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Electricnewb 03-07-2009 01:08 PM

Maximum wattage question
 
hey-I just bought a house and it was built in the 60's.i know the very basics about how many watts you can use with 15, 20 amps and so forth.(about 80% of the max ampage, correct?) My questions for the electric gurus are: 1.how long will the wiring in a house this old be good for with constant use of electricity? 2.If Im using all of a 20 amp breakers capacity on a single 120 volt outlet, will the wiring at this outlet become too hot and hazardous, as a result of all of the power going to one outlet?? 3. what is the difference/benefit to using a 120v power cord from an appliance as opposed to a 240v plug? 4. If i use a solid quality extension cord, could I run a 1500 watt portable A.C. off of it safely? If not, why? Please respond only if you know your stuff-Thanks.
J

Bocolo 03-07-2009 03:39 PM

I am hesitant to post answer because of your quote "Only respond if you know your stuff", understand that I am by no means an electrician and some of the answers may not be complete.

1. Wiring in the house should be good as long as there are no shorts or loss of power. If you are referring to being up to code then you need to check what code your municipality is on. As long as you do not do any major (I don't know if major means adding 1 circuit or 25 circuits) renovations you do not need to worry about bringing the house of to code. Any new wiring you do has to be to code.

2. If you are using a 20 amp breaker with 12 gauge wire you should not have a problem. You should not use a breaker that is rated higher than your wire. For example: use 15 amps for 14 gauge, 20 amps for 12 gauge, 30 amps for 10 gauge. Do not use 20 amps on 14 gauge wire.

3. I really don't understand the question.

4. I don't believe it is recommended. The main reason for spacing requirements for receptacles is to avoid the use of extension cords. Why? That is what code states and it could be a fire hazard.

As to watts to amps, I believe a 15 amp breaker will give you a max of 1800 watts and a 20 amp breaker will give you a max of 2400 watts. The is something about the 80% rule but I am not to clear on it. I believe it means that you should not go over 1440 watts on a 15 amp and 1920 watts on a 20 amp. I am sure someone will let us both know if any of this information is incorrect. Thanks.

williswires 03-07-2009 03:41 PM

Quote:

Originally Posted by Electricnewb (Post 241451)
1.how long will the wiring in a house this old be good for with constant use of electricity?

Wiring is not affected by "constant use of electricity" within the design limits of the conductors. However, wiring conductors and insulation are damaged by situations where:

-- excess current was allowed to occur (putting a 30A breaker on a 14AWG circuit),
-- where damaged by heat from loose connections, repeated surges, or use in areas with high ambient temperatures, etc.
-- where susceptable to mechanical damage
-- and many other improper uses of wiring

Quote:

2.If Im using all of a 20 amp breakers capacity on a single 120 volt outlet, will the wiring at this outlet become too hot and hazardous, as a result of all of the power going to one outlet??
Not if the wiring was correctly installed, used, and maintained.

Quote:

3. what is the difference/benefit to using a 120v power cord from an appliance as opposed to a 240v plug?
You'll have to clarify what your are asking. The entire circuit components must match - meaning if your appliance is rated 240VAC, you must use the proper power cord, with a listed 240V plug on the end of it, plugged into a listed 240V receptacle, connected to a properly sized and wired 240V circuit, which is fed from a properly installed double pole breaker which is sourced by each of your two legs of power in the panel.

If your appliance is dual rated, meaning it can be wired for either 120V or 240V, you can wire it to a 120V receptacle if the circuit is large enough.

Quote:

4. If i use a solid quality extension cord, could I run a 1500 watt portable A.C. off of it safely? If not, why?
Your portable air conditioner probably has instructions that tell you NOT to use an adapter plug or an extension cord. This is for basic electrical safety.

Your unit must be fed from a grounded receptacle, that's why they tell you to not use an adapter plug (although a there is no guarantee that a 3-prong outlet is properly grounded just because it has 3-prongs).

Also, many people would try to use any old extension cord they have which would be too small for the current draw of the unit.

If the air conditioner is 120V, I would not worry about using a SHORT length of 10AWG extension cord to run it. Don't use a 50 footer and run it from one room into the next where it will become a tripping hazard or where it would likely be damaged.

If it is 240V, no extension cord. :no:

Electricnewb 03-07-2009 07:25 PM

cool-that about answers all of my questions-Thanks to you both for taking the time to answer.anybody else-feel free to chime in. by the way-how would one be able to tell whether or not the house was wired correctly, and whether or not there is hazardous wiring in the house?

Electricnewb 03-07-2009 07:30 PM

In regards to my third question(apparently it didnt make sense-my apologies-my knowledge is limited on these topics) Im basically wondering what is the difference between a 120 v and a 240v power cord. Ive heard that you can pull more amps off a breaker with a 240 then with a 120, but that the wattage amounts to the same..Is this accurate?

Bocolo 03-07-2009 07:40 PM

I don't think you mean power cord. If you are talking about a 240 volt circuit then you are talking about multi wire in the circuit. Typically you have a white, black, red, and ground. The black carries 120 volts, the red carries 120 volts which equal 240 volts. You can get more wattage if you use bigger cable and a bigger breaker. For example 10/3 cable (10 gauge, 3 wires, 1 ground) with a 30 amp breaker. In regards to inspecting your electrical wiring best is to hire an electrician. I guess you can check wire size to correct amp breakers and voltage coming in at receptacles, and fixtures, check wires for continuity and so on but you will not be able to see what is hidden behind the walls unless you take them down.

Electricnewb 03-07-2009 08:35 PM

for example, my a.c. unit has an option to use a 240v three pronged plug or a 120v.same with my ballast for an aquarium bulb.supposedly id be using less amperage with the 240 power cord than the 120?

Bocolo 03-07-2009 09:06 PM

I am not too informed on 240 volts. Others in here are very knowledgeable on electricity. Give it time and someone will answer your question. Thanks.

Gigs 03-07-2009 09:31 PM

Electricnewb it depends on how the device is designed, but if it uses the same amount of watts at 240 volts that it uses at 120 volts, then the amps will be halved at 240 volts.

Regarding "pull more amps off a breaker at 240v than 120v"... no that is not how it works. A 240 volt breaker is twice as wide as a 120 volt breaker and is more similar to two 120 v breakers that trip at the same time. Also, while you can get more wattage at 240v, the ampacity of the wires for a given size does not vary by voltage.

Electricnewb 03-07-2009 09:46 PM

Gigs, so if I have a 15 amp breaker, and I want to use a 15 amp A.C. unit without maxing out my amps, and the a.c. is designed to use the same amnt of watts with both 240v and 120v, then the a.c. will use half of the amps?In other words id be using 7.5 amps but the same amount of wattage if i was using 120v?
Quote:

Originally Posted by Gigs (Post 241649)
Electricnewb it depends on how the device is designed, but if it uses the same amount of watts at 240 volts that it uses at 120 volts, then the amps will be halved at 240 volts.

Regarding "pull more amps off a breaker at 240v than 120v"... no that is not how it works. A 240 volt breaker is twice as wide as a 120 volt breaker and is more similar to two 120 v breakers that trip at the same time. Also, while you can get more wattage at 240v, the ampacity of the wires for a given size does not vary by voltage.


jamiedolan 03-07-2009 10:37 PM

Quote:

Originally Posted by Electricnewb (Post 241654)
Gigs, so if I have a 15 amp breaker, and I want to use a 15 amp A.C. unit without maxing out my amps, and the a.c. is designed to use the same amnt of watts with both 240v and 120v, then the a.c. will use half of the amps?In other words id be using 7.5 amps but the same amount of wattage if i was using 120v?

Generally the amps used would be the same, but with 240v, it will be split between 2 breakers. So you would be drawing 7.5A on 2 separate breaker slots (via one double pole breaker) for a total of 15A.

Jamie

Gigs 03-07-2009 10:38 PM

Newb, Just so you know, most devices will not function on either voltage, they are either designed for one or the other.

Gigs 03-07-2009 10:40 PM

Quote:

Originally Posted by jamiedolan (Post 241668)
Generally the amps used would be the same, but with 240v, it will be split between 2 breakers. So you would be drawing 7.5A on 2 separate breaker slots (via one double pole breaker) for a total of 15A.

I don't like this explanation. You don't count the amps in a single circuit twice.

jamiedolan 03-07-2009 10:47 PM

Quote:

Originally Posted by Gigs (Post 241671)
I don't like this explanation. You don't count the amps in a single circuit twice.

I think the OP was thinking the Air cond. would use less power if it was run on a 240v feed.

I was trying to illustrate that the Air cond. would use the same amount of power (amperage) if it is run on 120v or 240v service.

120v Air Cond Draws 15A

Same Air Cond running on 240v circuit, still draws a total of 15A, it is just split between 2 circuits.

Am I being confusing? sorry.

Jamie

Gigs 03-08-2009 12:06 AM

Quote:

Originally Posted by jamiedolan (Post 241674)
Am I being confusing? sorry.

Jamie

Yes, it's not the same amperage at all, it's half the amperage. It is the same wattage, however.


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