Feeder Taps: How to size junction box and conductors out of it?

[Prototype1]

Hey, I am trying to size the tap conductors size. Can anything tell me how? Ignore the dimension of the junction box it is wrong.

150 AF is 50 HP motor and 500 AF is 250 HP motor.
I am on 240.21(B) section.

Thanks

 

[petersonra]

It appears to me that the 350 MCM conductors are sized for 600 Amps so is not a tap conductor.

If the junction box where it splits off to the 3 motors is within the allowed distance to the OCPD for each respective motor, it would appear you would be OK to run these tap conductors.

Depending on the distance the conductors to the respective OCPDs would need to have an ampacity that is a minimum of either 33% or 10% of the rating of the 600 A OCPD upstream.

 

[Prototype1]

It appears to me that the 350 MCM conductors are sized for 600 Amps so is not a tap conductor.

If the junction box where it splits off to the 3 motors is within the allowed distance to the OCPD for each respective motor, it would appear you would be OK to run these tap conductors. They would be the normal size EXCEPT the conductors going to the 40 A CB would need to have a minimum of 10% of the ampacity of the upstream OCPD rating, so they would need to be upsized IMO.

The far right one goes to lighting XFMR and panel. (sorry, I did not mention that).

 

[petersonra]

The far right one goes to lighting XFMR and panel. (sorry, I did not mention that).

I was adjusting my post a little to be more correct while you were responding. At least I hope it is more correct. The tap conductor ampacity rating does not much care what the load is. Only the rating of the OCPD it lands on and the conductor length.

 

[augie47]

I don't recall what size your utility OCP device is ahead of your ATS.... assuming it is 600 also.
As petersonra notes, the 10 ft and 25' tap rule will require your conductor to have an ampacity of 10% or 33% of the upstream devicend, of course, be rated to carry the load and protected at their ampacity by the downstream device.

 

[Prototype1]

I don't recall what size your utility OCP device is ahead of your ATS.... assuming it is 600 also.
As petersonra notes, the 10 ft and 25' tap rule will require your conductor to have an ampacity of 10% or 33% of the upstream devicend, of course, be rated to carry the load and protected at their ampacity by the downstream device.

Yes, it is 600 Amp also ahead of ATS.

 

[Prototype1]

I don't recall what size your utility OCP device is ahead of your ATS.... assuming it is 600 also.
As petersonra notes, the 10 ft and 25' tap rule will require your conductor to have an ampacity of 10% or 33% of the upstream devicend, of course, be rated to carry the load and protected at their ampacity by the downstream device.

upstream device provide short circuit protection and downstream device provides overload protection.

 

[Prototype1]

I was adjusting my post a little to be more correct while you were responding. At least I hope it is more correct. The tap conductor ampacity rating does not much care what the load is. Only the rating of the OCPD it lands on and the conductor length.

Okay, then tap conductors ampacity lands on OCPD are 150AF, 500AF, and 40AT/150AF (XFMR). How can I find the length of the tap conductors? ( Should I do voltage drop calculations?).

 

[augie47]

Okay, then tap conductors ampacity lands on OCPD are 150AF, 500AF, and 40AT/150AF (XFMR). How can I find the length of the tap conductors? ( Should I do voltage drop calculations?).

The tap conductors are governed by the tap rules. In your case likely the 10' or 25'. I doubt VD will be an issue considering the wire size and length

 

[Prototype1]

Okay. The tap is less than 3 M. So, I did 10% of 600 = 60 A. for 40 AT (XFMR & Panel) (After going over 240.21 B(1)). Should I do the same thing for 500AF-250HP and 150AF-50HP?

 

[Prototype1]

The tap conductors are governed by the tap rules. In your case likely the 10' or 25'. I doubt VD will be an issue considering the wire size and length

Here is the photo of what I did. Should I get the ground, conduit size from AWG tap conductor size or from branch circuit breaker 150AF, 500AF,40AT?

 

[augie47]

Okay. The tap is less than 3 M. So, I did 10% of 600 = 60 A. for 40 AT (XFMR & Panel) (After going over 240.21 B(1)). Should I do the same thing for 500AF-250HP and 150AF-50HP?

Yes. Each tap must meet the appropriate tap rule. appears you may be paralleling #4s on the 150 amp OCP. You can not parallel conductors smaller than 1/0 {310.10(H)}

 

[Prototype1]

Yes. Each tap must meet the appropriate tap rule. appears you may be paralleling #4s on the 150 amp OCP. You can not parallel conductors smaller than 1/0 {310.10(H)}

Okay, does this conduit, ground and conductor size looks right to you?

 

[augie47]

Where's my engineering check ?

The quipment grounding conductors would be based on your 600 anmp OCP device so normally a #1 in each conduit however they don't need to be larger than the phase conductors {250.122(A)}.. so #1 on your 500kcmil and equal to your phase on the other,

 

[Prototype1]

Where's my engineering check ?

The quipment grounding conductors would be based on your 600 anmp OCP device so normally a #1 in each conduit however they don't need to be larger than the phase conductors {250.122(A)}.. so #1 on your 500kcmil and equal to your phase on the other,

I think I am misunderstood you sentence. Correct me if I am wrong. Isn't #1 conduit is small for 500 Kcmil? #3 conduit is right size for 500 kcmil. Or did I understand you wrong?

 

[augie47]

definite misunderstanding on one of our parts... no mention of conduits, #1 AWG equipment grounding conductor would be the normal size for protection on the load side of a 600 amp OCP device, however, the EGC does not need to be larger than the phase conductors of the tap.

 

[Sberry]

upstream device provide short circuit protection and downstream device provides overload protection.

Good to see a guy grasp that concept in simple terms.