100A residential appliance - preferred installation method

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type11969

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Greetings. I'm an electrical engineer working at a product development firm in Philadelphia. We are designing a novel water heating device for a client that will be drawing up to 100A, either 240 or 208VAC. We are familiar with the UL requirements for the device, and some of the NEC implications, but ultimately we would like feedback from professional electricians about how they would prefer to connect to such a device (especially considering that the wire gauge is not trivial from a routing perspective). To be clear, I read the FAQs and I'm not looking for specific instructions, installation methods, etc, I would merely like to get the opinion of actual electricians regarding the following:

1) Is a single 100A+ circuit in a residential retrofit application a non-starter (assuming there is not sufficient capacity in the existing box)? If we subdivide to two 50A circuits (understanding that we can't parallel the connection at the device per NEC), does this buy us anything from an installation perspective - or since a subpanel will need to be added, does it not matter?

2) If the installation is occurring in a new home build, does that lessen any of the concerns from #1?

3) Assuming the existing panel has sufficient capacity, or a subpanel has been installed, is routing cable capable of carrying 100A too much of an installation headache, or not that big of a deal? We are assuming this would be aluminum wire and would take the appropriate precautions at the connection point. Again, does two 50A circuits make this problem more manageable/cost effective?

4) Assuming 100A+ cable is being run to the device, is it more common to find cable bundles including a neutral line (3 wire plus ground), or without (two wire plus ground)?

Any insight appreciated! Please let me know if this is still in violation of forum rules and I will pull the post.

-Chris
 
type11969 said:
1) Is a single 100A+ circuit in a residential retrofit application a non-starter (assuming there is not sufficient capacity in the existing box)? If we subdivide to two 50A circuits (understanding that we can't parallel the connection at the device per NEC), does this buy us anything from an installation perspective - or since a subpanel will need to be added, does it not matter?
Click to expand...


Adding a 100 amps is adding a 100 amps to the total load calculation. This is a lot of load for most esisting services and would require a service upgrade ($$$) to many existing homes.

To tell the truth I often recommend gas water heaters as a cheaper option.


For a new home I don't see a problem as the added load can be calculated before the service is installed.
 
type11969 said:
1) Is a single 100A+ circuit in a residential retrofit application a non-starter (assuming there is not sufficient capacity in the existing box)? If we subdivide to two 50A circuits (understanding that we can't parallel the connection at the device per NEC), does this buy us anything from an installation perspective - or since a subpanel will need to be added, does it not matter?
Click to expand...


Adding a 100 amps is adding a 100 amps to the total load calculation. This is a lot of load for most existing services and would require a service upgrade ($$$) to many existing homes.

To tell the truth I often recommend gas water heaters as a cheaper option.


For a new home I don't see a problem as the added load can be calculated before the service is installed.
 
Two 50A will be a lot easier to wire than 100A in existing construction.
It will need a two pole breaker in any case, so space in the panel may be an issue for the two circuit solution. But a service change would take care of that too.
Will this ever be a continuous (full current for three hours or more) load?

Sent from my XT1585 using Tapatalk
 
Welcome. A 100A appliance in a new home with other high draw electrical loads (oven/range, dryer, HVAC w/elec emergency heat, spa) is going to draw more than 200A and need a 400 (320A) service.

From a cost standpoint, a 100A breaker is more expensive than 2x the cost of 2 50A breakers. It would be much easier to pull two runs of #6 NM/#8 MC than one of #2cu (#3 NM doesn't appear available) or 1 1/0 AL as well as cheaper, design permitting (as you wrote, you can't parallel conductors of that size; your unit would be 2 50A heaters in one appliance.

A 100A disco is more expensive than 2 60A discos. I dont see why you'd need a subpanel.

Subdividing the unit into 2 50A circuits that cannot be run at the same time (or making a half sized unit) would get you into older panels.

If you are considering more than a one-off production, you will probably want to make units that are sized for 208 and 240. Running a 240V resistive load on a 208V supply only gives 75% of the power as a 240V supply.

Mfg that make water heater units with such electrical demands as yours utilize 2 circuits w/o neutrals:

https://www.titanheater.com/proddetail.php?prod=Tempra-24
 
GoldDigger said:
Two 50A will be a lot easier to wire than 100A in existing construction.
It will need a two pole breaker in any case, so space in the panel may be an issue for the two circuit solution. But a service change would take care of that too.
Will this ever be a continuous (full current for three hours or more) load?

Sent from my XT1585 using Tapatalk
Click to expand...

Thanks, this is an instant hot water heater, so no, the load will not be continuous for 3 hours or more. It will be continuous for the duration of hot water usage however, so I suppose someone could take a 3 hour long shower . . .
 
Apologies for my ignorance, what is a disco (besides a 70s throwback)?

This is a product heading towards production, understood regarding the variation in output based on input voltage, but my understanding is that in most residential situations, 240 will be available. Is this not correct?

JFletcher said:
Welcome. A 100A appliance in a new home with other high draw electrical loads (oven/range, dryer, HVAC w/elec emergency heat, spa) is going to draw more than 200A and need a 400 (320A) service.

From a cost standpoint, a 100A breaker is more expensive than 2x the cost of 2 50A breakers. It would be much easier to pull two runs of #6 NM/#8 MC than one of #2cu (#3 NM doesn't appear available) or 1 1/0 AL as well as cheaper, design permitting (as you wrote, you can't parallel conductors of that size; your unit would be 2 50A heaters in one appliance.

A 100A disco is more expensive than 2 60A discos. I dont see why you'd need a subpanel.

Subdividing the unit into 2 50A circuits that cannot be run at the same time (or making a half sized unit) would get you into older panels.

If you are considering more than a one-off production, you will probably want to make units that are sized for 208 and 240. Running a 240V resistive load on a 208V supply only gives 75% of the power as a 240V supply.

Mfg that make water heater units with such electrical demands as yours utilize 2 circuits w/o neutrals:

https://www.titanheater.com/proddetail.php?prod=Tempra-24
Click to expand...
 
type11969 said:
Apologies for my ignorance, what is a disco (besides a 70s throwback)?

This is a product heading towards production, understood regarding the variation in output based on input voltage, but my understanding is that in most residential situations, 240 will be available. Is this not correct?
Click to expand...

Disco = disconnect. Most residential is 240V true, however apartment buildings, condos, and other multi-unit buildings may be 208V. Titan just makes one unit and accepts the power loss on a 208V circuit.
 
Thanks again everyone for your feedback. The Titan product is competition, although the tech used is different. The titan product is required by UL499 to subdivide, our product is not subjected to the same limitation. Based off of your feedback, however, it seems like subdividing is still the preferred approach.

JFletcher said:
Welcome. A 100A appliance in a new home with other high draw electrical loads (oven/range, dryer, HVAC w/elec emergency heat, spa) is going to draw more than 200A and need a 400 (320A) service.

From a cost standpoint, a 100A breaker is more expensive than 2x the cost of 2 50A breakers. It would be much easier to pull two runs of #6 NM/#8 MC than one of #2cu (#3 NM doesn't appear available) or 1 1/0 AL as well as cheaper, design permitting (as you wrote, you can't parallel conductors of that size; your unit would be 2 50A heaters in one appliance.

A 100A disco is more expensive than 2 60A discos. I dont see why you'd need a subpanel.

Subdividing the unit into 2 50A circuits that cannot be run at the same time (or making a half sized unit) would get you into older panels.

If you are considering more than a one-off production, you will probably want to make units that are sized for 208 and 240. Running a 240V resistive load on a 208V supply only gives 75% of the power as a 240V supply.

Mfg that make water heater units with such electrical demands as yours utilize 2 circuits w/o neutrals:

https://www.titanheater.com/proddetail.php?prod=Tempra-24
Click to expand...
 
Question for clarification:

At 100A, what is the normal wiring bundle? 3x wires (leg1, leg2 and ground)? 4x wires (leg1, leg2, neutral and ground)? Or are both equally common (no pun intended)?

-Chris
 
type11969 said:
Question for clarification:

At 100A, what is the normal wiring bundle? 3x wires (leg1, leg2 and ground)? 4x wires (leg1, leg2, neutral and ground)? Or are both equally common (no pun intended)?

-Chris
Click to expand...

it would be unlikely that a neutral would be required for such an appliance. One would think the manufacturer of such a device would know such a thing though.

personally, I think the chances of someone retrofitting this into an existing installation is remote as it will almost certainly require some kind of service upgrade and that may well cost as much as the appliance. so, I would not think how easy it is to wire into an existing residence matters much.
 
The current product is connected to 50A service, so the manufacturer does not have an answer to this question. Neutral is not required, but if it is common to be included in a bundle capable of handling 100A, then we can take advantage of it in the new device (transformerless, non-floating, low voltage power supply w/ neutral reference)

petersonra said:
it would be unlikely that a neutral would be required for such an appliance. One would think the manufacturer of such a device would know such a thing though.

personally, I think the chances of someone retrofitting this into an existing installation is remote as it will almost certainly require some kind of service upgrade and that may well cost as much as the appliance. so, I would not think how easy it is to wire into an existing residence matters much.
Click to expand...
 
type11969 said:
Not readily, no. I don't think we can guarantee that the breakers won't be paralleled within the system, and as far as what I understand, paralleled breakers is forbidden per NEC. Am I mistaken?
Click to expand...
You are mistaken if you think it is up to the manufacturer of an appliance to ensure the installer abides by the national electrical code.
 
type11969 said:
The current product is connected to 50A service, so the manufacturer does not have an answer to this question. Neutral is not required, but if it is common to be included in a bundle capable of handling 100A, then we can take advantage of it in the new device (transformerless, non-floating, low voltage power supply w/ neutral reference)
Click to expand...

A neutral conductor would only be provided if the device requires it. Don't spec it if you don't really need it. It's a large additional expense for the customer if the run is long. It would be especially annoying if you specify a neutral, but only use it for control power (tiny current) since the customer will be forced to run a very large neutral conductor to match the 100A breaker. Most electric tankless water heaters are 240V only (no neutral required).

You'll find that high power electric heating devices in residential installs often have the heating elements internally segmented into separate circuits of 50-60A max. Furnaces and tankless water heaters usually do this. It's much easier to work with on the customer/installer side. Whether you can allow the separate circuits to interact electrically inside the device will depend on what your applicable UL/ETL/other testing lab standards require. You might be allowed to do it. If there is any possibility of one circuit backfeeding the other, you would have to warn about this and require a handle tie between the multiple breakers.

It seems odd that you couldn't segment the heating elements and keep them electrically separate, though. Why not?
 
Another advantage of two separate heating elements is that if one fails, usually the second is still operational, giving some remaining warm water.
 
Good point about the neutral, I didn't think about conductor sizing.

Tankless hot water heaters are subdivided to 48A due to UL compliance. We are using a different technology that won't require this subdivision. Due to this tech, we cannot guarantee subdivision within the device (even if we give the option to connect two 50A services) - backfeeding will result.

NEC code states that you cannot parallel breakers, I assumed that this would extend in to the device - in other words, you are not allowed to parallel the two 50A circuits at the device design level. I suppose NEC doesn't cover device design, I will check with UL regs to see if they have a requirement.

mpoulton said:
A neutral conductor would only be provided if the device requires it. Don't spec it if you don't really need it. It's a large additional expense for the customer if the run is long. It would be especially annoying if you specify a neutral, but only use it for control power (tiny current) since the customer will be forced to run a very large neutral conductor to match the 100A breaker. Most electric tankless water heaters are 240V only (no neutral required).

You'll find that high power electric heating devices in residential installs often have the heating elements internally segmented into separate circuits of 50-60A max. Furnaces and tankless water heaters usually do this. It's much easier to work with on the customer/installer side. Whether you can allow the separate circuits to interact electrically inside the device will depend on what your applicable UL/ETL/other testing lab standards require. You might be allowed to do it. If there is any possibility of one circuit backfeeding the other, you would have to warn about this and require a handle tie between the multiple breakers.

It seems odd that you couldn't segment the heating elements and keep them electrically separate, though. Why not?
Click to expand...
 
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