[heywire321]

I have not been this field long enough to understand why this is such a complex topic among seasoned electricians and inspectors. However, there is a disagreement on if this design will satisfy 2017 NEC code due to derating/Ambient Temperature Correction Factors.

Would this not be to code?

 

[backstay]

Without more information, it is difficult to size the circuit/feeder. Does the manufacturer list the heater as continuous? What is the actual load, I’ve wired a few and I have not seen a 100 amp. I remember a 111 amp unit that took four 40 amp circuits. Anyway, if it’s rated continuous, feeder is 125 %.

#2 AL at 60 degrees is only good for 75 amps.



Interior Installations. In addition to the provisions of this article, Type SE service-entrance cable used for interior wiring shall comply with the installation requirements of Part II of Article 334, excluding 334.80.
For Type SE cable with ungrounded conductor sizes 10 AWG and smaller, whereinstalled in thermal insulation, the ampacity shall be in accordance with 60°C (140°F) conductor temperature rating. The maximum conductor temperature rating shall be permitted to be used for ampacity adjustment and correction purposes, if the final derated ampacity does not exceed that for a 60°C (140°F) rated conductor.

 

[Almost Retired]

actually im surprised that there is a debate.
as @backstay said the 100A feeder fails immediately without any calculations required

the breaker it is connected to is not 90*C rated
it is 60*C rated

so the wire could be a million degrees rated and it wont matter

 

[Almost Retired]

now if you put that wire on a 70A breaker, that works perfectly
but now the breaker is too small to feed the water heater

 

[heywire321]

Without more information, it is difficult to size the circuit/feeder. Does the manufacturer list the heater as continuous? What is the actual load, I’ve wired a few and I have not seen a 100 amp. I remember a 111 amp unit that took four 40 amp circuits. Anyway, if it’s rated continuous, feeder is 125 %.

#2 AL at 60 degrees is only good for 75 amps.



Interior Installations. In addition to the provisions of this article, Type SE service-entrance cable used for interior wiring shall comply with the installation requirements of Part II of Article 334, excluding 334.80.
For Type SE cable with ungrounded conductor sizes 10 AWG and smaller, whereinstalled in thermal insulation, the ampacity shall be in accordance with 60°C (140°F) conductor temperature rating. The maximum conductor temperature rating shall be permitted to be used for ampacity adjustment and correction purposes, if the final derated ampacity does not exceed that for a 60°C (140°F) rated conductor.

Thanks for replying. The tankless runs at 100A @ 100% demand so I would not consider that continuous.

At 1/3 demand it would use 33A, at 1/2 demand it would be 66A.

To your point for #2, this is not underground nor is it 10 AWG so how does that derating apply to what I have shared?

Let me know if I am missing something or have not shared enough. Still confused.

 

[Almost Retired]

Read the mfg info carefully, the last tankless i saw ran at 100% electrical all the time it was on.
if that was too hot they added cold water back to the mix to regulate temp.

if your tank runs at a full 100A then your 3 - 40A breakers are no longer sufficient
sure it would actually work for 6 months to a year, but at that point the panel and all of the breakers would be badly over stressed at minimum

every panel and breaker everywhere can only be used at 80% of its rating
very few breakers are 75*C rated, and none are 90
3 x 40 x 0.8 = 96

A 125A panel and main is required at 80% and 60*C rating

 

[oldsparky52]

#2 AL is not allowed to be used at the 90C rating in the installation you are describing. If you are allowed to use it at the 75C rating then it is undersized by at least 10-amperes.

 

[heywire321]

if your tank runs at a full 100A then your 3 - 40A breakers are no longer sufficient
sure it would actually work for 6 months to a year, but at that point the panel and all of the breakers would be badly over stressed at minimum

every panel and breaker everywhere can only be used at 80% of its rating
very few breakers are 75*C rated, and none are 90
3 x 40 x 0.8 = 96

A 125A panel and main is required at 80% and 60*C rating

This is what the manufacture shows, 3 x 40A. Are you stating that these should be using greater than 3 x 40A breakers?

 

[joe-nwt]

if your tank runs at a full 100A then your 3 - 40A breakers are no longer sufficient

How does 33A over stress a 40A breaker? If you look at a few spec sheets they all say 40A breakers on #8 wire.

 

[backstay]

Thanks for replying. The tankless runs at 100A @ 100% demand so I would not consider that continuous.

At 1/3 demand it would use 33A, at 1/2 demand it would be 66A.

To your point for #2, this is not underground nor is it 10 AWG so how does that derating apply to what I have shared?

Let me know if I am missing something or have not shared enough. Still confused.

How you look at continuous use is not useful. What does the manufacturer say in the installation instructions? SE is treated like NM in the code. If you put 100 amps on that cable for any length of time, it will fail. The 90 degree column is for de rating, not for general use. Can you guarantee the water heater won’t be turned on by flow for less than a certain amount of time? Is there some internal device that limits its time?

 

[backstay]

How does 33A over stress a 40A breaker? If you look at a few spec sheets they all say 40A breakers on #8 wire.

33 amps x 1.25 % = 41.25 amps.

 

[Almost Retired]

This is what the manufacture shows, 3 x 40A. Are you stating that these should be using greater than 3 x 40A breakers?

View attachment 162128

i am not saying you are wrong, i edited the post you replied to and added to read the mfg info carefully
i am saying to be cautious. if your tank is capable of running less than 100A (which would be new to me, but certainly possible and a good idea)
3 @ 40 may be just fine

24kW @ 240V is 100A ... the kW statement may be intentionally misleading
the statement "100A max draw" may be unintentionally misleading. normal full load may be less

but if you barely crack open a HW faucet and it pulls 100A regardless, i would use 50A and #6 at my house
and i would size my panel and feeder accordingly

none of my above statements have included what ever derate may be required by ambient temp for the feeder wire or anything else

 

[oldsparky52]

33 amps x 1.25 % = 41.25 amps.

What makes this a continuous load?

 

[backstay]

What makes this a continuous load?

An open faucet. I suggest reading the instructions, not the brochure.

 

[Almost Retired]

What makes this a continuous load?

that was what i was trying to get to in post 12.
the code way of determining continuous load is not how i would look at it in this situation

my question is whether it is capable of running at less than full load or reduced amps

and like backstay said ,,, the instructions not the brochure would be much more helpful here

 

[backstay]

So instead of the OP doing the leg work, I looked at the install manual. All I got was this.


“All wiring (wire gauge) and circuit protection (breakers) must comply with the U.S. National Electrical Code (NEC) in the USA, or the Canadian Electrical Code (CEC) in Canada. Failure to do so could result in property damage and/or personal injury, and void your warranty. Note: The Canadian Electrical Code generally requires that all supply wires and corresponding circuit protection used for domestic hot water heating and hydronic heating applications be sized to a minimum of 125% of the maximum current rating of the heater (see model specifications below for details).
Before installing this product, ensure that the home has sufficient electrical power available to handle the maximum amperage load of the applicable model.”


So the branch circuit breakers would be fine, but no way can you load #2 AL to 100 amps in this application. Not even getting into the cable through the attic.

 

[heywire321]

that was what i was trying to get to in post 12.
the code way of determining continuous load is not how i would look at it in this situation

my question is whether it is capable of running at less than full load or reduced amps

and like backstay said ,,, the instructions not the brochure would be much more helpful here

The heater has 3 heating elements. They are activated based on demand. The unit is rated for 5 GPM

To make this simple, if a shower head is rated at 1.6 GPM then each concurrent shower head to activate a heating element.

1 shower = 33A
2 showers = 66A
3 showers = 99A

 

[Almost Retired]

The heater has 3 heating elements. They are activated based on demand. The unit is rated for 5 GPM

To make this simple, if a shower head is rated at 1.6 GPM then each concurrent shower head to activate a heating element.

1 shower = 33A
2 showers = 66A
3 showers = 99A

now we are learning some stuff !!!
so maybe your 100A subpanel and breakers and #8 are fine

but still not the feeder

 

[heywire321]

So instead of the OP doing the leg work, I looked at the install manual. All I got was this.


“All wiring (wire gauge) and circuit protection (breakers) must comply with the U.S. National Electrical Code (NEC) in the USA, or the Canadian Electrical Code (CEC) in Canada. Failure to do so could result in property damage and/or personal injury, and void your warranty. Note: The Canadian Electrical Code generally requires that all supply wires and corresponding circuit protection used for domestic hot water heating and hydronic heating applications be sized to a minimum of 125% of the maximum current rating of the heater (see model specifications below for details).
Before installing this product, ensure that the home has sufficient electrical power available to handle the maximum amperage load of the applicable model.”


So the branch circuit breakers would be fine, but no way can you load #2 AL to 100 amps in this application. Not even getting into the cable through the attic.

"The Canadian Electrical Code generally requires that all supply wires and corresponding circuit protection used for domestic hot water heating and hydronic heating applications be sized to a minimum of 125% of the maximum current rating of the heater"


Thanks but that does not help I am looking for NEC code, not CEC.

 

[oldsparky52]

Note: The Canadian Electrical Code generally requires that all supply wires and corresponding circuit protection used for domestic hot water heating and hydronic heating applications be sized to a minimum of 125% of the maximum current rating of the heater (see model specifications below for details).
Before installing this product, ensure that the home has sufficient electrical power available to handle the maximum amperage load of the applicable model.”

I think the CEC is spot on in the way the look at this.

How many homes have an extra 100-amps sized into the service?