is tankless water heater considered a "continuous" load ?

[RustyShackleford]

Wondering if a tankless water heater is considered a "continuous" load, for purposes of increasing (or not) the amperage by 25% ? I have heard of the 3-hour rule (but can't seem to locate it in NEC), which of course a tankless could be run for 3 hours, but is certainly not expected to be.

The issue is adding a 6kW or so tankless to a sub-panel which is protected by a 50 amp breaker (in main panel) and already has a 4500 watt storage-tank water heater (but nothing else) connected.

 

[petersonra]

Wondering if a tankless water heater is considered a "continuous" load, for purposes of increasing (or not) the amperage by 25% ? I have heard of the 3-hour rule (but can't seem to locate it in NEC), which of course a tankless could be run for 3 hours, but is certainly not expected to be.

The issue is adding a 6kW or so tankless to a sub-panel which is protected by a 50 amp breaker (in main panel) and already has a 4500 watt storage-tank water heater (but nothing else) connected.

I would say no. It is not a storage type water heater.

422.13 Storage-Type Water Heaters. 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.

 

[RustyShackleford]

Thanks, yeah I know a storage tank is (although anything 50 gallons or less and 4500 watts or more can heat up a full tank of cold water in less than 3 hours), but NEC doesn't seem to say one way or the other for tankless.

 

[charlie b]

I have heard of the 3-hour rule (but can't seem to locate it in NEC). . . .

Try looking in the Article 100 definition of "continuous load."

 

[RustyShackleford]

So, to follow up, there are still two questions in my mind:

1. Just because the NEC says storage-tank water heaters ARE continuous (422.13), does that necessarily imply that tankless are not ?

2. Why are storage-tank style "continuous", since section 100 defines that as "expected to run for 3 hours or more", when most any residential storage tank will heat up from completely cold in less than 3 hours (ok, maybe an 80 gallon one would take 3 hours if it was set really hot or the incoming water was icy). Yes, you could leave the tap running, just as you could for a tankless; the key word is "expected".

I guess #2 is moot, the code says what it says. But I truly wonder about #1.

 

[JFletcher]

Wondering if a tankless water heater is considered a "continuous" load, for purposes of increasing (or not) the amperage by 25% ? I have heard of the 3-hour rule (but can't seem to locate it in NEC), which of course a tankless could be run for 3 hours, but is certainly not expected to be.

The issue is adding a 6kW or so tankless to a sub-panel which is protected by a 50 amp breaker (in main panel) and already has a 4500 watt storage-tank water heater (but nothing else) connected.

Dunno about TWH, but I know spas can be, at least on startup... 400 gallons of 60* water heated up to 100*F... pretty sure the heater would run 3 hours+ to get there.

Tankless could run that, maybe not at a residence, but commercially, in a hotel, wouldnt surprise me at all, since such a setup probably lacks mixing valves like a traditional boiler/water tank setup has.

 

[mwm1752]

you'll quit running your hot water in 3 hour incriments after the first electric bill -- tankless electrics are not intended for that type of use -- not continuous IMO

 

[Jraef]

So, to follow up, there are still two questions in my mind:

1. Just because the NEC says storage-tank water heaters ARE continuous (422.13), does that necessarily imply that tankless are not ?

2. Why are storage-tank style "continuous", since section 100 defines that as "expected to run for 3 hours or more", when most any residential storage tank will heat up from completely cold in less than 3 hours (ok, maybe an 80 gallon one would take 3 hours if it was set really hot or the incoming water was icy). Yes, you could leave the tap running, just as you could for a tankless; the key word is "expected".

I guess #2 is moot, the code says what it says. But I truly wonder about #1.

In #2, you are assuming the use will be such that someone uses hot water, it is replaced by cold, they shut off the hot and it takes a while, maybe less than 3 hours to re-heat.

But what if someone leaves the hot water running for 4 hours? Now think about that with an "instant" water heater.

Remember, the code is written by the National FIRE PREVENTION Association, so it is there primarily to prevent fires. The simple act of forgetting to turn the HW tap off could cause a heater to run continuously. If the tankless has a feature that automatically cuts power if it runs to 2 hours and 59 minutes, requiring a manual reset, that I think might qualify it as non-continuous.

 

[RustyShackleford]

Remember, the code is written by the National FIRE PREVENTION Association, so it is there primarily to prevent fires.

Doesn't using the right size wire, for the over-current protection device, cover that base ?

If the tankless has a feature that automatically cuts power if it runs to 2 hours and 59 minutes, requiring a manual reset, that I think might qualify it as non-continuous.

But I think the consensus here, and elsewhere, is that tankless IS non-continuous.

Anyhow, my point is that a person can leave the tap running forever regardless of the type of water heater - thereby letting it run for 3 hours. But Article 100 says "expected" to run for 3 or more hours.

 

[dwjhatton]

Try looking in the Article 100 definition of "continuous load."

Charlie, appreciate your insight to NEC rules for sure! I've been questioning the application of demand factor for IWH in nondwelling unit applications. Routinely applying 8 KW devices in restrooms located sporatically across manufacturing facilities. How does NEC require the demand factor for these appliances. Sizing branch circuit to requirement is easy, the cumulative load on the service and feeders is what I question.

 

[charlie b]

This is a four year old thread, and should not be extended with an unrelated question. I sent dwjhatton a message asking to have this new question posted on a separate, new thread. I will now close this one.