Electrical help needed

[Dave Lehnert]

I am going to wire this electrical heater from Northern tool.

http://www.northerntool.com/webapp/wcs/stores/servlet/product_6970_595_595

They recommend 10/2 wire and 30 amp breaker for the install.

I have a 240 volt circuit that ran to and old air compressor that has been removed. Nothing is now on this circuit. This circuit has a 40 amp breaker and 8 ga wire. I am going to run off this box for the heater. I need to extend the wiring about another 30 or 40 feet.

My question......

-Is it ok to run the recommended 10/2 wire off the box that has 8 ga wire?
( I have 10ga on hand)

-I am going to replace the breaker in the box from the old 40 amp to the recommended 30 amp. This OK?

- There is no ground wire in the box. The box is all connected with metal conduit. I am assuming the metal box is grounded. How do I test for sure?
The breaker box has a ground running from it to the ground (dirt) outside. All the electrical outlets are 3 prong. This is in a block garage so everything is in conduit.

[Rob Russell]

While all you need with a 30 amp circuit is the #10, it's considered poor practice to extend a circuit such as you're doing with a downsized conductor. The problem is that, in the future, someone could see the #8 and think the entire circuit was OK for a 40A breaker.

To test the continuity, use one of the hot conductors after you have extended it. Disconnect the conductor from the breaker and land it on the Equipment Grounding Conductor buss. You can then check for continuity between the end of the conductor and the box at the end.

[Dave Lehnert]

While all you need with a 30 amp circuit is the #10, it's considered poor practice to extend a circuit such as you're doing with a downsized conductor. The problem is that, in the future, someone could see the #8 and think the entire circuit was OK for a 40A breaker.

To test the continuity, use one of the hot conductors after you have extended it. Disconnect the conductor from the breaker and land it on the Equipment Grounding Conductor buss. You can then check for continuity between the end of the conductor and the box at the end.

Thanks for your help.

I understand what you are saying about the wire but if I understand correct no issue with the 10 from 8 as far as safety.

[Rob Russell]

As long as you are using a 30 amp breaker, that works in terms of conductor ampacity vs. the breaker.

One thing you don't mention is the overall distance from the panelboard to where the heater will be. If you're looking at a long run, I'd use #8 so you deliver full voltage to the heater. Resist the temptation to go cheap and use the #10 just because you have it. A load like a heater is something that is likely to run continuously for a lot longer than a machine. You don't want to skimp on the conductors feeding the heater.

[Dave Lehnert]

As long as you are using a 30 amp breaker, that works in terms of conductor ampacity vs. the breaker.

One thing you don't mention is the overall distance from the panelboard to where the heater will be. If you're looking at a long run, I'd use #8 so you deliver full voltage to the heater. Resist the temptation to go cheap and use the #10 just because you have it. A load like a heater is something that is likely to run continuously for a lot longer than a machine. You don't want to skimp on the conductors feeding the heater.

Thanks again.

The 8 ga from the panel is about a 20ft run. From there the run to the heater with 10ga will add about 30-35 feet.

The recommended wire from the manufacture is 10ga.

[Rob Russell]

If you're going to insist on tying the #10 to the #8, I'd put a tag on the #8 in the panelboard which indicates that - down circuit - the conductors are downsized to #10 and that nothing more than a 30 amp breaker should be used.

[Chris Padilla]

I have a length of #8 going from the main panel to a j-box, and then #10 going from there to my dust collector (5 hp). I have the whole circuit protected by a 30 A breaker and it is dedicated. I had a decent length of #8 leftover and decided to go ahead and use it....

[Tom Veatch]

...
To test the continuity, use one of the hot conductors after you have extended it...

No intention to hijack the thread, but is a simple continuity check sufficient to determine the effectiveness of the conduit as an equipment grounding conductor? I don't know any other way, but I see a potential problem, even if it's not a "real" problem.

To do its job, the grounding conductor should be able to carry a fault current long enough to trip the breaker. I imagine that's why codes specify minimum sizes for grounding conductors. A simple continuity check doesn't say anything about the ampacity of the path.

For example, a 1/2 amp cartridge fuse will pass a continuity check as would any other conductive path with a similar current capacity. But there's no way such a path would pass fault current long enough to trip even a 15 amp breaker. From that, I infer that a continuity check alone can't determine whether a given path will function as an equipment grounding conductor. Is this a problem, or am I just seeing phantoms in the mist?

[Shaddy Dedmore]

For me, I had #12 / 2 to my lights, then #14 / 3 between them, and the inspector told me to change one of them so they match ( I went #12 the whole way).

if you're getting it inspected, I'd give them a call and ask.

[Rob Russell]

Tom,

You raise an interesting question. I don't know how, short of running some sort of load test, that we could do much other than a continuity test to validate that the conduit is installed correctly.

I'm not even sure how you would run a load test on conduit to ensure it's installed correctly.

Dave,

Another option for you - since you have the #10, is to remove the #8 and run #10 from the panelboard all the way out to your heater. That would at least make it crystal clear in the panelboard what conductor sizes are used for that circuit.

Rob

[Dave Lehnert]

Dave,

Another option for you - since you have the #10, is to remove the #8 and run #10 from the panelboard all the way out to your heater. That would at least make it crystal clear in the panelboard what conductor sizes are used for that circuit.

Rob

Thanks so much for your help.
One the way home from work I stopped in to an Electrical supply close to my house. I was going to "play it safe" and continue the wiring with the 8ga. I ran past the workers at the supply house what I was doing and they advised to not use 8ga at all. Only 10ga. Said bigger is not always better. 8ga may not let the breaker trip if a problem happened with the heater. Something about the resistance the 8ga would or would not let happen????

Thought I would add this for an FYI to everyone.

[Tom Veatch]

...Said bigger is not always better. 8ga may not let the breaker trip if a problem happened with the heater. Something about the resistance the 8ga would or would not let happen????...

I'm sorry, but the guy who told you that displayed some serious lack of understanding. Whether or not the breaker trips is completely determined by the current through the breaker and the amount of time that current flows. The size of the conductor has absolutely zero influence (as long as it's large enough to conduct the current without self-destructing before the breaker trips).

But as others have advised, it's bad practice to downsize the conductor extension. Better in the long run to either extend with 8ga or pull it and replace with 10ga throughout.

[Josh Mountain]

Yes, Dave that is totally false. He may have been referring to not putting in a larger breaker than what is called for (will let too much current pass for too long although breakers protect the system not so much the appliance.

Absolutely no advantage to using a smaller conductor electrical speaking (non-electrically: easier to pull, conduit fill limitations, more flexible, smaller hole through structurual members, etc.)

Concur with Rob and Tom. . . continue the 8AWG or replace the whole thing w/ 10AWG is the right answer in my opinion.

As far as the ampacity of the conduit, I can't think of a good test that could determine the characteristics of a high(er) current load. I mean if the continuity test works, we know a high amperage can travel on that path. The questions is whether it is a better path to complete the circuit than you or something else (more than likely) and whether or not said current will cause the conduit, its contents, or its surroundings any undue heat. Hmmmmm. . .I'm no expert on conduit as a ground; maybe somebody has a more concrete discussion or advice. I assume this heater has an equipment ground path?

[Kevin Stricker]

So you are saying that there are only two wires in the conduit currently? I have a hard time imagining anyone running a 220 40A circuit without some type of ground. While conduit can act as a ground, an open circuit fault would melt the insulation off your wires in a second. That is why you always run an insulated ground in conduit. If you indeed only have two insulated wires in your conduit I would at the least run a insulated #10 ground wire back to your panel regardless of the gauge wire you use.

[Rollie Meyers]

So you are saying that there are only two wires in the conduit currently? I have a hard time imagining anyone running a 220 40A circuit without some type of ground. While conduit can act as a ground, an open circuit fault would melt the insulation off your wires in a second. That is why you always run an insulated ground in conduit. If you indeed only have two insulated wires in your conduit I would at the least run a insulated #10 ground wire back to your panel regardless of the gauge wire you use.

The conduit is a code compliant grounding conductor that being said, codes are just a minimum standard.