Current Carrying Conductors

[LWT]

I have a twinned furnace each requiring its own 20A ckt. The FLA for each is 12.5A. The current wiring is 3ea #10, (H, N, G). Can I install another hot and neutral #10 thhn and be within code in the same raceway? The existing panel is a 4w 3ph 208 wye connected system and the existing raceway is 3/4" EMT.

According to 310.15 (B) (3) (b) the neutral is considered a current carrying conductor making (4) hot wires and one ground in a 3/4" raceway. I am sure there needs to be a calculation done for deration on the cable, but I get lost in the translation......lol. Can anyone offer some help on this?

 

[kwired]

I have a twinned furnace each requiring its own 20A ckt. The FLA for each is 12.5A. The current wiring is 3ea #10, (H, N, G). Can I install another hot and neutral #10 thhn and be within code in the same raceway? The existing panel is a 4w 3ph 208 wye connected system and the existing raceway is 3/4" EMT.

According to 310.15 (B) (3) (b) the neutral is considered a current carrying conductor making (4) hot wires and one ground in a 3/4" raceway. I am sure there needs to be a calculation done for deration on the cable, but I get lost in the translation......lol. Can anyone offer some help on this?

The EGC doesn't count as a current carrying conductor. The other four do. So you adjust ampacity of them based on the table entry for four current carrying conductors.

 

[winnie]

I have a twinned furnace each requiring its own 20A ckt. The FLA for each is 12.5A. The current wiring is 3ea #10, (H, N, G). Can I install another hot and neutral #10 thhn and be within code in the same raceway? The existing panel is a 4w 3ph 208 wye connected system and the existing raceway is 3/4" EMT.

According to 310.15 (B) (3) (b) the neutral is considered a current carrying conductor making (4) hot wires and one ground in a 3/4" raceway. I am sure there needs to be a calculation done for deration on the cable, but I get lost in the translation......lol. Can anyone offer some help on this?

You have 3 conductors, H, N, G. 310.15 (B) (3) (b) does not apply because you have a single hot conductor using the neutral, so both count; the EGC doesn't count. 2 CCCs.

If you pull in an additional H and N, you add a completely separate circuit. Both H and N count, 310.15 (B) (3) (b) doesn't apply. Total 4 CCCs.

If you pull in an additional H, from a different phase in the panel, 310.15 (B) (3) (b) would apply. In this case you have a shared neutral, H-N-H, but the neutral counts, 3 CCCs.

With 4 CCC your derating factor is 80%. #10 is still good for 12.5A unless there is some other factor involved.

-Jonathan

 

[LWT]

Thank you for your feedback.

I know the EGC is not a current carrying conductor but am I correct in saying the neutral for each is? It has always been said to only allow 3 current carrying conductors in a single run of conduit. I was not thinking about the neutral being current carrying. If this is true I will need to run another conduit for the other ckt?

 

[kwired]

You have 3 conductors, H, N, G. 310.15 (B) (3) (b) does not apply because you have a single hot conductor using the neutral, so both count; the EGC doesn't count. 2 CCCs.

If you pull in an additional H and N, you add a completely separate circuit. Both H and N count, 310.15 (B) (3) (b) doesn't apply. Total 4 CCCs.

If you pull in an additional H, from a different phase in the panel, 310.15 (B) (3) (b) would apply. In this case you have a shared neutral, H-N-H, but the neutral counts, 3 CCCs.

With 4 CCC your derating factor is 80%. #10 is still good for 12.5A unless there is some other factor involved.

-Jonathan

But if you only have 3 CCCs you have no adjustment factor that is required.

 

[LarryFine]

Welcome to the forum.

I know the EGC is not a current carrying conductor but am I correct in saying the neutral for each is?

Yes. Each two-wire circuit is two CCCs.

A three-wire from 1ph is two CCCs; from 3ph it's three CCCs

It has always been said to only allow 3 current carrying conductors in a single run of conduit. I was not thinking about the neutral being current carrying. If this is true I will need to run another conduit for the other ckt?

No. Three CCCs is the limit before derating, not the absolute limit.

#10 cu, which starts at 40a, is still rated at (0.8 x 40) 32a, even with four CCCs.

 

[LarryFine]

With 4 CCC your derating factor is 80%. #10 is still good for 12.5A unless there is some other factor involved.

 

[LWT]

Thank you for your help.

 

[infinity]

Am my I the only one reading the OP's question to be 8 CCC's? He has 3-two wire circuits (6 CCC's) and wants to add a fourth??

 

[LarryFine]

Am my I the only one reading the OP's question to be 8 CCC's? He has 3-two wire circuits (6 CCC's) and wants to add a fourth??

Good question. Reading it again, it's not clear whether he has three #10 conductors or three circuits.

OP, we need clarification. How many wires are presently in the conduit?

Note that one EGC may serve many loads.

 

[LWT]

I have one existing ckt. for a twinned furnace. the inspector is asking for each to have its own ckt and means of disconnect. So two ckts total in one 3/4" EMT conduit.
Two #10 hots
Two #10 neutrals
One #10 ground

5 wires total

 

[LarryFine]

I have one existing ckt. for a twinned furnace. the inspector is asking for each to have its own ckt and means of disconnect. So two ckts total in one 3/4" EMT conduit.
Two #10 hots
Two #10 neutrals
One #10 ground

5 wires total

Absolutely okay.

If you really wanted to, you could use a 2p breaker (or a handle tie) and add only one more wire.

If the EMT is intact from metal enclosure to metal enclosure, you don't have to have the wire EGC.

 

[LWT]

Sorry for any confusion. I really appreciate the knowledge sharing.

 

[winnie]

You did the calculation. 4 CCC #10 has a derated ampacity of 32A, which is greater than 12.5A.

Of course if the conduit goes through a 180F heated room.....

 

[wwhitney]

You did the calculation. 4 CCC #10 has a derated ampacity of 32A, which is greater than 12.5A.

Of course if the conduit goes through a 180F heated room.....

Yes, that would require separate conduits, as the temperature derating factor for 82.2C ambient with 90C conductors is 0.36, so the #10 has a temperature corrected ampacity of 14.4A, but with an 80% adjustment that's only 11.5A.

Turn the temperature down to 175F (0.419 temperature correction factor), and one conduit is still fine. : - )

Cheers, Wayne

 

[LarryFine]

All what-ifs aside, the OP's loads only require 20a circuits, so even the derated #10 is overkill.

 

[Dennis Alwon]

Btw, you only need #12 awg conductors. If you want you can add one extra conductor and share the neutral. The new multiwire branch circuit would need to have a dp breaker or 2 single pole units with a breaker tie.

 

[kwired]

Btw, you only need #12 awg conductors. If you want you can add one extra conductor and share the neutral. The new multiwire branch circuit would need to have a dp breaker or 2 single pole units with a breaker tie.

Keep in mind sometimes they parallel the 24VAC control circuits when twinning furnaces. Supplying them with a MWBC on a 208/120 supply would never allow you to get those two transformer secondary outputs in phase with each other.