Electrical Question

pburchett · Apr 5, 2018

Which breaker would trip first (1 or 2) if the load on the motor caused it to draw more than 30 amps (and why would this breaker trip before the other)?

john *.?-!.* cub owner · Apr 5, 2018

It is a 50/50 toss up. Most breakers have slight differences in their sensitivity to overloads, how tight connecting screws are, etc. so it is anyone's guess. Electricity travels to fast for mechanical breakers in line to make the difference as to physical location, plus it is AC, so it is always reversing.

Mike Groom · Apr 5, 2018

Probably # 2 would trip first,it's closest to the load source,but sometimes one breaker is more sensitive than the other so it could vary.

Ken Macfarlane · Apr 5, 2018

If they are identical breakers, manufacturing tolerances and
wear are what determine which one trips.

If they are different models or brands, the design response
time will be largest factor.

Breakers can be made fast or slow.

JCinKY · Apr 5, 2018

I don’t see the point for breakers in series with no other branch circuits upstream from #2. Even then #1 would be undersized or #2 is oversized.

JMOR · Apr 5, 2018

all the above plus temperature difference in panels where breakers are located.

Crazy Horse · Apr 5, 2018

Interesting question PB ....... but I'm curious. There must be a reason that you are asking, and since it appears that one or the other or
both are going to trigger, the system is going to do its job regardless. So, why do you pose the question? Just curious or maybe you and
someone else have different opinions? Or maybe some other reason?

Hay maker2 · Apr 5, 2018

If both breakers #1 and #2 are identical, then breaker #1 will trip first as it is the closet breaker to the panel (source of power). The current must pass through breaker #1 before it can get to breaker #2. Therefore breaker #1 will protect breaker #2

Bob · Apr 5, 2018

A better question is WHY is there a "neutral" wire run to the alleged 220 Volt motor?

Hay maker2 · Apr 5, 2018

Example:

A modern day electric dry is a 4 wire circuit. They separate the neutral from ground as there may be a 120VAC digital display.

boler76 · Apr 5, 2018

the distance between the breakers will make the difference

Ozlander · Apr 5, 2018

(quoted from post at 14:10:52 04/05/18) the distance between the breakers will make the difference

NOT

Ozlander · Apr 5, 2018

(quoted from post at 13:41:35 04/05/18) If both breakers #1 and #2 are identical, then breaker #1 will trip first as it is the closet breaker to the panel (source of power). The current must pass through breaker #1 before it can get to breaker #2. Therefore breaker #1 will protect breaker #2

Since it's AC, you don't know which way the current is flowing the moment of the overload.

Hay maker2 · Apr 5, 2018

(quoted from post at 20:23:58 04/05/18)

(quoted from post at 13:41:35 04/05/18) If both breakers #1 and #2 are identical, then breaker #1 will trip first as it is the closet breaker to the panel (source of power). The current must pass through breaker #1 before it can get to breaker #2. Therefore breaker #1 will protect breaker #2

Click to expand...

Since it's AC, you don't know which way the current is flowing the moment of the overload.

Regardless of which way the AC is flowing at any giving moment in time the current is being generated from the source and being drawn by the load. The breaker closet to the source will sense the overload first and protect the load that is applied to the secondary side (load side) of the breaker.

Janicholson · Apr 5, 2018

Reality check:::: Electricity moves the electrons in the wire depending on load. It moves them in a continuously alternating stream. The RMS amps in the loaded circuit
components will all have the same flow rate at all points. Variability between those breakers will determine the outcome. Not a recommended schematic. Jim

dick1234 · Apr 5, 2018

Take either one out of the circuit. The other is redundant. Just add a switch, if you need another point to stop motor. Good Luck, Dick

Stephen Newell · Apr 5, 2018

I'm wondering why there is two breakers in the circuit. Only one is needed. Then if there is no lamps or 110v motor at the load end I don't see what the neutral is for.

Steve@Advance · Apr 5, 2018

Since this is all hypothetical, let's assume the breakers are set to trip at exactly the same amps, both are in the same ambient temperature, all connections are tight, both lines are the same gauge wire from source to motor.

What we don't know is the distance from breaker 1 to 2. Assuming there is a considerable distance, my conclusion is breaker 2 will trip first...

Voltage drop!

paul · Apr 5, 2018

I assume this is all theoretical and not an actual design, as it would be silly. The breakers act identical, and the wires are the same size, etc.

So there appear to be 2 lines of thinking.

#1 gets the full draw first and would trip first.

#2 sees a dirtier electricity after voltage drop and going through the first breaker connections and so would see the heat up and feel like tripping
frost.

Which way is correct?

JMOR · Apr 5, 2018

(quoted from post at 18:06:04 04/05/18) I assume this is all theoretical and not an actual design, as it would be silly. The breakers act identical, and the wires are the same size, etc.

So there appear to be 2 lines of thinking.

#1 gets the full draw first and would trip first.

#2 sees a dirtier electricity after voltage drop and going through the first breaker connections and so would see the heat up and feel like tripping
frost.

Which way is correct?

either

Hay maker2 · Apr 5, 2018

This circuit is more common than we want to think. How many gutter cleaners and silo unloaders were wired up with two circuit breakers just as shown here? The first 30 amp breaker is in the main panel and the second 30 amp breaker is next the the gutter cleaner or silo unloader. And yes, there most likely would not of had a nuratral wire.

Hay maker2 · Apr 5, 2018

If voltage drops then ampacity increases to try to maintain the required watts. Low voltage will draw more current which creates heat. If more current is being drawing in order to compensate for low voltage then that current must pass through breaker #1 first which will sense the overload before it reaches breaker #2.

Crazy Horse · Apr 5, 2018

Hay Maker ...... is the second (non panel) breaker you mention really necessary or just there for convenience for resetting (if it triggers
off first)? Maybe you could explain what its purpose is in your examples.

jeffcat · Apr 5, 2018

Here is what I have seen in the field. Most of the time I bet they
both trip. Now I would say #1 first. Sometimes #2 depending on a
whole bunch of things that others have said. You want to see a dish
machine running on 480 3ph. Trips the breaker in the machine, the
breaker in the wall box, and the main breaker on the sub pannel three
floors up. Makes a big BOOM when they all trip off. A lot of people
think the new breakers trip too easily.

JMOR · Apr 5, 2018

(quoted from post at 19:34:26 04/05/18)
If voltage drops then ampacity increases to try to maintain the required watts. Low voltage will draw more current which creates heat. If more current is being drawing in order to compensate for low voltage then that current must pass through breaker #1 first which will sense the overload before it reaches breaker #2.

et real! At the speed of light? How many light years apart are the breakers? How fast does a magnetic or thermal breaker operate? :roll:

MarkB_MI · Apr 5, 2018

Kirchoff's Current Law: The sum of all current flowing into a node is equal to the sum of all current flowing out of the node. In other words, the current through both legs of circuit breaker one is exactly the same as the current flowing through circuit breaker two. Assuming both breakers are functionally identical, there is no way to predict which one will trip first.

If you want one CB to trip before the other, they must have different electrical characteristics. Either they must have different amp ratings, or one must be "slower" than the other.

pburchett · Apr 5, 2018

Ok here is the story behind this:

My house breaker box is the power source with a 30 amp breaker feeding a subpanel in the barn which is 200 ft away. In the barn subpanel I have a welder connected to a 30 amp breaker (it came free with the subpanel so I used it for the welder). From welder to subpanel it is 20 ft, including power cord.

When I do any serious welding it trips the breaker in the subpanel but not the house. I did not know why and thought someone could explain it. Whatever the real reason I appreciate the fact that it does because I do not have to walk up to the house to reset the breaker.

K Effective · Apr 5, 2018

Well, pburchett, you must live in the Bermuda Triangle of electrical service, because that violates Murphy's Law of Electrical Service, whereby the most likely breaker to trip is ALWAYS the one less convenient to reset. :wink:

MarkB_MI · Apr 5, 2018

> My house breaker box is the power source with a 30 amp breaker feeding a subpanel in the barn which is 200 ft away. In the barn subpanel I have a welder connected to a 30 amp breaker (it came free with the subpanel so I used it for the welder). From welder to subpanel it is 20 ft, including power cord.

When sizing a breaker, it should be sized appropriately to protect the downstream wiring. Not necessarily the load device. Welders usually have thermal protection that shuts them down when they overheat; since the 30 amp breaker is tripping before the welder's thermal protection, it sounds like you need bigger circuit breakers. And if the 30 breaker in your house is protecting a 10 gauge feeder to the barn, that cable is way undersized for running most welders 200 feet away from electrical service.

teddy52food · Apr 5, 2018

Try switching the two & see what happens. Maybe one is weaker than the other.

John T · Apr 5, 2018

Which breaker would trip first (1 or 2) if the load on the motor caused it to draw more than 30 amps

Whichever has the time and current curve that would cause it to trip faster then the other one. Subject to brand and age and electrical design one could easily trip before the other one.

John T Retired Electrical Engineer

Bob · Apr 5, 2018

(quoted from post at 14:09:34 04/05/18) Example:

A modern day electric dry is a 4 wire circuit. They separate the neutral from ground as there may be a 120VAC digital display.

Hay maker 2, in the original post, the O.P wrote "motor".
He didn't write "dryer", or (for that matter) welder.

Kansasredneck · Apr 5, 2018

At 200 ft you have considerable voltage drop that would not be sensed by the breaker in your house. Hopefully you have larger wire than #10 which is rated for 30 amps, but given the distance, it would be derated to about 20 amps.

Ken Macfarlane · Apr 6, 2018

Breakers don't sense voltage, only amperage, which is the same everywhere in the circuit for our intents here.

buickanddeere · Apr 6, 2018

Depends on the breaker specs and what they are set to. There is instantaneous and short term which are short circuit protection. Then there is long term trip settings which are thermal protection. Also there is the breaker’s interrupting capacity which maybe 4KA, 10KA, 50KA or what ever you chose to pay for . Sometimes breakers blowup because they had more short circuit current through them than what they were rated for .
Then the trip curves matter . I tried to tell the company engineers that they were screwing up the set points on the new 600 and 1200 amp breakers in the new buss gear . I pointed to the factory instructions on for the” how many times “ the rated current of the downstream MCC panels for instantaneous, short term and ground protection . The engineers decided to be “safer” and set the breakers to just over the normal operating current of the combined loads .
Some of you know where this is going .
Months later a crew was installing a new 2Kw 600V three phase heater in the control room . They did not megger the circuit prior to energization and failed to find the shorted heating element . They closed the 15amp branch circuit breaker which exceeded the instantaneous protective circuit set point on a 1200amp supply breaker . Which took down half of the facility. The 15amp breaker never tripped as the current was interrupted upstream before enough cycles occurred to trip the 15amp breaker .
Needless to say I spent a lot of time rubbing it in and saying “I told you so”.

RJKeith · Apr 6, 2018

You are correct John, it depends upon the tolerances in the individual breakers. Other factors that might affect it though are ambient temperature.

John T · Apr 6, 2018

Yeppers, even if the time and current temps were identical when manufactured (Never exactly perfect and the same) age and use and as you say temperature is gonna determine which will trip first.

John T

pat sublett · Apr 6, 2018

It is un-predictable in the real world.

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