Circuit Breaker Tutorial-why you should not use a 20A breaker.

LCHIEN · #1 12-30-2006, 09:06 PM

Seems to me like Circuit breakers are commonly used by everyone and little understood by most. I will explain Circuit breakers and why you should not use a 20A breaker with your BT3x saw.

Technical basis for this tutorial comes from the Square-D company that makes circuit breakers used in their panels for industrial and residential uses. These are designed to the same guidelines of the National Electrical code as other manufacturers' breakers so the operation should be the same.

I think the typical person thinks circuit breakers are black and white devices. That is, for a 15A breaker that if the current is 14.9 amps it will continue to conduct and if the current rises to 15.1 Amps it will open up and disconnect the circuit.

But in reality, circuit breakers are both time and current sensitive. This is a very useful thing because it allows us to use devices which vary in current pull so that the average is 15 amps and have peaks (say when your saw encounters a knot, or when a motor or light bulb turns on) very much in excess of 15 amps.

If you look at the end of the reference document, it has trip-point curves for the breakers , time vs. normalized current (e.g. the rated, or handle current is considered to be "1"). The curves and the text say that the circuit breaker should conduct the rated "handle" current (e.g. the value marked on the handle) pretty much forever if the ambient temperature is less than 40°C (~104°F). If the current is over about 1.5 times the rated current for longer than 10-100 seconds the breaker will eventually trip.

The curves approach the rated current when the time exceeds 10-100 seconds, this is because one trip mechanism is a bimetal element that is heated by the current flow. The element takes time to heat up to the trip point, allowing brief overcurrent surges since the heating takes time. And, devices shifting quickly from 10-20 amps and back but averaging 15 will also work forever.

The other feature of the curve is that around 9-10 times the rated current (about 135 Amps in this 15A breaker example), the breaker trips within one AC cycle (a few milliseconds) due to the electromagnetic trip element based solely on current. This is sufficient to let motor and light bulb starting curges through.

Between the two portions of the curve, like from 1.5x to 9x the rated current, the time it takes to trip decreases from 10 seconds to less than .3 of a second.

Thus you see that circuit breakers will trip immediately on gross overcurrent levels starting around 9x the rating, but allow brief, surges in the 1-10 second or more range. this is consistent with motors which can stand brief surges without overheating due to the thermal mass of the motor.

If you understand how these work, the people who put a 20A breaker on their dedicated circuit for their BT3100/BT3000 are just fooling themselves and taking a risk that the saw will melt down when overloaded. A 15A breaker for the BT3100/BT3000 is more than sufficient, given the leeway designed into the breakers. A 15A breaker is more than sufficient, allowing current surges consistent with startup and brief surges in operation associated with knots and feeding inconsistencies and other short term irregularities which do not exceed 15A on average. It will also protect the 15A-rated motor properly since the saw itself does not use any current overload protection.

Reference:
http://ecatalog.squared.com/techlib/...01&action=view

Stormbringer · #2 12-30-2006, 09:30 PM

Valuable information Loring. Thanks for taking the time.

Greg

lcm1947 · #3 12-30-2006, 09:47 PM

OK you got my interest Loring. So then with that knowledge am I correct in thinking all my circuits for my tools like my DeWalt chop saw, band saw, drill press, etc should be on a 15 amp breaker? How about the HF DC? It's suppose to be rated at 20 amps so I assume that's really the only tool where I should use a 20 amp breaker then right?

Ed62 · #4 12-30-2006, 10:00 PM

Good post, Loring. Thank you.

Ed

maxparot · #5 12-30-2006, 10:01 PM

A bit of additiional info for optimizing your dedicated circuits.

Although Loring's info on breakers is quite correct there is a couple of things that can help support a high draw device on a circuit:

Increase the gauge of wire you use to run your circuit. Even though a 15 amp circuit calls for 14 awg cables using 12 or 10 awg lowers circuit resistance. Likewise running conduit and using stranded thhn wire instead of solid wire will lower resistance. Using the combination of a larger gauge and stranded wire will insure you never choke your tool.

Sawatzky · #6 12-30-2006, 10:49 PM

I had my saw plugged into a 15 amp circuit. It was always tripping, especially in hot weather. I used no extension cord either. Even if I had other devices plugged into the same circuit (the circuit powered additional plugs in my garage) but did not turn them on, the circuit still tripped when I ran my BT. I added a 20 amp circuit because of advise on this forum. I now plug my BT and miter saw (both with 15 amp motors) into this outlet. I have not had any problems. Will I really damage my tools? If so, how do I avoid tripping the breaker all the time?

and207 · #7 12-30-2006, 10:59 PM

Hi!
Thanks.
Your posts are always informatives.

Andre

cgallery · #8 12-30-2006, 11:16 PM

Quote:

Originally Posted by Sawatzky

I had my saw plugged into a 15 amp circuit. It was always tripping, especially in hot weather. I used no extension cord either. Even if I had other devices plugged into the same circuit (the circuit powered additional plugs in my garage) but did not turn them on, the circuit still tripped when I ran my BT. I added a 20 amp circuit because of advise on this forum. I now plug my BT and miter saw (both with 15 amp motors) into this outlet. I have not had any problems. Will I really damage my tools? If so, how do I avoid tripping the breaker all the time?

Did you try replacing the original 15a breaker before adding the 20a circuit?

LCHIEN · #9 12-31-2006, 01:59 AM

Quote:

Originally Posted by cgallery

Did you try replacing the original 15a breaker before adding the 20a circuit?

That would be my question - perhaps your 15A breaker was bad.

My point is that a 15 Amp breaker will allow at least 15amps average to be delivered to those motors for a long time. A 20A breaker will allow 20 Amps average to be delivered to those devices for a long time, it has no conscience.

If you push a 15A motor to draw 19Amps it will burn out quickly. Because the magnetics of the motor will be saturated and the current will just end up heating the motor rather than doing mechanical work. The 20Amp breaker will allow it to self destruct.

LCHIEN · #10 12-31-2006, 02:03 AM

Quote:

Originally Posted by lcm1947

OK you got my interest Loring. So then with that knowledge am I correct in thinking all my circuits for my tools like my DeWalt chop saw, band saw, drill press, etc should be on a 15 amp breaker? How about the HF DC? It's suppose to be rated at 20 amps so I assume that's really the only tool where I should use a 20 amp breaker then right?

A 20 Amp breaker would simply be to protect the dwelling from fire due to overcurrent in the wiring, caused by a fault (likely catastrophic short) in one of the loads or perhaps the wiring itself.

It (15 Amp breaker used with single tools) should be adequate and provide maximum protection. You would want to use a larger breaker if you were running several loads on a single circuit, whose total was more than 15A but less than 20A. But the possibility exists, unless the individual loads were fused or protected by internal circuit breakers, any device on the line could be damaged by overcurrent.

I believe you are incorrect about the HF 2HP DC. It is rated to draw 14 or 15 Amps and I have confirmed mine draws 14.7 amps (67 Amps on startup) when running with no hoses attached - pretty much the maximum airflow condition and hence maximum current condition. I run mine on a 15A circuit without tripping any breakers.

from the HF webpage:
Powerful 2 HP dust collector creates a dust free working environment. Develops over ten times the suction of most shop vacuums. Works with a 4'' hose to pick up large chips from jointers, saws, shapers and planers. Hose sold separately.