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lostagain View Drop Down
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Posts: 2595 		Direct Link To This Post Topic: 50 or 30 amp
    Posted: 30 Jan 2019 at 7:51am
Thanks for the very thorough discussion offgrid.  The risks and the reason for them are much more clear.  In a nutshell, violating Ohm's law has consequences and one is above the law.
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Posts: 5290 		Direct Link To This Post Posted: 30 Jan 2019 at 6:06am
Just to make things less confusing, we can call "impedance" "resistance".  Impedance and resistance aren't quite the same thing but they're close enough for the purposes of this discussion. 

Going back to ohm's law, V= I*R or better stated for our purposes, I=V/R. So current flow equals voltage divided by resistance. Our voltage is 120Vac and effectively doesn't change, so we can just worry about current which changes inversely with resistance. 

Now suppose you start with the case of  a dead short (low resistance fault) between your hot and neutral or between hot and ground conductors.   For example if someone accidentally drives a staple through a piece of Romex. (I'll admit it, I've done it).  Let's say this is a 15A circuit run with 14 AWG Romex. The resistance R between the wires and the staple is very low so current I is very high. Hundreds of amps probably.  Bad, right? No, not so bad, your circuit breaker will see a very high current and trip immediately. You'll have a dead circuit and a burnt up staple but the conductors won't have time to get hot so any damage will be localized.  Your electrical system is doing its job keeping you safe. 

Now, suppose instead of a staple driven through the wire you have a break in the insulation say caused by the cable abrading over time or by being pierced without direct metal to metal contact to the conductors. This might cause a parallel arc fault where in the electricity is flowing as a hot ionized gas discharge through the air between the conductors (line to neutral or line to ground).  We've all seen electrical arc faults probably. The resistance of that type of fault is much higher than a dead short  caused by a staple driven through the Romex, so the fault current is lower  . 

So now you might have say 10A flowing through that fault rather than hundreds of amps. Your circuit breaker won't trip until 15A is reached so this current will just continue to flow (for an indefinite period of time until something changes in the area of the fault). But, like in the case of the nail driven through the Romex, any damage is localized to the area of the fault. The conductors won't overheat because they are rated for 15A and only have 10A flowing through them. Your electrical system is still doing its job protecting you. 

If you now turn on that 10A hair dryer you will have 20A flowing through that circuit, 10A from the fault and 10A from the dryer.  Your conductors will begin to get warm but before long your circuit breaker will trip. Your electrical system is still doing its job. You'll be safe and now you have a hint that something is very wrong somewhere. 

This was exactly what happened to me on my outdoor circuit when I turned on my skillsaw and the breaker tripped, except that instead of an arc fault I had corrosion inside a receptacle which created a similar high resistance path for current to flow. If I had't tried to run the skillsaw I wouldn't have known the fault existed, at least not for awhile.

Now suppose you combine the high resistance fault in case 2 with a mistake by a previous homeowner where he installed a 20A breaker on that 15A circuit. You don't have a problem other than right where the fault is as long as you haven't turned on that hair dryer. Once you do you now have 20A flowing through those conductors which will heat up. But the cb won't trip to protect them. Not good, eventually a fire could start anywhere on that circuit run.

So back to our rPod case with the 50A receptacle feeding a circuit rated at 30A via the 50 to 30A dogbone. Now add on a high resistance fault from abraded wire or corrosion or something else, somewhere between the dogbone and the 30A breaker in the rPod panelboard. Let's say that high resistance fault is 10A. Now go turn on your a/c and microwave. You will have around 40A  flowing through those conductors and connectors which are rated for 30A. The 50A cb back at pedestal or wherever the park has put it won't trip, so those conductors will begin to overheat. 

Or, you could just have a "medium" resistance fault that allowed 40A to flow and the same thing would happen without any loads being on in the trailer. Or a 20A fault current with just the a/c turned on, etc. 

Those are the kind of scenarios which you are not protected from when you use that dogbone unless you add in a 30A circuit breaker like GlueGuy and I are suggesting. The 30A breaker in the rPod panel doesn't protect you in these cases because the fault is upstream of that point.  

Re StephenH's suggestion of using an arc fault breaker that would certainly add additional protection as it is intended to identify arc faults with currents below the trip rating of the conventional breaker. Because they can see arcs at lower current levels these can also help with lostagain's concern about the lamp cord, in fact that's the kind of scenario they were developed to protect against.

The last time I tried arc fault breakers was about 15 years ago when they were a new code requirement and they were really prone to nuisance tripping, I would imagine they are better by now. The AFCI's are code required for most dwelling outlet circuits which are 15-20A circuits so you might not be able to find them them with a 30A rating. You could look into putting them in the trailer panel though.  Its interesting that apparently they aren't yet required for RV use even though that's used as a dwelling.

To clarify when the NEC comes into play its whenever new work is done. So new construction but also if you have an electrician in to say replace your electrical panel he won't be able to do that without bringing that system up to current code. 
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Posts: 6417 		Direct Link To This Post Posted: 29 Jan 2019 at 9:10pm
What I was reading is that GFCI protects against personal injury and AFCI protects against fire. One of the examples was insulation breakdown or cracking that allows current to arc between conductors. For homes, it looks like the NEC does now require it but that probably applies to new construction (second link below). The dead short in an undersized wire is just one of the situations that an AFCI would protect against.

Yes, it would likely be overkill and it is highly unlikely that such protection would be available at the pedestal. What is available though are the breakers in the RPod's panel and the GFCI outlet. Between them both, it would seem that the likelihood of fire risk is minimal even if a 30A breaker is not in the 50A to 30A dog bone. It is difficult to conceive of a situation where the 30A breaker in the panel will not trip long before the wires between it and the dog bone would get hot enough to cause a fire.
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Posts: 2595 		Direct Link To This Post Posted: 29 Jan 2019 at 8:03pm
Oversizing conductors is never really going to be a problem.  It's undersized conductors that are grossly mismatched to the circuit breaker that presents the fire danger.  If the mismatch is not too disproportionate it's not likely that a undersized conductor would be an issue unless there was a dead short or a very heavy load on it.  

Seem to me, as GlueGuy suggests, the simple solution is to look for a dog bone converter that has a 30A overcurrent protection right before the point where the wire drops in size, or at the pedestal.  A simple and probably inexpensive solution to the fire risk issue.
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Posts: 2702 		Direct Link To This Post Posted: 29 Jan 2019 at 7:12pm
Originally posted by StephenH

It would seem that to mitigate the situation described of the lamp cord, an Arc Fault Circuit Interrupter would be the more appropriate protection device. Am I correct?
If the high impedance fault creates an arc, most definitely. However, producing an arc is not necessarily what we're worried about. 

Thinking about the situation, a 30 amp cord will handle something between 30 and 50 amps for a "little while". The question is whether it would handle it long enough to cause a problem. In many situations, it just might be. Would it cause an arc? Don't know. If it did, then we would be done here. 

The idea with circuit breakers in general is to protect the wires from seeing current that is beyond their specification. The problem being that going over their capacity "just a little" will cause them to heat up and potentially cause a fire before they fail completely.

So my feeling is that if you build or use a dogbone to allow you to use a 50 amp receptacle it should include a breaker, or even just a fuse that will protect the wire in the connecting cord beyond the dogbone.
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Posts: 3419 		Direct Link To This Post Posted: 29 Jan 2019 at 6:59pm
SH,

In my mind that's overkill, do you have that in your house? Most rely upon the CBs and GCFI to ensure we don't have electrical issues. The design of your electrical system covers outlets around sinks and water supplies, outside outlets, high current devices (stove), you name it   When was last time your spouse used a hair dryer, that caught fire? RVs, houses, the protection is there. I think under new codes there is a reg to add to certain places in the house, it's an added protection, but not mandatory to old units with GCFI.
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Posts: 6417 		Direct Link To This Post Posted: 29 Jan 2019 at 6:47pm
It would seem that to mitigate the situation described of the lamp cord, an Arc Fault Circuit Interrupter would be the more appropriate protection device. Am I correct?
StephenH
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Posts: 3419 		Direct Link To This Post Posted: 29 Jan 2019 at 6:04pm
Lostagain,

Understand better. Hair dryer is an example, heaters, microwaves, anything that has a high current draw. The key in my mind is the CB and/or the GCFI, I'm just talking about a regular outlet device. In a shop for an example, heaters, welders any high current device. The key is the protective devices. Taking the example if I was to take a 50A stove and put it on a 30A circuit with 30A wire, is it going to cause a fire, in my opinion Nope. It's going to kick the 30A CB, due to over current, the chances of fire are less than the fact the apparatus won't work. Adding 50A wire won't effect the load of a 30A CB. In a standard RV, regardless of source, internally the RV is controlled by CBs on the outlets, for example. Example I can run one cube heater in my Pod if I run 2, I trip the GCFI. If you could hook a hair dryer up to a 50A circuit and the cord shorted the wire would burn in two before the CB would kick.
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Posts: 2595 		Direct Link To This Post Posted: 29 Jan 2019 at 5:44pm
Thanks, Mike, those are effectively dead short scenarios and in both, as was observed earlier, they would burn up the undersized cable possibly before the 50A breaker would trip since not a lot of heat would make it to the breaker due to limited wire size.  That certainly presents a safety problem since there is no effective overcurrent/heat protection for the undersized cable.  Of course in the RV setting we are talking about 10awg wire in the 30A compatible cable which is a lot closer to the 6awg for a 50A circuit so the danger is diminished.

Wow, you had quite a danger there with the mis-wired plug.  I had a similar experience when I worked in the grape sheds and a field box nailing machine arrived from Delano with a mis-wired 220.  My co-campesino touched it and grounded himself when he touched a properly wired box skid.  It nearly killed him.

My question, though, is not the simple overcurrent dead short situation but other forms of impedance that offgrid alluded to.  What other high impedance situations are within the world of possibilities that could also make an undersized conductor dangerous.  I can think of, for example, running a hair dryer on high with 22awg wire.  It may not trip the breaker, but it certainly would get the wire pretty warm.  I got the impression there are other examples that it would be nice to know about.
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Posts: 3419 		Direct Link To This Post Posted: 29 Jan 2019 at 4:44pm
Example: You have a standard outlet, we know most are circuit breakered at 15 or 20 A. Somebody wired this outlet to a 50A CB (STUPID), you plug a lamp into it. The lamp has a shorted wire. The wire will get HOT, it could cause combustible materiels near it to light. If the current draw exceeds the CB it will kick, probability is wire will burn in two before CB kicks at somewhere less than 50As.

Example 2: When I first moved into my building, I was tracing outlets back to CBs in panel. I came across a standard outlet under a work bench that had 220 volts on it. Standard outlet. Traced back to panel and it had a double 100A CB on the line. I removed the double 100s, put in a single 20, went back to the outlet feed and removed the now disconnected 120. Now I had a standard 20A outlet.
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