Can I run THHN for separate washer and dryer circuits through the same conduit?

Question

I am relocating a 120V washer and 240V dryer from the basement on south side of my house to the basement north side.

• The current wiring is one 110V 20amp circuit with dedicated circuit breaker and one 240V 40amp circuit with dedicated 40 amp dual breaker.
• The wiring for both circuits runs out of the panel box into one 3/4" conduit to the receptacles (first receptacle in line is the 110V and terminating with the 240V 3 prong receptacle for the dryer.)
• I also plan on removing the old circuits.

I have two questions:

1. Is it still within code to run all the THHN wiring (12 gauge for 110 and 10 gauge for 240) or should I run two separate conduits for each circuit as one will be 20 amp and the other will be 30 amp?
2. My current dryer has a 3 prong receptacle (2 hots and 1 neutral). The NEC code since 1996 states that all dryer circuits 220V for house dryers need the 4 wire (2 hots, 1 neutral, and 1 ground). My house is 1925. So in converting from an 3 wire to 4 wire plug and receptacle where do I attach the new ground wire on my old dryer? And does the ground connect into the neutral bus or the ground bus?

I will need to get a permit here in Oregon and I am not a licensed electrician.

Answer 1

Multiple circuits in conduit is not a problem

OK, so you want the details. Yes, you can run multiple circuits in conduit if:

• they meet physical conduit fill rules, which is easy with THHN/THWN wire at your sizes
• the wires are electrically derated, which is irrelevant below 3-4 circuits at these sizes.

One ground can be shared amongst all circuits, it must be large enough for the largest circuits. With EMT, IMC or rigid conduit, the conduit is the ground and no wire is needed.

For the physical fill, just use a calc, and you use it on the actual physical wires you have in the pipe. For you, that's 2 #12 and 4 #10 unless you're using pipe as the ground, in which case only 3 #10. Any conduit size will handle that. 3/4" will give easier pulls. But if you use stranded THHN, the pulls won't be bad at all.

For the electrical derating, in residential, don't worry about it if have 3 circuits 60A or smaller. Also don't worry about if you have 4 circuits in 15/20A.

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Oh, you want the gory details:

OK. We start by counting conductors in the conduit. Here's the important part: Grounds do not count. In 120/240V split circuits with 2 hots (including MWBCs), neutrals don't count either. So here is your list of wires, and what counts and what doesn't: 120-Hot 120-Neutral / 240-Hot1 240-Neutral 240-Hot2 Shared Ground ... Total 4 wires.

Then proceed as in my answer here.

Table 310.15b3a says 80% derate (don't panic) then 310.15B16 says #12=30A=derate 24A. Well, that makes no difference, since we're already limited to 20A. #10=40A=derate 32A. Again no difference. This logic applies up to 3 circuits per conduit. At 4 circuits (8 countable wires) we need a 30% derate, which doesn't bother #12 or #14 wire. At 5-10 circuits (10-20 wires) we hit a 50% derate, which means bumping at least 2 numerical wire sizes - no fun.

On the dryer's ground strap

You must follow the labeling and instructions. Modern dryers, when wired for a 3-wire connection, instruct you to add a bootlegging jumper on the dryer. Either this procedure will imply how to "roll it back" to 4-prong, or the instructions will explicitly say. There is no substitute for following them.

The 3-prong configuration is dangerous because if the neutral wire breaks, the dryer chassis is energized. If you are stuck at 3-prong and can't retrofit ground, the safer thing is convert the dryer and receptacle to 4-prong anyway and fit a GFCI breaker. Label the receptacle "No Equipment Ground".

Answer 2

1. It is permissible to run more than one circuit in a conduit as long as the conduit is sized to carry all of the current carrying conductors. In your case a 1/2" conduit should do it, but I would go with a 3/4 for ease of pulling.

2(a). One of the advantages of pulling in one conduit is that you would only need a single grounding conductor sized to the largest circuit.The ground is attached to the receptacles and any metal enclosures and there are places marked for that purpose. You will attach the ground to the neutral bus in the main panel. You might check to see if there is a proper ground in the panel and make any necessary upgrades.

2(b). Most jurisdictions require any work to be permitted and installed by qualified personnel. We really can't be certain of every jurisdiction since codes vary by state and municipality. A quick call to your AHJ can solve that problem.

Good Luck.

Answer 3

The shorter answer ..... You can run conductors for different breakers (branch circuits) within the same conduit regardless of the amperage, as long as you follow what it says in the conduit fill tables of the Code. Do not run conductors from two different transformers in the same conduit. Do not mix power conductors with communication or speaker wires in the same conduit. If you are using metal conduit then the metal itself acts as the ground or bonding conductor. If you are using a non-conductive conduit, like PVC, you will need to add a bare copper bonding conductor. You will connect this bond conductor to the bond bus in the panel, not to the neutral bus. N.B.; If this panel is a sub-panel then the neutral bus MUST be kept seperated from the metal panel and the bond (ground) bus. Neutral and Ground can meet up at only one place; the main panel, otherwise you may end up with circulating currents. Do not use 40 amp breakers on # 10 gauge wires. Replace the 40 amp breakers with 30 amp breakers.

Answer 4

My dryer is currently on a 3-wire cable, i.e., no separate ground wire. The neutral and the equipment ground are bonded by a jumper on the connection block of the dryer. To change to a 4-wire cord and cable the jumper is disconnected at the contact with the chassis. The ground wire of the new 4-wire cord is connected to that point on the chassis.

The instructions on my dryer are to connect the freed end of the jumper to a designated spot to keep it from coming into contact with something it should not.