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I have a three phase panel fed from a three phase 100A breaker on another panel.

I found the 100A breaker tripped on the main panel, but none of the breakers in this panel were tripped. My understanding is limited but if I understand correctly, the four breakers in this panel do not add up to above 100 on any phase.

electrical panel showing four breakers and no main breaker. Three two phase breakers: 20A, 30A, 40A and one single phase 20A.

The breakers are installed as follows:

Upper left: 30A 2 phase
Lower left: 40A 2 phase
Upper right: 20A 2 phase
Lower right: 20A 1 phase

Total breaker limits per phase:

  1. 30+20+40=90
  2. 30+20=50
  3. 40+20=60

What am I missing or misunderstanding? How could none of these trip while the 100A tripped? If this is impossible, what should I look for in terms of failures that might allow this situation to occur (i.e., bad breakers, weird loads, etc)

The main panel has four wires coming in, and the subpanel is connected directly to the white wire shown with two wires, one being smaller than the other which I assume is ground. The other three wires are connected to the 100A breaker.

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    Depending on the type of load and breakers I have seen a inverse time breaker 3 x the size of a thermal overload class 10 trip before the smaller breaker. It only has to be overloaded on 1 leg to cause this. – Ed Beal Apr 20 '16 at 16:21
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It could have to do with the trip curves of the breakers, and/or the ambient temperature differences between panel locations.

Trip Curve

For example, let's say all the circuits on Phase A are drawing 3 times the rated current. The trip curve for these breakers, say they will trip between 10 and 30 seconds at 3 times current. So the total current on Phase A at the main panel is 270 amperes, or 2.7 times the rated current (100 A). The trip curve for the breaker in the main panel, says that it should trip between 12 and 35 seconds at 2.7 times current. If the breaker in the main panel is a bit more sensitive, you can see that it might actually trip before any of the secondary breakers.

Ambient Temperature

As well as a trip curve, circuit breakers will also have an ambient temperature curve. Breakers are designed and calibrated to operate at 40°C (104°F) ambient temperature. If the breaker is operated at an ambient temperature of -10°C to 24°C (14°F to 75°F), the breaker will be able to carry more current. However, if the breaker is operated at an ambient temperature between 41°C to 60°C (106°F to 140°F), it will carry less current and could lead to nuisance tripping.

For example, if the 100 ampere breaker is operating at 140°F ambient, you might find that it actually trips at only 85 amperes.

High Altitude

When breakers are installed at higher altitudes, the current has to be adjusted due to the reduced cooling effects of the thinner air. According to the documentation I could find, the adjustment multipliers are as follows:

  • 0 - 6600 ft. (0 - 2011 m) -> 1
  • 6600 - 8500 ft. (2011 - 2591 m) -> 0.99
  • 8500 - 13000 ft. (2591 - 3962 m) -> 0.96

If you were installing a 100 ampere breaker at 10,000 ft., the breaker should only be expected to carry 96 amperes (100A * 0.96 = 96A).

Though in your situation, the breakers are likely all installed at a similar altitude. So this it likely not the problem, unless the air near the main panel is significantly thinner for some other reason.

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Breakers trip on overload or short circuits. A dead short could easily trip a main breaker.

  • So a dead short on the 30A circuit might trip the main 100A breaker but not the 30A breaker? That's what I'm trying to figure out - if it was total load that caused the trip, or a problem with a single circuit. If I can trip the main breaker without tripping the smaller breakers it makes finding the problem much harder. – Adam Davis Apr 20 '16 at 16:26
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    Yes, that is entirely possible. The fault current is different from the overload current. – Speedy Petey Apr 20 '16 at 16:27
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Could be something as simple as a short between the 100A breaker and the subpanel. Have you inspected the entire length of the wire for e.g. signs of rodents chewing on insulation?

Also, your photo is unreadable and all I can tell is that you have four breakers, one of which is single-pole and I'm not clear about the others. What are the ratings and phase assignments of each breaker?

  • I've added a little list showing the breakers on each side. As far a inspection, it's been installed for less than a month now, and is completely enclosed in metal conduit from box to box. Further, it tripped while I was not present and upon reset it hasn't tripped again. – Adam Davis Apr 20 '16 at 18:50
  • I see. Yes, individual breakers don't seem to add up to 100 in any phase. It might be some combination of having the 100A breaker with actual trip current below nominal (within tolerance) and individual breakers above nominal. Or the 100A breaker is significantly hotter than the subpanel (higher temperature -> faster trip at the same multiple of trip current). Seems unlikely though (if all breakers have the same design.) Especially since even a relatively high overcurrent on one branch equates relatively low overcurrent on main breaker. Maybe condensation/dew inside conduit or boxes? – Eugene Smith Apr 20 '16 at 20:45

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