When can 12 gaugecopper carry 30 AMPS???
PatchogPhil
| Posted in Energy, Heating & Insulation on
Saw this on a new electric water heater:
5500 WATT elements @ 240Volts, 30 AMP breaker required with 10 gauge feeders from the CB. So why are the connector wires INSIDE the water heater only 12 gauge? The wires are marked 12 gauge! This is on a brand new GE water heater made by Rheem. 5500 WATTS divided by 240Volts = 22.9 AMPS. 12 gauge is supposed to be limitted to 20 amps, right?
Or does this fall under the same reasoning that 200 AMP service feeder wires from the utility pole are so small since they are “open air cooled”? Yet the service entrance wires down to the meter have to be 4/0?
Puzzled,
Phil
Edited 9/2/2002 11:52:11 PM ET by Patchogue Phil
Replies
There are 2 heating elements in the water heater right? So #12 is good for 20 amps, and there are 2 heating elements, therefore the 2 circuits together are capable of carrying 40 amps with no problem.
There are TWO heating elements, yes. Both are 5500 WATTS each. They operate separately. Meaning only one is "ON" at a given moment. Still, each one will pull 22.9 AMPS.
Appliance manufacturers follow a different set of rules. The environment that the wires are run is tightly controlled and defined when the appliance or fixture is made. #12Cu will carry 30A without melting the insulation, the real controlling issue in ampacity, if the insulation is rated at 90 degrees centigrade.
Even though Romex is made with 90 degree insulation the NEC, with agreement of most experts, it is limited to being put on a 20A breaker. There are many reasons for this but it can be summed up by saying that the conditions existing in any particular installation are so variable that the limiting #12Cu to 20A makes great sense.
For examples or further explanation post back and I will go into this in greater detail.
Better safe than sorry, right?
Patchogue Phil,Pretty much. Cooling, containment and ambient temperatures are all highly variable from location to location for the Romex run in the typical house.
CAP,From what I understand appliances in England have fuses in the cord cap sized to protect the cord
For examples or further explanation post back and I will go into this in greater detail
I'm all ears!
I misunderstood that the elements were 5500 watts each. I work with high temperature silicone and teflon insulated wiring frequently so I see your point. These types of insulation will not burn under a heavy load. What type of wire insulation is used in appliances such as ranges and water heaters?
Would you believe that #12 is rated for 54 amps.
That is type PFAH or TFE and is rated to 250 C. I believe that those are teflon with mineral fill.
In the distant past they used asbestose.
There are many different ratings that depend on the specifics of the application.
The UK system is interesting. They use a 30A/240V "ring circuit". It is a loop that starts and ends at the CB pannel. As current can feed both ways so they can use a much smaller wire guage. Then the plug for each device is fused. Basically it makes the whole wiring system into a sub-pannel.
From what I have heard, outside of the kitchen and other appliances a typical home migh only have 2 ring branch circuits.
It would be a great way to wire a shop.
"Another one to ponder: why can you can plug a 16 or 18 gage extension cord into a 20 amp circuit? That sort of leaves the extension cord running hot if the load on the cord is 15 to 20 amps."
Best not use this for running a full 15 or 20 amps, it'll heat up. And then you'll be glad the circuit has #12 or #14, thinking about that heat in your walls.
"Even though Romex is made with 90 degree insulation the NEC, with agreement of most experts, it is limited to being put on a 20A breaker"
I don't understand this, Ive seen Romex up to #8 and wasn't aware of a restriction on connecting them to circuit breakers? NEC Reference?
I'm all ears if you want to go into detail about this
James
Edited 9/30/2002 12:24:42 PM ET by EXLRRP
"I don't understand this, Ive seen Romex up to #8 and wasn't aware of a restriction on connecting them to circuit breakers? NEC Reference?"
Actually this is a mismash of two different requairesments.
In the section on the use and application of NM it limited you the 60 degree column in the ampacity table. 336-26. However, if it needs to be derated you can start with the 150 column ampacity before the derating.
There is a separate section (but I could not find it) that limits #14 to a 15 amp protectionm #12 to 20, and #10 to 30. That is true for all types of wiring, not just NM.
Phil,
Another one to ponder: why can you can plug a 16 or 18 gage extension cord into a 20 amp circuit? That sort of leaves the extension cord running hot if the load on the cord is 15 to 20 amps.
How about putting a fuse holder at the plug end of the cordset, configured to protect the cordset and designed so that you couldn't overfuse? That would take care of overloads and arcing faults (I think) more effectively than a standard thermal trip molded case breaker, without cutting power to the whole circuit. Is this not done because of expense, or inconvenience?
Several years ago there was a UL-listed product called a "fire plug" that was an add-on fuse holder. It was supplied with a 5 amp fuse and designed to go between the TV cord and the receptacle outlet. The idea was that the small fuse would protect against fire if a short occurred in the TV's "instant on" circuitry. I know that this wouldn't work for motor loads etc, but how many fires of electrical origin start in properly installed fixed appliances vs cord-and-plug connected lights and appliances?
Cliff
"Or does this fall under the same reasoning that 200 AMP service feeder wires from the utility pole are so small since they are "open air cooled"? Yet the service entrance wires down to the meter have to be 4/0?"
Yeah, this one really gets my goat. I put in my own 200 amp RV service pole. Inspected out the wazzoo. I have no problem with that, I want it done right, even more than they do.
But... I had to put in copper wire that is right about 5/8" in diameter. (Per single cable) From the masthead, down to the meter box. Then they come along and put up wire from the masthead, out to the pole at the street that is barely a quarter of an inch thick. Aluminum, to boot. What the heck is with that ?
Quittin' Time
Think of it this way..
30 AMP breaker required with 10 gauge feeders from the CB.
You got that part right, the purpose of the breaker is to protect the feeder, the wire concealed in the wall, not the appliance, So, if the water heater wants to malfunction, burst into flames, or do a tap dance across the basement, the 30 amp breaker is gonna let it do what it wants.
no turn left unstoned
There is also 260C wire now available (20 AWG can carry 25A). In addition to the type insulation systems Bill mentioned, the conductors are nickel plated to avoid oxidation at those temps as are the 250 C wires. Termination methods are critical.
One step farther are the specialty ceramic coated conductors for operation at 500C and above for use on jet and rocket engines and internal to some transformers. Some of the more esoteric use Pt plated wire.
You won't find'em in NEC.
The meltdown, burst into flames part..... I think I may wanna avoid that part. Yeah, I think so. :-)
Phil you brought up a good question. I would like to comment that even though you have two elements and two thermostats in water heaters only one element is on at a given time. It would be good to find out about the 12 AWG wire on a 30 amp circuit though. Larry