Conductor Derating Question

[offroadsteve]

I think I understand the requirements, but I wanted to run this by the experts.

I'm getting ready to run power to my garage, I have 2" PVC buried from the house to the detached garage, right about 100' (for this calc I don't believe this matters).

My plan is to run 2-2-2-4 MHF as the feeder to the garage sub-panel, and I'd also like to run 4 pieces of 12 AWG THHN/THWN (Bl, R, W, G) for a 3-way switch back in the house.

Looking at the "conductor proximity adjustment factors" table, i'm at 6 current carrying conductors, which means I have to derate to 80% of the 90 degree C column, so the max breaker I can put on the MHF is 80 amps, correct?

I was hoping to protect the feeder at 90 amps (big difference of 10 amps I know...), and the adjustment seems unnecessary in this instance since i'm well under my conduit fill requirements for 2" PVC.

So... the question is what would you do? protect at 90 amps and see if the inspector catches it, or just install the 80 amp breaker and be done with it?

 

[sberry]

Are you running a light or 2?

 

[Cmreschke]

Umm, don't put it all together, iirc you cannot pull more than 4 wires between separate buildings. So no Threeway. Also your plan for your 3 way switch looks like your wanting 2 travelers a neutral and a ground, missing your common there.

 

[offroadsteve]

Sberry - Yes, I'd like to be able to control some of the outside lights on the garage from a switch near the back door. My read of NEC section 225.30, sub-section D allows the following "Additional feeders or branch circuits shall be permitted for different voltages, frequencies, or phases or for different uses, such as control of outside lighting from multiple locations."

 

[pattenp]

Umm, don't put it all together, iirc you cannot pull more than 4 wires between separate buildings. So no Threeway. Also your plan for your 3 way switch looks like your wanting 2 travelers a neutral and a ground, missing your common there.

That only applies if the additional wires are supplying power to the outbuilding. Switch loops are allowed if the power source comes from the outbuilding.

To the OP: Code now requires a neutral in the switch box even if not used, so you need to run 4 conductors plus a ground.

 

[ishiboo]

I think I understand the requirements, but I wanted to run this by the experts.

I'm getting ready to run power to my garage, I have 2" PVC buried from the house to the detached garage, right about 100' (for this calc I don't believe this matters).

My plan is to run 2-2-2-4 MHF as the feeder to the garage sub-panel, and I'd also like to run 4 pieces of 12 AWG THHN/THWN (Bl, R, W, G) for a 3-way switch back in the house.

Looking at the "conductor proximity adjustment factors" table, i'm at 6 current carrying conductors, which means I have to derate to 80% of the 90 degree C column, so the max breaker I can put on the MHF is 80 amps, correct?

I was hoping to protect the feeder at 90 amps (big difference of 10 amps I know...), and the adjustment seems unnecessary in this instance since i'm well under my conduit fill requirements for 2" PVC.

So... the question is what would you do? protect at 90 amps and see if the inspector catches it, or just install the 80 amp breaker and be done with it?

I have a 2" conduit between my house and barn as well, but I used Insteon for the lighting control instead. The barn is 100' from the house, but the signal actually travels to the pole and back to the barn (which has a separate service), so it's got some pretty good range over power line.

A thought as using Insteon I also have the capability of having the barn lights shut off automatically, turn them on from the house, etc.

 

[aandpdan]

Just run the switch wiring in a separate conduit, in the same trench.

As Patten says, the power has to come from the outbuilding for the lights.

 

[toplessHO]

you could derive the power from the outbuilding if you pulled a switch leg in with the other conductors. you would have 2 travelers ,switch leg,neutral and ground. Alot of care must be taken when pulling such a big difference in wire sizes in same conduit. smaller wires can be broken because of the tension on the larger ones when installing. Jacketed multiwire cable is useful for this.

 

[theoldwizard1]

That only applies if the additional wires are supplying power to the outbuilding. Switch loops are allowed if the power source comes from the outbuilding.

Good to know !

To the OP: Code now requires a neutral in the switch box even if not used, so you need to run 4 conductors plus a ground.

I always thought this was an odd requirement. I know it is because "smart" switches require a neutral and you can't use the ground. Sigh. I won't tell, if you won't tell !

 

[theoldwizard1]

So... the question is what would you do? protect at 90 amps and see if the inspector catches it, or just install the 80 amp breaker and be done with it?

According to the Southwire calculator, the max you can run is 70A.

 

[offroadsteve]

To the OP: Code now requires a neutral in the switch box even if not used, so you need to run 4 conductors plus a ground.

Pattenp - good call on the neutral, would have forgotten that. I'm working my way towards a 500' spool of black THWN and just labeling each end of the two travelers instead of cut lengths of different colors.

Thanks for all the suggestions, I will look into the Instion type stuff at Depot this morning.


Sent from my iPhone using Tapatalk

 

[offroadsteve]

You know it just occurred to me... I was planning to transition to NM-B for the final run to the switch, and I've not seen 14/4 romex... I'm sure it exists in BX for commercial work. Looking like wireless is going to be the way to go.


Sent from my iPhone using Tapatalk

 

[pattenp]

14-4 NM-b does exist. You can get it at an electrical supply company.