Main Service panelboard full. Couple of questions

[Strahan]

I'm currently working up an estimate for a customer who has installed a tankless water heater and needs electric run to it. His main service panelboard is full. I want to istall a 100amp "sub-panel" in order to provide enough room for the water heater. I'm finding lots of reference to panels installed at separate buildings but not necessarily same building. I'm treating this as a feeder and the panel will not be very big circuit number wise. I know to keep the grounded conductor and EGC separated at the feeder panel, but should I also run a bonding wire to the water heater since this panel is feeding it? I'm pretty sure I do.

Also, does the feeder panel require a main OCPD? reading 408.36 I'm thinking as long as the panel is rated for 100amps and is less than 42circuits which it will be I don't need one. Before anyone talks about load study being done I have looked at what is currently there and his main panel now is a 200amp but very small in number of circuits. Instead of replacing the main panel with a bigger circuit one the homeowner is requesting a feeder panel be installed. He doesn't want to go through the hassle of dealing with replacing the main panel. Any help would be appreciated. Thanks!

 

[Cletis]

I'm currently working up an estimate for a customer who has installed a tankless water heater and needs electric run to it. His main service panelboard is full. I want to istall a 100amp "sub-panel" in order to provide enough room for the water heater. I'm finding lots of reference to panels installed at separate buildings but not necessarily same building. I'm treating this as a feeder and the panel will not be very big circuit number wise. I know to keep the grounded conductor and EGC separated at the feeder panel, but should I also run a bonding wire to the water heater since this panel is feeding it? I'm pretty sure I do.

Also, does the feeder panel require a main OCPD? reading 408.36 I'm thinking as long as the panel is rated for 100amps and is less than 42circuits which it will be I don't need one. Before anyone talks about load study being done I have looked at what is currently there and his main panel now is a 200amp but very small in number of circuits. Instead of replacing the main panel with a bigger circuit one the homeowner is requesting a feeder panel be installed. He doesn't want to go through the hassle of dealing with replacing the main panel. Any help would be appreciated. Thanks!

First of all, how much does tankless water heater draw ?

Not sure what you mean a bonding wire to water heater ? you mean a ground (L1,L2, G) ?

No main required


All the tankless electric water heaters i've installed have been between 90-120 amps so...better check that and see if 200 amp main is enough for house now. Number of breakers doesn't mean anything

 

[Strahan]

First of all, how much does tankless water heater draw ?

Not sure what you mean a bonding wire to water heater ? you mean a ground (L1,L2, G) ?

No main required


All the tankless electric water heaters i've installed have been between 90-120 amps so...better check that and see if 200 amp main is enough for house now. Number of breakers doesn't mean anything

I'm talking GEC from sub panel to copper water line run to heater. Main water line is already bonded to main service panel I was just wondering if there were any requirements for this at a sub panel located in the same dwelling.
I know I need to run an EGC to the unit just not sure if any bonding requirements for sub in same building.

 

[jumper]

You do not need a GEC from sub panel to water line.

 

[Strahan]

You do not need a GEC from sub panel to water line.

Thank you I didn't think so because it's all handled at main service panel just wanted to make sure.

 

[Strahan]

First of all, how much does tankless water heater draw ?

Not sure what you mean a bonding wire to water heater ? you mean a ground (L1,L2, G) ?

No main required


All the tankless electric water heaters i've installed have been between 90-120 amps so...better check that and see if 200 amp main is enough for house now. Number of breakers doesn't mean anything

This is my first time dealing with these and I just asked homeowner for more detailed info on the unit and it turns out it requires two 60amp circuits!! Wow!!! Why two? Anybody know? This will for certain cause an issue with what H.O wants to do.

 

[lakee911]

This is my first time dealing with these and I just asked homeowner for more detailed info on the unit and it turns out it requires two 60amp circuits!! Wow!!! Why two? Anybody know? This will for certain cause an issue with what H.O wants to do.

User manual or installation manual will probably tell you. It's likely because it has two different elements and either two or three stages of fire depending on the flow rate.

 

[Strahan]

User manual or installation manual will probably tell you. It's likely because it has two different elements and either two or three stages of fire depending on the flow rate.

Yeah need to get ahold of that info. I'm tired of wasting time going by info provided by H.O instead of looking initially for myself.

 

[kwired]

This is my first time dealing with these and I just asked homeowner for more detailed info on the unit and it turns out it requires two 60amp circuits!! Wow!!! Why two? Anybody know? This will for certain cause an issue with what H.O wants to do.

Why two? Some require even more then two circuits. The reason is it has to raise the water temp from anywhere from 50 to 80 deg F instantaneously as it is being used. On top of that the faster the water flow the more watts of heat are needed to raise the water temperature.

But it draws nothing when no water is being used. This is where the energy efficiency concept comes in, no wasted heat when no water is being drawn, but energy demand will be at same time as water demand.

A tank style heater heats water in the tank even when no water is being used, but if the usage rate is fast enough or long enough it will not be able to keep up with demand for hot water. A tankless has no choice - it must keep up with demand from the start.

 

[GoldDigger]

They use two or three 60A circuits instead of one 120 or 180A because of NEC limitations, available branch breaker sizes, ability to better balance on a three phase supply, and probably several more reasons.

 

[Strahan]

They use two or three 60A circuits instead of one 120 or 180A because of NEC limitations, available branch breaker sizes, ability to better balance on a three phase supply, and probably several more reasons.

Now I'm concerned about main service size. Main panel full. Guess next step is to do a load study. I understand the concept of these heaters now just curious if it will draw full load amps under normal water flow/demand. FLA is listed at 50amps x 2.

 

[GoldDigger]

Now I'm concerned about main service size. Main panel full. Guess next step is to do a load study. I understand the concept of these heaters now just curious if it will draw full load amps under normal water flow/demand. FLA is listed at 50amps x 2.

The current drawn will be proportional to the product of the flow rate and the temperature rise from input temp to the set output temp.
Typically if the demand flow rate times temperature rise exceeds the maximum heat input, some tankless units will throttle back the flow.
If less than maximum best input is needed, I would hope that it either use proportional control or cycle the heating element very rapidly.

 

[GoldDigger]

The current drawn will be proportional to the product of the flow rate and the temperature rise from input temp to the set output temp.
Typically if the demand flow rate times temperature rise exceeds the maximum heat input, some tankless units will throttle back the flow.
If less than maximum best input is needed, I would hope that it either use proportional control or cycle the heating element very rapidly.

In short, if you open the tap in the tub wide and the pipes are big enough you will get the maximum flow rate that the electrical input can support.

 

[kwired]

Proper sizing of the unit will take into consideration a maximum design flow rate, as well as how much temp rise is necessary. Temp rise will vary from location to location as incoming water temp does vary. Use more flow then max design and you will have lower water temp.