REQ:Breaker Maximum...6AL...?

[Joey_Bitchn']

Greetings could someone post an anwser to the group for the following:

I have a home that is about 20 years old. The sub-panel in the basement is a 100 AMP panel. I have an aluminum wire that runs from the panel to a new airconditiong unit I had installed recently. The wire reads: SE Cable Style U Type XHHW CDRS 600V 3 CDRS 6 AL (UL) 1983

I currently have a 40 AMP breaker installed that keeps tripping after the AC unit cycles on and off a few times. The AC guy that installed it said to put in a 50 AMP breaker but I can't find the information to see if this wire can take that size breaker. If someone can help me find the info or if you know for sure that a 50 AMP breaker is OK, could you please let me know?

PS The AC unit says it can take a max breaker of 60 AMP.

Thanks

Joe

[Joey_Bitchn']

Greetings could someone post an anwser to the group for the following:

I have a home that is about 20 years old. The sub-panel in the basement is a 100 AMP panel. I have an aluminum wire that runs from the panel to a new airconditiong unit I had installed recently. The wire reads: SE Cable Style U Type XHHW CDRS 600V 3 CDRS 6 AL (UL) 1983

I currently have a 40 AMP breaker installed that keeps tripping after the AC unit cycles on and off a few times. The AC guy that installed it said to put in a 50 AMP breaker but I can't find the information to see if this wire can take that size breaker. If someone can help me find the info or if you know for sure that a 50 AMP breaker is OK, could you please let me know?

PS The AC unit says it can take a max breaker of 60 AMP.

Thanks

Joe

[John Grabowski]

Check out table 310.16. in the National Electrical Code. If all of the terminals are rated for 75 degrees Celsius, than you can install a 50 amp circuit breaker. The electrician probably should have run a 60 amp line since that is what the nameplate says is the maximum. You might want to put an ammeter on the line to see how much current the unit is actually drawing.

[Joey_Bitchn']

Turtle, The length of the wire is about 30 feet on the conservative side. I didn't get a chance to check the draw on the circuit though. I'll try getting that tomorrow assuming I can figure out how to do it correctly. Thanks for the help Joe

[TURTLE]

This is Turtle.

Joey , i know just about all # 6 AL. good wire like you yhave here will run good on 50 amp breakers on them. Now one thing you did not say and that was how long of a run was the #6 Wire as to distance fromn the breaker to the hvac units. Untill you give this info. i can not say the wire would be OK or not. So for now No it will not work till you give the wire distance to concider.

TURTLE

[TURTLE]

This is Turtle.

The Ugly Book says you have up to about 50 feet or a little more for the 50 amp ability and 50 amp breaker. Your well under with 30 feet.

TURTLE

[CL (dnoyeB) Gilbert]

This is odd, could you explain that to me? I am an automotive systems engineer and I design the wiring for automobiles. The length of wire has nothing to do with the fuse rating. The device on the end of the wire also has nothing to do with the fuse rating.

Well ok, some devices can terminate a few different sizes of wire, and this is a physical thing, nothing to do with current amount but with diameter, etc.

So why does wire length matter for breaker size in a home?

[Percival P. Cassidy]

On 08/03/05 03:19 pm CL (dnoyeB) Gilbert tossed the following ingredients into the ever-growing pot of cybersoup:

I assume that he means that a longer wire will result in a larger voltage drop, so one would use a larger wire to reduce the resistance and the voltage drop -- and a correspondingly higher-rated breaker to go with the larger wire.

Perce

[Joey_Bitchn']

Turtle

I installed the 50 AMP Breaker and so far so good.I've had the A/C on all day. The wires aren't even warm. The A/C unit read 37.5 AMP minimum so I guess that with the 40 AMP breaker the initial voltage drop over a few on/off cycles was enough for the relay not to re-engage. Thanks for the tip. If you lived in Fallbrook, Calif. I'd buy you lunch for saving me the call to an electrician :-)

Cheers! Joe

[Nathan Pralle]

This is my understanding as well, and I've seen it in practice. Running a 16A compressor on a 30A circuit done in 10/2 wire for 60 feet saw too much drop across it to power the compressor in the winter. Bringing the compressor within 6 feet of the box solved that problem.

Nathan

[TURTLE]

This is Turtle.

In the World of Electricians we have a simi-rule book called a Ugly Book which will tell use what size wire, load size, and breaker size that matches a load and keeps the voltage drop to less than 2% of Voltage. If you have 120 volts service. You don't want more than 2% or 2.4 volt drop caused by the wirte size being too small for the ampcity. The fastest way to burn up a hvac system is to run it on low voltage.

TURTLE

[Chris Lewis]

According to CL (dnoyeB) Gilbert :

It doesn't matter directly, but there are some indirect effects.

First of all, and this applies to your engineering, the fuse/breaker should be no larger than the current carrying capacity of the wire.

Second of all, in AC power wiring, devices are tested and approved for connection to a maximum circuit ampacity. So, you can't put a

15A 120V device on a 60A circuit.

And thirdly, which is where wire length comes into play: certain devices (especially A/Cs and other large motor-powered devices) have very large startup surges. The wiring has to be large enough so that coupled with wire length, there isn't excessive voltage drop to the device. The more the voltage drops, the more the startup surge is prolonged. Coupled with the fact that breakers and fuses have time-delay factors built in, it's entirely possible for a large startup motor load (like a central A/C) to trip a breaker on 100' of wire when it wouldn't trip the breaker on 10' of the _same_ wire.

Often the wire size has to be a size or two larger than the ampacity would dictate to reduce voltage drop. With A/Cs and certain other loads, not only the wire sometimes has to be larger than you'd expect (with long runs), the breaker has to be somewhat larger than the steady-state draw of the A/C would suggest.

Central A/Cs often have somewhat oversize wires and breakers, with a fuse local to the device closer to the A/C's continuous draw.

[TURTLE]

This is Turtle.

Your Elkcome.

The most you can put on a 40 amp breaker is 32 amps. The most you can put on a

50 amp breaker is 40 amps without running it hot . So your good on the 50 amp breaker and a 37.5 amp load.

TURTLE

[Bud]

Rules change a little for motor circuits. In general the wire has to have a rating 125% of the motor rating. Because of the high starting current of motors, time delay fuses/circuit breakers can have a rating of 175% of the motor rating; this is for short circuit protection. For overload protection motor starters with a narrow range overload trip unit or thermal trip units in the motor are used. For low duty-cycle welders the fuse can be much larger than the wire rating. (Don't use higher ratings if you don't know what code requirements are and what you are doing.)

Home A/C units will be marked with a maximum overcurrent device size. If it says maximum fuse size, the device must be a fuse.