Question: Bundled Conductor Derating Calculation

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PietroAZ

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Location
Santa Cruz Valley AZ
Occupation
Retired
Goal: To limit roof penetrations for installation of 3 Mini Splits

Each condenser would be fed by a 2p breaker at 20 amps (via 12/2 MC cable)
Also require an 120v 20 amp GFCI on roof, fed by a 1p breaker at 20 amps (via 12/2 MC cable)
All below roof and within structure (panel MC ...) before daylighting to condensers
Condensers have MCA of 14

Would like to take all through the roof with 3/4 or 1 inch IMC.
Could combine all with a 4457-DC (below).
Conductors dont appear to overfill the 3/4.

We are at 110F here for Design Ambient High Temperature.
I see THHN 12AWG at 30a and 0.87 as the derating multiplier for 106-113F.
And apparently an adjustment is also required 80% with up to 6 and 70% up to 9 conductors.

Math
30*0.87*0.8 = 20.88
With that the 120 GFCI would need to daylight on its own.

Did I get that even close to correct?





BPFTNG_MP_4457-DC.jpg
 

infinity

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Location
New Jersey
Occupation
Journeyman Electrician
You only need an adjusted conductor ampacity of 14 amps for the units and 15 amps if you use a 15 amp OCPD on the receptacle circuit. With 8 CCC's your adjustment value is 70%.
 

PietroAZ

Member
Location
Santa Cruz Valley AZ
Occupation
Retired
You only need an adjusted conductor ampacity of 14 amps for the units and 15 amps if you use a 15 amp OCPD on the receptacle circuit. With 8 CCC's your adjustment value is 70%.

Thank you

I had used the 0.8 as I was leaving the GFCI wiring out of the conduit, and going with 6 CCC's.
When I now look at the plan from the engineer, there is a note for the receptacle itself to be at 20 amps
that note is likely just copied and pasted from something else she had done, but it is on the approved plan :(
I will run the GFCI' in its own conduit.
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
How long are your circuits? Maybe this would apply and you can abandon the derating completely.

310.15(A)(2) Selection of Ampacity. Where more than one ampacity
applies for a given circuit length, the lowest value shall be used.
Exception: Where different ampacities apply to portions of a circuit, the
higher ampacity shall be permitted to be used if the total portion(s) of
the circuit with lower ampacity does not exceed the lesser of 3.0 m
(10 ft) or 10 percent of the total circuit.
 

PietroAZ

Member
Location
Santa Cruz Valley AZ
Occupation
Retired
How long are your circuits? Maybe this would apply and you can abandon the derating completely.

310.15(A)(2) Selection of Ampacity. Where more than one ampacity
applies for a given circuit length, the lowest value shall be used.
Exception: Where different ampacities apply to portions of a circuit, the
higher ampacity shall be permitted to be used if the total portion(s) of
the circuit with lower ampacity does not exceed the lesser of 3.0 m
(10 ft) or 10 percent of the total circuit.

If I understand what you are thinking and also what code allows for that would work

From panel to point of transition to conduit ~25 feet
Once conductors are together in conduit (sealed attic space), distance to daylight at rooftop is 3 feet
So the 7 feet remaining allows for much more than is needed above the roof line
 

Dennis Alwon

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Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
How long is the run from panel to connections where bundling does not apply? If the distance is 35 feet then you can only use 10% or 3.5 feet max for the bundling. If your run is 100' or more than 10' can be used (100 x 10%
 

tortuga

Code Historian
Location
Oregon
Occupation
Electrical Design
If you have a good spot you could also set a sub panel on the roof and just run a feeder up there to do you condensers and GFCI.
 

PietroAZ

Member
Location
Santa Cruz Valley AZ
Occupation
Retired
If you have a good spot you could also set a sub panel on the roof and just run a feeder up there to do you condensers and GFCI.

That would be grand wouldnt it!
Unfortunately, plans approved and permits already issued
Even though I am her customer, the engineer is not the shall we say the most accommodating individual ...will not be using her services again
 
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