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Not trying to hijack this thread, but after reading it, I just want to know something...

What's the difference between, running a 30amp heater off a 50amp circuit, and running a lamp off a 20amp circuit?
POWER!!!

This outlet will provide 12,000 (yes THOUSAND) watts of power and will not trip the breaker. By comparison, a 20 amp circuit can only supply 2400 watts of power.

Think of it as the difference between being given the keys to a mopped vs. an Indy Car.
 

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POWER!!!

This outlet will provide 12,000 (yes THOUSAND) watts of power and will not trip the breaker. By comparison, a 20 amp circuit can only supply 2400 watts of power.

Think of it as the difference between being given the keys to a mopped vs. an Indy Car.
That in no way answers the question of why a light bulb & lamp that uses .5a can be plugged into a 20a circuit & a 30a heater can't be plugged into a 50a circuit

.5a/20a = using 2.5% of the circuit
30a/50a = 60% of the circuit
 

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That in no way answers the question of why a light bulb & lamp that uses .5a can be plugged into a 20a circuit & a 30a heater can't be plugged into a 50a circuit

.5a/20a = using 2.5% of the circuit
30a/50a = 60% of the circuit
Assuming what jbfan is saying is NEC code, I'm sure there's specific reasons, and if you know the reasons then please share them.

Otherwise, I'm trying to think of reasons why the NEC says what it says when they don't always provide reasons for what they say.

In this case, I'm drawing a comparison with a 50 amp circuit = Indy Car while a 20 amp = Mopped. In the case of a mopped, it doesn't mater if you run it at 0.5% or 100%, the potential for damage is less compared to turning someone loose with an Indy Car regardless of whether you are driving it at 5 mph or 300 mph... cause even if you're only driving at an Indy Car at 5 mph, it only takes a small press on the pedal to unleash the power to jump to 300 mph.


At least that's on general reason to try to explain in a non-technical terms that I can think of.
 

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Comparison is invalid since a 20a circuit can burn a house down just as easily as a 50a
Its about what can be plugged into what circuit, and how much power a device is rated to handle
 

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Comparison is invalid since a 20a circuit can burn a house down just as easily as a 50a
It's not totally invalid, after all you can still kill yourself on a moped, it's just a lot easier to do so in an Indy Car.


But I'm sure there are other reasons and other comparisons that can be made.
 

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Thanks for throwing the numbers out there scuba.

I always thought that amperage through a circuit would only increase as more devices, wired in parallel, are used.

V=I/R right? R=I/V? This is what I can remember from AP physics from 5 years ago (can't believe it's been that long since I graduated).

So, 240=I/whatever the resistance rating is of the heater. So if it's a 30amp heater, then it has a resistance of .125 ohms, correct? That resistance isn't going to change, the only variables are voltage and current. Since the resistance is already defined, and we want it on a 240v circuit, then the current draw will be 30 amps. I don't think I can arbitrarily plug 50amps in the formula, because that would change the resistance, which is impossible unless something physically happens to it.

Or am I wrong here?

The 50amp circuit will supply a MAXIMUM of 50 amps, and not constant, correct?
 

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Discussion Starter · #27 ·
No "in theory" about it...if you feed 50a to a 30a heater you WILL fry it
Series resistance ? for an extra 2400w of power ?
There isn't anything you can do that will be safe & meet code using a 50a circuit & a 30a heater
You burn down that Apt you will be found at fault & liable for all damages

You MIGHT be able to connect a 30a & 20a heater to use the 50a
I'm not sure how or if that will work or meet code/be safe
Just because a circuit is capable of delivering up to 50 amps does not mean it will always be putting out 50 amps to the load. The load must be factored in to the equation. V/R = I , if the load is sufficiently resistive it will limit the current well bellow the rated 50 amps. In fact, V/R = 240V/(Heater Resistance + Wire Resistance) = (Current to Heater). By this equation the current to the heater should be very similar between a 240V-50A circuit and a 240V-30A circuit.
 

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Discussion Starter · #28 ·
Thanks for throwing the numbers out there scuba.

I always thought that amperage through a circuit would only increase as more devices, wired in parallel, are used.

V=I/R right? R=I/V? This is what I can remember from AP physics from 5 years ago (can't believe it's been that long since I graduated).

So, 240=I/whatever the resistance rating is of the heater. So if it's a 30amp heater, then it has a resistance of .125 ohms, correct? That resistance isn't going to change, the only variables are voltage and current. Since the resistance is already defined, and we want it on a 240v circuit, then the current draw will be 30 amps. I don't think I can arbitrarily plug 50amps in the formula, because that would change the resistance, which is impossible unless something physically happens to it.

Or am I wrong here?

The 50amp circuit will supply a MAXIMUM of 50 amps, and not constant, correct?
See my previous post.
 

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Now I am at a bit of a roadblock here. If its true that 30A is the largest space heater one can find for 240v, then my 50A circuit will not be ideal for the space heater (could burn up the space heater in theory). Perhaps adding a series resistance to the circuit would limit the current draw enough to allow me to safely use a 30A heater on a 50A circuit. Any other ideas? A single 1500W heater is not going to be nearly enough. I need to heat a space up to ~120F.
A current limiting resister is totally wrong way to go here. You would be creating way too much heat in the resistor AND dropping the voltage across the heater. That's worst than just running the 30amp heater on a 50 amp circuit. After all, if the 30 amp heater is designed for 240v, and THERE ISN'T ANYTHING WRONG WITH THE HEATER, then it's only going to draw 30 amps even if it's connected to a circuit capable of supplying 300 amps.

But still, this whole idea just doesn't sound safe: Trying to artifically heat a space to 120F inside an apartment?
 

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Discussion Starter · #30 ·
A current limiting resister is totally wrong way to go here. You would be creating way too much heat in the resistor AND dropping the voltage across the heater. That's worst than just running the 30amp heater on a 50 amp circuit. After all, if the 30 amp heater is designed for 240v, and THERE ISN'T ANYTHING WRONG WITH THE HEATER, then it's only going to draw 30 amps even if it's connected to a circuit capable of supplying 300 amps.

But still, this whole idea just doesn't sound safe: Trying to artifically heat a space to 120F inside an apartment?
I completely agree with you. The series resistor would be unneeded complexity. I also agree with you about the fact that the 30amp heater should only draw up to 30amps on 240v, assuming its properly designed and not defective. However, the above posters seem to be saying that this does not meet standard code. I don't really give a damn about standard code as long as I can account for all possible issues.
 

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Possible issue is burning down the Apt because the heater is NOT rated to be connected to a 50a circuit
There is a BIG difference between 30a wire & 50a wire
Have the owner of the Apt setup the heat correctly

You can't use the existing 50a plug & circuit with a 30a device

-closed-
 
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