I have a shed 100' from my garage, the another 40' or so from the garage entry point to the circuit breaker.

I have an unused double-pole 30A 240 V breaker with enough 10G NM-B to reach the garage entry.

can I extend the NM-B 10G to the shed with 8G THHN (underground conduit)? Or does the entire run need to be 8G?

I want to supply a 240V electric heater as high of wattage I can safely, maybe 6,000W (25A)

thank you!

 

You may find a voltage drop calculator to help you decide what is the right size of the cable. like this: https://www.southwire.com/calculator-vdrop
I will suggest to use a voltage drop less than or maybe 4% to be sure that the breaker will be acting quick enough.

 

How many conductors will you be using.
You can't use NM outdoors even in conduit.

 

At 140ft you may want to consider aluminum wire for cost savings. For inside without conduit use SER cable and outside underground in conduit use individual conductors of XHHW-2. This will require junction boxes for transition between wire types.
To handle a 30A full load with allowable voltage drop I recommend using #6 aluminum. You need 4 conductors (Hot,Hot,Neutral,Ground).
For the upper limit I'd go no more than a 40A breaker on the #6Al because of voltage drop getting to around 4% .

 

What ever gauge wire you use in conduit you have to use THWN.
Also I believe the ground can not be bare. It must be insulated when outside in conduit. When you refer to "B" in your designation in wiring "B" refers to "Bare" as in the ground is not insulated. The "NM" refers to non-metalic

 

THWN is not the only option to use in conduit, so the OP doesn't HAVE to use THWN. A bare copper ground can be used in conduit but is not a common nor recommended practice.
Oh.. and "B" does not mean bare ground. It indicates the temp rating of 90C.

 

Thanks for all the suggestions and information to everybody!Just to clarify, I already have 10 gauge Romex that runs from the circuit breaker to the entry point in the garage, and it runs along the rafters and in the studs.
So my main concern is, whether connecting another hundred feet of 8 gauge (or 6G) wire onto 40' of 10G would be acceptable; I don't want to overload the Romex and make it a fire hazard.
I figure if I have heavy enough gauge THWN for the 100' stretch through the conduit then it shouldn't put a burden on the 10 gauge in the house.

it may be just as simple as figuring out the total amperage drop in the line and make sure that my breaker is the right amperage rating

more thoughts?

 

Yes it is just as simple as figuring out the total Voltage drop in the full length of the line. Figure out the voltage drop for each section of wiring of a given size and add up the voltage drops to come up with the total. The breaker would be the same as if the same heater (or other load) were a short distance away. The Amperes drawn would be the same.

It is possible that the required wire size for continuous loads (includes heaters) versus intermittent loads is adequate to satisfy voltage drop requirements in a given circuit without upsizing more.

A thinner gauge run coming first out of the breaker box will not nullify the benefits of a fatter gauge run continuing on. But if the thinner gauge wiring was insufficient to satisfy the 125% continuous load rule all by itself, then adding on the fatter gauge wiriing to put the heater further away will not fix or make up for that.

 

Perfect, thanks!

 

Your #10 NM-b can only yield 24A @ 240v based on continuous load.
That limits you to a 5000w(21A) or 5500w (23A) heater.

 

The B in NM-B refers to the 90 degree rated insulation. Older NM had a lower rated insulation.

 

Yet more good info, thanks!

 

I'm planning to put in 240v to a shed (earlier thread by me) for a heater. I realize the max continuous load allowed will be 80% = 24A

in the summer I would like to tap into that power to run a small 120v air conditioner.

I expect that it is single phase, so I have the option to buy a converter, which is quite pricey, or possibly run a neutral wire and install a switch between 240V 30A to a single 120V 15A circuit.

My questions are

1. Will the converter actually convert the 240V 30A to two 120V 15A circuits?

2. Is my assumption correct that I can get a single 120V 15A circuit from the 240V using a switch box of some sort?

I plan to enlist a licensed electrician for the work, I'm just planning based on what is possible / practical / cost-effective.

I realize I could just run a separate 120V line, but I'd like to explore options to use existing wiring. adding 140' more wire is not cheap, but may be more cost effective in the long run

thank you!

 

I combined your threads. Many questions that would get asked again were already asked.

So far you're running a 2x with ground cable. That will only yield a 240v circuit.
It's not easy to covert the wiring back and forth when needed.
It would be so much better to run the circuit as a 120/240v circuit now.
You could set a little sub panel in the shed.
One 2P breaker for the heat.
One 1P breaker for A/C and one 1P breaker for service receptacles.
OR just a single breaker for all 120v receptacles.

You could consider a 240v to 120v transformer but you'd be looking at several hundred dollars in parts.