Any way to do this?

[suemarkp]

I'm trying to find a solution to provide backup power for a heat pump system I want to install. The heat pump will be 4 tons and have 15KW of resistance heat. I can buy the air handler such that it takes two power feeds -- one for the fan and a 5 KW heating element, and the other for the 10 KW heating element. The circuit raw amp loads are about 23A for the first and about 42A for the second.

I have a 100A subpanel and generator transfer switch. However, I can't run 65A of load through this panel, as there are too many other things that must run on it. I believe I can swing the 23A to run the fan and 5 KW element. The compressor won't be run from the subpanel, so the heater would have to work in "emergency heat" mode when in power fail conditions.

Now for the problem. It is my understanding that an appliance must be fed from one panel if it has multiple feeds. But I don't have enough capacity in the 100A panel to run the full up air handler when using all the resistance heat. Is there a legal way I can feed the 10KW from one panel and the 5KW from the transfer panel? Would a separate 30A transfer switch at the air handler allow this (just switching the 5KW circuit between the main panel and the subpanel)? Further complicating this is that the main panel feeding the 10KW heat is not the same panel that provides the feeder to the 100A subpanel (I have a 400A service with 2 main panels).

Or should I just abandon this complexity and keep 4 120V space heaters on hand for the rare occasions when the power fails?

[Wgoodrich]

Sue Markup, if it were me I would not limit myself. I would go outside by the meter base. You have I am guessing 2 parallel 200 amp panels in your home fed by a common meter base. If you installed a 200 amp transfer box between the meter base and the first 200 amp main serivce panel now then you could install any size generator to the generator side of that 200 amp transfer box. Then you could later when you can afford it install a second 200 amp transfer box to the second main service rated panel. Then as time goes buy you can buy a larger generator and larger generator etc. till you get the generated power you need a little invested at at time all the time having the choice to use the generator to feed any part of the house at any time. Remember you house brought up to heat will maintain for a long time with little temp change. During that time you could be maintaining your fridge and freezer and any other such as normal lighting load etc. If you start with a 60 amp rated generator you could run one at a time anything in your home including light use of electric range to cook by, water heater, furnace, general lighting loading only what you need shutting off what you need then changeing as needed as to what will run using your branch circuit breakers in that 200 amp panel as your controller what will be energized by that generator.

Then you could parallel a second generator to power the second 200 amp service controlled together as one generator in face with each other.

That is you best use of money not limiting yourself doing as you can afford over the years having something when you are done.

Hope this helps

Wg

[suemarkp]

Unfortunately, with where I had to place my panels, I could maybe put a 200A transfer switch in front of one of them. No way to ever put one in front of the other panel. That would have been a better solution in the end, but I already had the 100A switch and subpanel before I did the service upgrade.

Because of the huge amp loads of the heat pumps, I was segregating all of them and their compressors to one 200A panelboard, along with some other minor loads. I'd have to try to change this if one 200A panel was a generator panel. It may be possible, but not easy, because of things that have to be fed from the generator panel whether or not I would ever run them on a generator. For example, the pool building must have backup power so the pool pump can run when it is freezing outside. But I'll never run the pool heat pump in freezing conditions. Unless I want dual feeders to the pool building, all of the pool equipment must be fed from the generator panel (and this comes to a 50A load). There are other items with similar issues.

I think I'll just have space heaters on hand for those rare power failures. If I can convince my wife that a wood stove would be a good backup heat source, that would solve the problem too. But she hates what wood stoves do to her nose.

[Ron]

suemarkp,
What is the problem with serving the two point connection heat pump system, with two separate sources?
Not to state the obvious, but the nature of a double throw switch (transfer switch) has two sources, and it is one piece of equipment.
In large computer equipment centers, there are lots of equipment that have two sources, even two power supplies within the computer itself.
Could someone point me toward a violation?

[Wgoodrich]

A generator is a power source same as a utilty transformer. It is against Code to serve one building with two power sources except for a few exceptions such as dissimalar services [three phase versus single phase] and industrial settings where load is too big of service for a power comany to handle.

See I believe 230.2 of hte NEC also same rule applies for outdoor circuits such as accessory buildings using a common service of one main building found in 225.30.

Wg

[Ron]

IMHO, a generator is not a service. The definition of a Service is "....from the serving utility....".
In this case, there is one Service.
This equipment may have two sources (not two services). They are not interconnected within the unit, so I see no problem.

[suemarkp]

This may be what I was remembering:

422.30 General. A means shall be provided to disconnect each appliance from all ungrounded conductors in accordance with the following sections of Part III. If an appliance is supplied by more than one source, the disconnecting means shall be grouped and identified.

424.19 (article specific to fixed heating equipment) has the same restrictions.

My main panels are grouped, but the generator subpanel is about 25 feet away. I suppose I could comply with this by placing two disconnects at the air handler, but I don't need to now because all of my panels are in sight of the air handler so the two breakers (if in my main panel) can be the disconnecting means.

[Ron]

Sounds good to me.

[Wgoodrich]

Ron

See if this hits what you seek;

Copied section NEC handbook commentary;

Article 705 sets forth basic safety requirements for the installation of generators and other types of power production sources that are interconnected and operate in parallel as distributed generation. Power sources include any systems that produce electric power. They include not only electric utility sources but also on-premises sources ranging from rotating generators (see Article 445) to solar photovoltaic systems (see Article 690) to fuel cells (see Article 692).

COPIED SECTION NEC 2002;

705.12 Point of Connection.
The outputs of electric power production systems shall be interconnected at the premises service disconnecting means.


COMMENT;

There are some ifs and & buts involved but read that section and see if this hits your concern. This rule does allow more than one source or sources connected in different locations of a certain structure but only per approval of AHJ and only if a directory is installed at the point of service.

Hope this helps

Wg

[Ron]

WG,
That infers that the subpanel type transfer switch panelboards are not acceptable since they are not at the point of service, but as a distribution breaker off the the main panelboard.
I'm not sure I buy it.
I'm not sure if you have looked to purchase a computer server recently, but the more reliable ones have two power cords, for connection to two sources. I don't think these are violations either.

[Wgoodrich]

I provided a copied section of the actual rule. It has not been edited.

The transfer boxes mounted beside the main service rated panel meets those requirements.

If you tapped this transfer box to a sub panel not at that main service rated panel then it would be a violation.

Look that NEC rule up in your Code book. See if you read something [part of the rule or an exception that may apply] that fits what you desire to do.

Good Luck

Wg