We just bought a cabin. It has a 100 amp service breaker, and a service
panel with 5 20 amp breakers.
I know on appliances, all I have to do to find out the amperage is RTFM, but
for lights and such, how do I calculate just how much wattage I can put on
any breaker circuit?
The lighting is inadequate. I will need more. I have a licensed
electrician friend who will come and wire everything, so it will be done
right and safe. I just need to get an idea how many lights we are talking
about so I can do some shopping. I don't want to max everything out and put
up a ton of lights. I just want to balance them, and not put so many that I
am approaching critical mass.
this is Turtle.
In normal lighting circuits it is hard to over load normal lighting wattages to
just light up a cabin. Just take 1 -- 20 amp breaker and circuit for lights only
and that gives you 20 -- 100 watt light bulbs to light up the cabin. One Circuit
is what I thinik you need for all lighting to the cabin.
Now really 19.6 -- 100 watt light bulbs to be exact.
Not the best idea: Pop the breaker an dthe whole place
goes dark. They should be mixed on at least two
breakers, and instead of 20 bulbs, that would be 16
bulbs. Over 80% usage will allow normal variations in
the grid, cabin, over time, breakers, etc. to begin to
heat the breaker, thus degrading it over time and
leaving no safety overhead. So with a min two lines
you've got 32 bulbs now, lots more than you'll need.
Not sure where the 80% figure comes from, nec, ul, mfg,
whatever, but it's reality.
This is Turtle.
He was asking what number of breakers would be need for lighting for the cabin.
I said 19.6 for a 20 amp breaker will support the 19.6 light bulbs and will be
the 80% of the amperate of the 20 amp breaker. He can split up the bulb in all
area , but all lighting would not need more than 1 --- 20 amp circuit, no matter
how he run it.
With two circiuts of 20 amps he could put not 32 -- 100 watt light bulbs but
39.2 --- 100 light bulbs. This would still be compliant of the 80% rule of the
two 20 amp breaker circuits.
Now the degrading of the breaker to trip at lower amperages is just a effect you
have to deal with 20 to 30 years from now and just wait 20 years or so and think
This is Turtle.
When Speaking of a Electrical breaker in a breaker box suppling electricity and
the 80% rule does not matter if the elctricity is supplied to a whole home or to
a one light bulb. If you have a 20 amp breaker in the box. You better not put
more that a 80% of 20 amp load on it. So You can have a 16 amp load to be
supplied to the 20 amp breaker.
i know now that your not a electrician for you would know exactly what I was
speaking about and you would be telling me about the 80% rules. So 80% Applies
to everything that will pull amps from a breaker and if you want to change
anything. Please be welcome to do as you please.
What you say here is completely wrong.
It's quite obvious that *you* are not an electrician, or you would know that
the 80% rule refers to continuous loads, defined by the NEC as "a load where
the maximum current is expected to continue for three hours or more." This
does *not* apply to residential lighting circuits, or indeed to most other
Doug Miller (alphageek-at-milmac-dot-com)
This is Turtle.
first i don't do residentiual electric work and only commercial HVAC which makes
me do the electric for them.
Second if you was running power to lights and the lighting wattage was 2,400
watts / 24 -- 100 watt light bulbs and then would you say you would hook all
them up to a 20 amp breaker which the load would be matching a 2,400 watt load.
Then or would you only connect 19.6 light bulb on the 20 amp breaker to not over
load the breaker and trip if all the lights were turned on at one time. Awwwww
that would be the 80% rule. When every you pull over 80% of the breakers rating.
Your asking it to trip and you have to come back to rebalance the loads.
Tell me what you would do with this problem !
Turtle, I specifically addressed residential lighting loads. Do you
have every light in your house on at the same time?
BTW maybe you could think of the "80%" rule as an absolute in your
business since HVAC can be called a continuous load but it is far from
universal in residential and never applies to the residential "general
lighting" loads. If you added up all the required receptacle circuits
and lighting in a residence at 125% of potential load a small bungalo
might need a 400a service. They use something called load diversity to
indicate that only a small part of the loads in a home will be on at
any given time.
As a related topic, I hope you understand the "minimum circuit
ampacity" on the nameplate of HVAC equipment already has the 80%
factor built into it so you don't need to size to 125% of that number.
A am sure you have noticed that the branch circuit overcurrent dervice
spec is quite a bit higher than 310.16 would seem to allow.
It's perfectly legal.
Ain't the NEC fun?
This is Turtle.
You missed the question of what you would do in this case. I stated that I would
not put but 20 -- 100 watt light bulbs on a 20 amp circuit / breaker. The 20
light bulbs would be the max i would put on the 20 amp breaker because of the
what is the max number of 100 watt light bulbs that you can put on a 20 amp
So , What is the answer to this question ?
Now, I am really confused. I never thought this would be such a complex
It seemed like a simple question, but now I am not so sure. I am thinking
of cancelling the sale because of all of this.
I mean, I lie awake nights now, thinking of amperage, wattage, voltage, and
things I didn't even know existed last week.
The current debate has caused me to think of a lot of related issues.
Will the atmospheric pressure change affect these bulbs? What if I buy them
here in Nevada and take them to Utah? Will the change in elevation or time
zone affect their performance?
Will the "long life" bulbs violate the religious beliefs of the area where
there are strong afterlife philosophies? Would I be in violation of local
traditions by bringing in these "long life" bulbs?
What happens if I violate this 80% rule? Say, by 2%? Was this one of the
commandments that was dropped by Mel Brooks playing Moses in "The History of
the World, Part One"?
If I use bulbs made outside of the US, can I, in good conscience, ask my
union electrician buddy to work on this project?
There are five 20 amp breakers. That will give me up to 20,000 watts in a
1200 square foot cabin. Or, a mere 16,000 if I listen to the 80%
philosophy. My question is, IF the Space Shuttle were flying, will it be
visible from space? That's a lot of light.
Should I just use Halogens?
Do you think I will need eye protection?
Do you think it will cause sunburn or carpet discoloration?
In the meantime, I believe I will just stick to flashlights, candles, and
All this other stuff is just so complicated and confusing.
Right now, I'm going to take six valiums and try to wind down.
This is Turtle.
I see you would load up a break to it's max. amps and if the people in the house
happen to turn on all 24 light bulbs. It will blow the breaker in about 2 or 3
hours and then call you to ask why did my break throw. You maybe able to do this
by NEC but I will not load anything up to the max. what so ever.
Now what I did up above by just putting 20 light bulbs on a 20 amp breaker is
Nothing wrong with doing it. You had said it was wrong well i tell you it is
nothing wrong with it if i wanted to do it. Now you can explain any wrong as NEC
goes please explain it to me. Let me explain to you what right and wrong is.
Right is you can do it. Wrong is you Can't do it. Now take these two words and
explain wrong as you say above here.
Please read more carefully, Turtle. I never said that putting only 20 100W
bulbs on a 20A breaker was wrong -- I said you were wrong to claim that 19
bulbs was the maximum permitted because of the 80% rule.
I've explained it several times already, but you're not paying attention.
Let's try again. The 80% rule applies to continuous loads. Residential
lighting is not a "continuous load" as defined in the NEC. Therefore the 80%
rule does not apply to residential lighting circuits.
Yep, and by that definition putting twenty-four 100W light bulbs on a 20A 120V
circuit is right. So is twenty bulbs. Or five bulbs. Or one.
There's nothing wrong with loading a circuit to less than its capacity. What's
wrong is your understanding of the capacity of a 20A circuit when used for
residential lighting, and your understanding of the 80% rule.
Doug Miller (alphageek at milmac dot com)
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