New sub panel

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  #1  
Old 05-04-13, 01:17 PM
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New sub panel

I have a 150 amp breaker box in my house which is a sub panel off a 200 amp
feed to my pole barn. The panel is currently loaded. I need to run another sub panel to the house to accommodate a 5 ton geothermal unit. I,m thinking along the lines of a 125 amp box. Two questions. Will 125 amp be
sufficient for the geothermal furnace and a 66 gallon elec. hot water? And the 2nd, what underground copper will I need to for a 150ft stretch?

The new furnace wil demand a 60 amp breaker for the 10k aux strip heat,
the data sheet from the manufacturer list the furnace as needing 45.8 FLA for the compressor,pumps and blower.
My water heater will require 30 amp's

Appreciate all inputs.
Thanks Larry
 
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  #2  
Old 05-04-13, 02:32 PM
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This is a subpanel in your home? Then you have another problem to deal with first. You can't run two feeders to your house. You'll have to upgrade the feeder between the buildings.

NEC 225.30 Number of Supplies. Where more than one building or other structure is on the same property and under single management, each additional building or other structure that is served by a branch circuit or feeder on the load side of the service disconnecting means shall be supplied
by only one feeder or branch circuit unless permitted in 225.30(A) through (E). For the purpose of this section, a multiwire branch circuit shall be considered a single circuit.

The exceptions are pretty much life safety type equipment.

Do a load calculation, see what you have and what you need for a feeder as depending upon your loads in the pole barn you might need to upgrade that too.
 
  #3  
Old 05-04-13, 03:18 PM
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Welcome to the forums!

I have a 150 amp breaker box in my house which is a sub panel off a 200 amp
feed to my pole barn. The panel is currently loaded. I need to run another sub panel to the house to accommodate a 5 ton geothermal unit.
Originally Posted by aandpdan
You can't run two feeders to your house.
But you can run a subpanel off the one you have now. Do a residential load calculation, as aand suggested, to see what you will need altogether. You may need to upgrade your existing feed and you may not, depending on what loads you have now.

If you haven't done a load calculation before, here's a tool - an Excel template - that some of have used and liked: Residential Load Calculations - Mike Holt Enterprises.

Two questions. Will 125 amp be sufficient for the geothermal furnace and a 66 gallon elec. hot water?
The new furnace will demand a 60 amp breaker for the 10k aux strip heat, the data sheet from the manufacturer list the furnace as needing 45.8 FLA for the compressor,pumps and blower.
My water heater will require 30 amps.
The breaker size is unimportant. The actual load is what matters. As one quick example, you've told us the FLA for the geothermal unit, but 30A sounds like the circuit size, not the draw, for the water heater. Plug those values, along with your other loads, into the calculator. Let us know what you find out.

And the 2nd, what underground copper will I need to for a 150ft stretch?
This will only need to be answered if you determine that your existing 150A feed needs to be upgraded.
 
  #4  
Old 05-04-13, 04:41 PM
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Electric water heaters require their branch circuit to be designed to 125% of the full load amperage of the heater. If you know you need a 30 ampere branch circuit then working backwards gives a maximum draw of 24 amperes on the heater. It could be somewhat less but let's go with 24 amperes.

Likewise, space heating branch circuits also need to be designed to 125% of the full load so if you have 10 kw of heaters the branch circuit must be designed to 52 amperes full load. However, when calculating the full load draw 42 amperes is the correct figure. Add that to the 24 amperes for the water heater and we get 66 amperes.

The full load amperage of the heat pump equipment is 46 amperes. I don't know if the branch circuit for that needs to be designed for 125% or not. Assuming not we add all three loads together and get 112 amperes, it seems as if a 125 ampere panel would be sufficient but if we throw in the design for heating devices multiplier we get 128 amperes and if the heat pump equipment also requires the 125% multiplier then the total would be 139.5 amperes. I personally would be reticent installing anything less than 150 amperes.

The price difference between a 125 ampere panel and a 150 ampere panel is minimal. Obviously the more important figure is the wire from the source to the panel. Honestly, since you need to upgrade the original feeder to the building it makes no sense to install anything smaller than a 200 ampere panel and you may want to even use two such panels with a common feed. To do so you may need to upgrade the service from the utility and the service panel in the pole barn.
 
  #5  
Old 05-04-13, 04:48 PM
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The OP has been told on another forum about having 2 feeders.

The new load will have a MCA of 114.1 not including the water heater.

Time for a service upgrade.
 
  #6  
Old 05-04-13, 05:18 PM
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My service to the barn is a larger panel with 200 amp feed. Very little
load in barn, some lighting and a couple outlets. Thanks for the advice,
looks like I'm back to the drawing board.
 
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