Neutral sizing

[hes 01]

I am having a hard time figuring out my neutral size on a 3 phase 120/240 service with a high leg. I will be installing a overhead service from a ct cabinet and thinking about 2 3" conduits parallel with 300 copper wire. There was a existing service there and have to meet the power company code as well as ahj.

 

[infinity]

Have you done any load calculation to come up with the parallel 300 kcmil conductors?

 

[Carultch]

I am having a hard time figuring out my neutral size on a 3 phase 120/240 service with a high leg. I will be installing a overhead service from a ct cabinet and thinking about 2 3" conduits parallel with 300 copper wire. There was a existing service there and have to meet the power company code as well as ahj.

Group all of your 120V single phase loads. Organize by phase A and phase C. Add up the phase A, add up the phase C. Select the maximum.

Any 240V loads, whether they be phase-to-phase, or three phase, will not affect neutral current.

Therefore:
1. "Add up" your 120V phase A loads with proper load diversity factors.
2. "Add up" your 120V phase C loads with proper load diversity factors.
3. No need to factor in 3-phase loads in to either of the above
4. Select the larger of the 1 and 2. Size neutral to at least this amount.
5. In no case shall the neutral be smaller than the equipment grounding conductor, whether a wire-based EGC is installed or not. And the EGC needs to be sized per the overcurrent device (plus upsized for distance if needed). The overcurrent device will account for all loads both single phase and three phase.

 

[Smart $]

Group all of your 120V single phase loads. Organize by phase A and phase C. Add up the phase A, add up the phase C. Select the maximum.

Any 240V loads, whether they be phase-to-phase, or three phase, will not affect neutral current.

Therefore:
1. "Add up" your 120V phase A loads with proper load diversity factors.
2. "Add up" your 120V phase C loads with proper load diversity factors.
3. No need to factor in 3-phase loads in to either of the above
4. Select the larger of the 1 and 2. Size neutral to at least this amount.
5. In no case shall the neutral be smaller than the equipment grounding conductor, whether a wire-based EGC is installed or not. And the EGC needs to be sized per the overcurrent device (plus upsized for distance if needed). The overcurrent device will account for all loads both single phase and three phase.

You forgot 120/240 loads. Perhaps semantics, or even that there aren't any on a high leg system... but still a consideration until verified.

There's also a potential reduction per 220.61(B).

 

[Dennis Alwon]

If this is a service then there is no equipment grounding conductor so substituted a grounding electrode conductor for the equipment grounding conductor in #5 above.

 

[Carultch]

You forgot 120/240 loads. Perhaps semantics, or even that there aren't any on a high leg system... but still a consideration until verified.

There's also a potential reduction per 220.61(B).

Can you give me an example of a "120/240V load"? A load that actively uses all three wires.

Not a panelboard feeder containing 120V loads on both phases

 

[Smart $]

Can you give me an example of a "120/240V load"? A load that actively uses all three wires.

Not a panelboard feeder containing 120V loads on both phases

Well the typical ones are cooking appliances and electric clothes dryers. May run into others in commercial or industrial applications.