conduit fill when sizing wire

[RDS]

We are wiring a large store with a 120/208 volt 3 phase service. Without the HVAC circuits, it is all duplex receptacles, track lighting and 3 tube T8 fluorescent fixtures.

When sizing wire and conduit fill for voltage drop, do the neutral wires count as a current carrying conductor? Also, when putting over four wires in a conduit, does the wire have to be derated for amperage draw. Say we are putting #8 wire for a 20 amp circuit that is pulling only 10 amps and running approx 240'. How many wires of #8 (3 hot legs and a neutral) can we put in a 1" and a 1 1/4" emt conduit?
The last questions is: Do we have to size strictly for a 3% voltage drop or would a 5% drop be of any significance in this situation?

I would greatly appreciate help with this. The engineer on this project did not size wire and conduit.

 

[roger]

Re: conduit fill when sizing wire

Start by reading all of 310.15(B)(2), and don't forget 250.122 if applicable.

The 3 and 5% voltage drop numbers are only recommendations and not code requirements.

For conduit fill go to Annex C

Roger

[ January 25, 2006, 10:31 AM: Message edited by: roger ]

 

[charlie b]

Re: conduit fill when sizing wire

Originally posted by RDS: When sizing wire and conduit fill for voltage drop, do the neutral wires count as a current carrying conductor?

Normally, not. Fluorescent lights (or specifically, the ballasts that control them) are generators of harmonics, as are computers that might be plugged into the receptacles. But in most store applications (and I should think most office applications), the harmonics won't be high enough to create a significant neutral current.

Originally posted by RDS:Also, when putting over four wires in a conduit, does the wire have to be derated for amperage draw. Say we are putting #8 wire for a 20 amp circuit that is pulling only 10 amps and running approx 240'. How many wires of #8 (3 hot legs and a neutral) can we put in a 1" and a 1 1/4" emt conduit?

The number of wires you can put into a conduit has nothing to do with the current you plan to run through the wires. It is entirely based on the cross-sectional area of the conduit and the wires. The answer comes from the Tables in Annex C. Assuming the wires are THHN, a 1" EMT can handle 9 #8 conductors, and a 1 1/4 can handle 16.

But derating will come into play, if you have more than 3 current-carrying conductors. A #8 THHN is good for 55 amps (per Table 310.16, 90C column). If you have four runs of conductors (i.e., 4 sets of A, B, C, Neutral), for a total of 16 conductors, and presuming the neutrals don't count, you have to derate by 50%. That gives you an ampacity of 22.5 amps. So you can use a 20 amp breaker for this application.

Originally posted by RDS: The last questions is: Do we have to size strictly for a 3% voltage drop or would a 5% drop be of any significance in this situation?

With the data you provided, I calculate a voltage drop of 3.2 volts, or 2%. So I don't think you have to worry about whether 5% would be significant. The "truth" of the situation is that the 3% and 5% are suggestions, and not requirements, so they are never significant. The "reality" of the situation is that motors (presuming there will be HVAC equipment) and other loads do not do well if the voltage gets too low. Most of us try to stick with the 3% and 5% numbers, just to give us confidence that there will be no problems.

 

[RDS]

Re: conduit fill when sizing wire

Thanks for the help.

 

[ramsy]

Re: conduit fill when sizing wire

Originally posted by RDS:
When sizing wire and conduit fill ..do the neutral wires count as a current carrying conductor?

That neutral counts on 3-wire circuits, w/2 hots originating from that 3? grounded wye. 310-15(b)4b