30 amp breaker. The breakers are listed to carry their full load for up to 3 hours. Then they are limited to 80 percent.
The hand-dryers that you see in rest rooms that say full-load 20 amperes can be installed on a 20 amp breaker and circuit. They are not continuous loads by Code. They are intermittent, thermostat controlled loads.

IP: 208.0.86.131

unregistered posted August 25, 2001 at 04:07 PM           Edit/Delete Message
Good post, rkeis.

IP: 205.188.198.48

unregistered posted August 29, 2001 at 05:46 PM           Edit/Delete Message
>How can you assume the water may be left running for days as you stated?

Because it is completely at the user's discretion. I knew of a lady who left the hot water running in the shower all the time to make moisture for her tropical plants. There's one of every type out there somewhere. There could be a plumbing failure while the usual occupants are on vacation. Small children are known to turn on the water and not shut it off. Water may run unnoticed from the tap for many hours.

>They are intermittent, thermostat controlled loads.
And the heater described has no automatic provision for limiting its duty cycle to 80% after three hours. Therefore, it is my personal opinion that it is a continuous load for the purpose of sizing the conductors.

Why? If the breaker pops after three hours, do I care? No.

If the conductors melt their insulation off after four hours, do I care? Yes!

Is there any conflict here? I don't see it.

IP: 162.119.240.100

Member

   
Name: Scott Headley
Email: sheadley@mcbrideelectric.com
Location: California
Title: Estimator_Project_Manager
In Trade Since: 1984
Registered: Aug 2001
Total Posts: 2

posted August 29, 2001 at 06:33 PM       Edit/Delete Message
Lets not loose sight of the fact that the code is the lowest limit of protection allowed. You are always welcome and advised to provide greater protection in an issue of safety.

[This message has been edited by sheadley@mcbrideelectric.com (edited August 29, 2001).]

IP: 205.162.11.178

Member

   
Name: John Steinke
Email: renosteinke@yahoo.com
Location: Nevada
Title: Electrician
In Trade Since: 1974
Registered: Jun 2001
Total Posts: 116

posted September 11, 2001 at 12:00 AM       Edit/Delete Message
I do not consider this to be a continuous load. However, I would reccomend a GFI breaker- any current leakage is going to be directed to whoever is washing their hands, and touching the plumbing at the same time!

IP: 134.197.158.41

Member

   
Name: Robert h Keis
Email: rkeis@dca.net
Location: Delaware
Title: Consultant
In Trade Since: 1960
Registered: Feb 2003
Total Posts: 9

posted September 11, 2001 at 12:16 AM       Edit/Delete Message
Have you ever used a GFCI on a cal-rod element? I don't think so or you wound't recommend it.

IP: 208.0.86.230

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