Need Help With Number of Wires in Conduit

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Old 05-12-16, 07:32 PM
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Need Help With Number of Wires in Conduit

I am just really confused with derating. I am trying to run a few simple circuits in a metal building/shop I built. It has a 200 amp single phase main. I want all circuits to have a separate neutral. I need 2, 20 amp, 110v circuits for my lights which will be on 4 switches, 2, 20 amp, 110v circuits for receptacles and 1 double 20 amp 220v circuit for my water well pump. I want to use THHN stranded #12 wire. I know if you go over 3 CCC you must derate the wire. My questions are:

1. Do i need a separate ground for the 110 and the 220 circuits or can I use 1 #12 as the equipment ground for the 220 and for the 110 circuits ground?

2. If my calculations are correct, that would be a total of 10 CCC and a ground plus the extra runs from the light switches back to the lights which is within the 15 limit for #10 or #12 wire in a 3/4 PVC. Since I believe that 10-20 CCC require 50% derate, would I need to use #10 wire rated at 40 amps and derate to 20 amps or can I still use #12 since they are not continuous load circuits?

Can anyone please offer some advice? I have no problem running cables and electric but want to be sure it is not a fire hazard. Thanks
 
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Old 05-12-16, 08:00 PM
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Welcome to the forums.

A question for your question.... is there a particular need to stay with 3/4" PVC ?
We try not to fill it to the max..... not even figuring in derating.
 
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Old 05-12-16, 09:45 PM
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Multiple runs of conduit are going to be easier to work with and have more flexibility.

One egc can be used for both the 240 and 120 loads. It needs to be sized based on the largest circuit.
 

Last edited by pcboss; 05-13-16 at 07:55 AM. Reason: egc was egg
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Old 05-12-16, 09:55 PM
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One egg can be used for both the 240 and 120 loads. It needs to be sized based on the largest circuit.
PC means one equipment grounding conductor, egc, not egg.

Why are you against running multi-wire branch circuits? In a MWBC the neutral is not counted as a current-carrying conductor since it only carries unbalanced current.

For derating purposes you use the 90[SUP]o[/SUP] C. column which for #12 type THHN means a starting point of 30 amperes. For nine current-carrying conductors you derate to 70% which would be 21 amperes. Since #12 must be protected to a maximum of 20 amperes, nine CC conductors is the limit for #12 THHN in a single conduit. That would be nine circuits using MWBC (actually eight since you won't have a single circuit but only pairs using MWBCs). Saves a fair amount of wire (neutral wiring) as well.

Also, voltages are 120 and 240 and have been in the US for the last fifty years or more.
 
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Old 05-13-16, 10:39 AM
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I don't need to stay with 3/4. It is in a area where I can run any size conduit needed, Just trying to keep cost under control. Thats also why I was trying to limit the conduit runs to as few as possible. Larger Conduit also means larger junction boxes. All this adds up to more money.

I am not against MWBC. I'm just not familiar with how to wire the receptacles & lights. I know you must use a double pole breaker or two singles with switch ties and both breakers are "stacked" on the same side of the main panel (1&3 slots or 2&4 slots, etc.) you also use only one neutral per 2 circuits. What I don't know is, 1. Do you still use a ground in addition to neutral on these circuits? I would think you would. 2. Do you wire the neutral on both circuits as though it was a single circuit but split the load side across the two hots to different breakers? Is that all there is to it? 3. Can you use MWBC on a GFI circuit? I know, from what I've read that on 3 phase MWBC the neutral doesn't count as a CCC but not sure if I count neutrals if using MWBC on single phase. I don't know ANYTHING about 3 phase circuits. I guess what I'm not understanding about counting neutrals is that if it doesn't count on single phase, why it counts as a CCC if used for 1 circuit but doesn't count if used for 2 circuits?

When considering the number of CCC, if you run a hot through a pvc. past a light, to a switch and then double back from that switch through the same pvc to go back to the light, is it counted as 1 CCC since its the same circuit or is it 2 CCC because it loops back through the same pvc after the switch? I will have 4 120v circuits and 1 240v circuit, which would be 6 hot and 2 neutral (since 240V doesn't have a neutral). That would be either 6 or 8 CCC depending if you have to count neutral. Thats within the 7-9 wires @ 70% (21 amps) which would make using #12 ok. But, if I have to add the 4 wires that double back through the conduit from the 4 switches, it would either 10 or 12 CCC depending if you count neutral which would mean that I need to move up to 10-20 wires @ 50% and use #10 wire? What would be correct?

I completely understand standard basic wiring, voltage and how to hook up old school circuits using Romex and no conduit, without MWBC. I'm willing to give MWBC a try if I get these questions answered. I've been certified in installing and terminating fiber optic cabling, phone cabling and computer cabling for many years. Physically doing this job is the easy part. Understanding MWBC for single phase (there isn't much on the internet), comfort in knowing the circuits are wired properly and figuring what wires count as a CCC and when is whats confusing. I appreciate the great info given so far. Please keep it coming. I'm almost there.
 
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Old 05-13-16, 10:51 AM
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1. Yes still use the ground on MWBC. The same ground can be shared for all circuits in the same conduit.
2. The same neutral is shared for both circuits. The hots are split between the two "phases" of the MWBC.
3. GFI is more complicated with MWBCs. There are two options: first is to use a 2 pole GFCI breaker; the second is to not share the neutral after the GFCI device. For example you could run a MWBC to a junction box on the other side of the shop. In that box put two GFCI receptacles, share the neutral on the LINE side, and put one hot per GFCI on the LINE side. From the LOAD side of the GFCI devices, treat them as completely separate circuits with separate neutrals.
4. The shared neutral does not count as a CCC -- the current in the neutral cancels out when it is shared between two hots of the MWBC. In other words, the three conductors of the MWBC count as two CCC.
 
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Old 05-13-16, 02:22 PM
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Great. Thanks. Just to clarify a couple points. If I do the GFIs as you stated, do I count the neutrals from the GFIs down the LOAD side? Also, in my last post, do you count the wires from the switch back to the lights? Therefore, would it be 6 not counting them or 10 counting them? Think I'll have it after this answer. Thanks again.
 
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Old 05-13-16, 09:03 PM
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...do I count the neutrals from the GFIs down the LOAD side?
Yes.

Also, in my last post, do you count the wires from the switch back to the lights?
Yes.

Therefore, would it be 6 not counting them or 10 counting them?
Correct, ten conductors.

The way to determine is if any conductor is carrying the entire current of the load. With a shared neutral (MWBC) the neutral only carries the unbalanced current between the two (single phase) hot conductors so it never "sees" the entire circuit current. In the case of the switched wires each conductor does carry the entire circuit amperage.

The purpose of derating is to allow for the internal heating of the individual conductors from the flow of the current. By reducing or eliminating the current flow in the neutral by using the MWBC you effectively remove the heating effect in that neutral the closer you get to a balanced load between the two hot conductors.
 
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Old 05-14-16, 07:44 AM
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Make it easy on yourself, keep each conduit to no more then 9 current carrying conductors and derateing will not be an issue. Also it will be much easier to pull in the wires.
 

Last edited by Tolyn Ironhand; 05-14-16 at 12:59 PM.
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Old 05-14-16, 12:09 PM
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Thanks for all the great info. I think I finally have a handle on this project.
 
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