Heating Effect of Multiple Current Carrying Conductors on Voltage Drop

[ToxiKing]

A construction site is being constructed. Among the loads, there are multiple 3-phase Branch Circuits. 14 to be exact. 7 of them (load current of 40 Amperes, the farthest is 70 meters away from the Panel Board) have successfully installed. The conductor and conduit size recommended on plan is 4 AWG THHN within 2" PVC Conduit each circuit. 4 AWG has 85 Ampere ampacity at 75 deg C and 95 Amperes at 90 deg C.

I calculated the Voltage Drop and found it out to be 4.59V (2 percent %VD)

Now, big miscoordination happened. 7 circuits were installed but there are no other way to install the other 7 circuits (load current of 40 Amperes, the farthest is 30 meters away from Panel Board) in an aesthetical way. My proposed solution is to insert each of this 7 circuits to the first 7 circuits successfully installed. Meaning, one 2" PVC will now contain 6 pcs 4 AWG current carrying conductors and 2 pcs 10 AWG ground conductors. 2" PVC Conduit has enough space to be filled with 16 pcs 4 AWG. 4 AWG will have an adjusted ampacity of 76 Amperes, still greater than the 40 Amperes load. All were good. Perfect!

But..

If you are on this situation, would you recommend the same? On this proposed solution, what would be the new voltage drop? Is the calculated 4.59V drop of the first circuit unaffected? But there would be a heating effect caused by second circuit right? We know that Voltage Drop is directly proportional to Resistance (which is affected by the temperature).

Your experience and knowledge are appreciated.

 

[ramsy]

Solving the formula in NEC 310.15(B)(2) for temperature gets the answer.

My calculator shows 41c with 3 ccc's, and 47c with 6 ccc's, with 230 Volt Drop barely changed from 3 ccc's. Add + 0.1 volt, or 0.04% over 210 feet.

However, individual 1-phase loads or harmonic loads, fed from 3-phase 4-wire Wye xfrmrs = 4 ccc's each branch Ckt.

4 ccc's x two 3-phase Wye branch Ckts = 8 ccc's burning 40 Amps each at 52c. Add + 0.16 volts, or 0.07% beyond 3 ccc's.

2" PVC-Sch-40 fits 16 MAX #4 AWG with #12 Awg EGC x 2, not #10.

 

[James Williams]

You only need a single #10 AWG EGC and it need not be insulated if that helps in conduit fill.

 

[suemarkp]

When conductors are upsized for voltage drop purposes, the equipment grounds need to be proportionately upsized. The hard part is determining what size is "normal" for a 40A circuit -- #10 or #8? Then see how much the #4 is oversized and upsize the ground accordingly.

Also, no neutrals in these branch circuits?

 

[ramsy]

When conductors are upsized for voltage drop purposes, the equipment grounds need to be proportionately upsized. The hard part is determining what size is "normal" for a 40A circuit -- #10 or #8? Then see how much the #4 is oversized and upsize the ground accordingly.

Also, no neutrals in these branch circuit

Good exercise. Delta 240v xfmr with no neutrals = 6 ccc's + 1 EGC per PVC conduit.

(load current of 40 Amperes

Assuming #8 with #12 EGC is normal for 40A load.

Per 250.122(B) #4 is 2.53 x larger than #8, so #12 @ 6530 cm x 2.53 = 16509 cm or #8 EGC

 

[suemarkp]

#12 is not the EGC for a 40A branch circuit per table 250.122, it would be a #10 copper (a 25A - 60A circuit uses a #10 EGC, so the #10 needs to be upsized 2.53X).

 

[ramsy]

per table 250.122, it would be a #10 copper (a 25A - 60A circuit uses a #10 EGC

Missed the word "NOT" in that table:

Table 250.122 Min. EGC for OCP Not Exceeding:
Amps -> AWG-cu
15 ---> 14
20 ---> 12
60 ---> 10