Can a 150 amp connect be made on a 200 amp service?

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Old 10-14-19, 11:51 AM
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Question Can a 150 amp connect be made on a 200 amp service?

I installed a ground source heat pump with electric emergency backup heaters 23 years ago and have been relying on it to keep the house warm if the pump went out in winter if I was gone. It blew a capacitor this weekend so I've been trying to keep the house warm with the backup heaters until the serviceman arrives, only to find it trips the breaker after half an hour even with the compressor disengaged. I finally traced it back to insufficient wiring. [And here I thought I was safe all these years! ]

There is a 90 amp service to a subpanel with a 60 amp breaker for the heat pump and another 60 amp breaker for the electric backup. However, the electric backup requires a 90 amp service by itself. To properly rewire this setup I'm thinking I could install a 150 amp breaker in the 200 amp service panel, restring heavier wire to the subpanel, and then use the 90 amp breaker there to the heater connection.

The one concern I have is about having a 150 amp subcircuit out of a 200 amp service. When I leave in winter I turn off the water heater and with no appliances running the current load is minimal. Since I keep the electric backup heaters disengaged during normal day-to-day operations, I don't see where this would cause an overload to the 200 amp service.

Your views, please. BTW, I can't speak highly enough for the GSHP, In 23 years I've used no more than 10,000 kW annually to heat and cool a 3400 sq.ft. house with 57 windows here in Montana. {<$700 annually.)
 
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Old 10-14-19, 04:33 PM
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Hi, if the backup electric heat requires 90 Amps why is there a 60 Amp suppling it now?
Itís very doubtful you will be able to install 150 Amp breaker into a 200 Amp panel.
What is the name plate rating for the electric heat?
Geo
 
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Old 10-14-19, 04:40 PM
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Normally a single heating load cannot be more than 60 amps. In most cases the heater will be divided into two or more circuits. The compressor should not run at the same time as the heat strips. The only thing that should run is the air handler.

What is the total amps or watts on the nameplate of the electric heater? This will determine the size sub-panel and branch circuits you will need. 150 amps sounds like overkill to me. You will also have a tough time finding a 150 amp two pole breaker for your panel.
 
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Old 10-14-19, 05:06 PM
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You need to read the specs for your panel as to the largest branch breaker allowed. You may find that 125A is the largest allowed.
 
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Old 10-14-19, 06:19 PM
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The reason it has 60 amps is that it was miswired! The heaters are a 3-staged, set of coils, each rated at 5000 watts and the electrician apparently only read the spec for the heat pump which was 60 amps and missed the 90 amp requirement for the heaters.
 
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Old 10-14-19, 06:53 PM
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15000 watts / 240 volts = 62.5 amps x 125% = 78 amps (rounded)

The branch circuit should not be required to be more than 80 amps (Unless required by the manufacturer) using #4 copper wire or #3 cable. As mentioned I suspect the heater is designed to be fed by more than one circuit. Are there any breakers or fuses on the heater? Is there more than one set of terminals to connect the wires to? Perhaps you can post a picture of the heater outside and inside?
 
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Old 10-15-19, 11:02 AM
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Thanks for all the replies. The reason for the post was that I recognize sometimes a person's logical ideas may be at variance with the NEC. None of the replies has stated that my approach constituted an illegal activity but that there are different ways of managing the problem.

The 3-stage heaters are designed "to provide supplemental and/or emergency heat." Although I've never needed it as supplemental, it apparently could engage under extreme conditions. The data label and manual state that a 90 amp breaker is recommended, which per my initial post, wasn't properly wired initially. So if I wanted to energize the entire system as designed, I'd need up to 150 amps as both components could be powered simultaneously.

The only way to furnish 150 amps from the 200 amp service box would be with a 150 amp breaker to feed the subpanel in an adjacent room to allow shutoff breakers at that location. Eaton makes a CH 4-space 150 amp breaker that should work in my service box. It's my understanding that no one breaker should exceed 85% of the primary capacity of the service, so this could work. To install an additional second feed and a separate service box would be very difficult.

In the meantime, the service technician arrived and, after examination and discussion of the wiring configurations, recommended a workaround by simply disconnecting one heating element to lower the load to align with the current 60 amp breaker. I guess this could still overload the 90 amp breaker at the service if both the heat pump and the auxiliary electric were fully engaged, but I still intend to use it only as an emergency backup.
 
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Old 10-15-19, 04:05 PM
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You need to check the bus stab rating for the maximum breaker size.
 
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Old 10-15-19, 05:53 PM
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I'd need up to 150 amps as both components could be powered simultaneously.
The unit should only either run the heat pump OR the heat strips, never both. That would be horribly inefficient.
 
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Old 10-15-19, 06:15 PM
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There were design rating methods that offered the customer the option of buying the largest system that would heat under the most extreme temperatures or buying one for the lowest typical temperature. This difference was often at least another ton of capacity with an upgraded cost. The cost difference in the units plus the additional loop might be significant and a Water Furnace offered this design option.
 
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