Measured amperage to 240V subpanel


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Old 12-10-07, 06:28 AM
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Measured amperage to 240V subpanel

Hi,

I am trying to determine if my subpanel is overloaded or not.
My main breaker box contains a 50Amp double pole breaker feeding a subpanel right next to it.

I have a clamp style amp meter which I am using to measure the current on one leg of the service to the subpanel at a time.

If I turn on all the major appliances in the subpanel and measure the amperage on one leg of the power feed then the meter is reading 75 Amps.

I would have thought that the 50Amp breaker in the main box would have tripped, but it does not.

My question is should the 50 Amp breaker have tripped, or not? I am wondering if a 50 Amp double pole breaker should trip at a measured amperage on each leg of 50Amps or 100Amps?

Thanks.
Andrew
 
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Old 12-10-07, 06:47 AM
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What major appliances do you have on a subpanel that are drawing 75 amps?

In theory , yes it should have tripped.

Are you sure the ammeter is on the correct range , and you arent measuring "7.5"
 
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Old 12-10-07, 07:04 AM
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The subpanel is feeding a well pump, a water pressure booster pump, pool equipment and a shop which has a glass kiln in it.

The meter only has an AC current measurement setting. I believe it is auto ranging. It is an Extech 830 meter:
http://www.extech.com/instrument/pro...s/EX800_UM.pdf

So why would the breaker not trip? Could it have something to do with only having 3 feet of wire to the subpanel and the next breaker in series?
 
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Old 12-10-07, 07:15 AM
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Yep, that'll do it.....

What kind of subpanel is this? Explain "Next Breaker wired in series" Are you using a "Backfed Breaker"?

If the wiring is correct, I would assume that you have a defective breaker.
 
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Old 12-10-07, 07:24 AM
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There are no special breakers involved.

The main panel contains a 50 amp double pole breaker which feeds a subpanel which is right next to the main panel. In the subpanel is a 30 amp double pole breaker feeding the kiln, a 40 amp double pole breaker feeding the pool equipment and a 40 amp double pole breaker feeding another subpanel down in the well pump house.

So when I said the breakers are in series I meant that each appliance in the subpane is at least behind two breakers. The one in the subpanel and the 50 amp one in the main panel.

Thanks for the help.
Andrew
 
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Old 12-10-07, 07:36 AM
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Ok----

"Each appliance goes thru its own breaker and the main"--2 breakers---I'll be alright in the morning....

The likelyhood of all that stuff running at once is slim, but 75 amps should have tripped the breaker.

Time for "Whole house load calcs".....If you are actually pulling 75 amps out of the sub, its time to upgrade the breaker, And of course the feeding wire to the sub. Just ensure your Main service can support the change.
 
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Old 12-10-07, 07:01 PM
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How long did you have the 75 amp load on?
If it was only for a short time, the thermal in the breaker may not have had time to heat up and trip.
 
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Old 12-11-07, 08:30 AM
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A 75A load should trip a 50A breaker _eventually_.

Here is a link for a common type of breaker, showing its 'time-current characteristic curve'. This is a graph of current level versus time to trip. Looking at the curve, you will see that the breaker responds more rapidly to higher overloads, but always takes some time to respond.

This 'inverse time' characteristic is essential for normal operation of your electrical system; many loads draw a large multiple of their normal operating current to start, and these loads would trip breakers if the breakers responded instantaneously.

The 'curve' is actually a band, showing the allowed manufacturing tolerance.

What this curve shows is that at a 50% overload, the breaker will delay at least 40 seconds prior to tripping, and may take up to 400 seconds to trip.

http://www2.sea.siemens.com/NR/rdonl...570A_2Pole.pdf

That said, you should either perform a proper load calculation on this panel, or get a recording amp meter and just see if over time the load exceeds its rating. Because of the time-current curve and tolerance, you could have an excessively loaded circuit without the breaker tripping. By excessive I mean 'not necessarily unsafe, but subject to higher than desired temperatures and reduced service life'. The _continuous_ load on a 50A circuit should be less than 40A.

-Jon
 
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Old 12-14-07, 10:55 AM
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The load was fluctuating between 68-75Amps for a period of 15 minutes. The load would fluctuate due to the water booster pump kicking on every few minutes to pressurize the system. I had a water hose on full at the time of the test.

I will try and re-arrange the panels to swap some of the heavy hitter circuits to the main panel and the lower current circuits to the subpanel.

Thanks.
Andrew
 
 

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