brybedo

1. I'm working in a salon located in a mall. I have a 3P 100A Main breaker at the switch gear supplying the salon sub panel. The salon panel is a SUB panel with no factory hardware for a main breaker to be installed. However, the 2 OT feeders from the switch gear are connected to a 100 A breaker landed on the bus bars, acting as an additional means of disconnect for the sub panel. Can I eliminate the breaker in the sub panel?

2. The salon service is 3P 277/480V and each phase amps out to approximately 36Amps. My A phase sub feed at the sub panel side is almost too hot to touch while the other two phases feel slightly warm. The 100 Amp sub panel breaker keeps tripping. The temperatures of the sub feed at the switch gear side are slightly warm. Why is my A phase sub feed HOT at the sub panel and fine at the load side of my main breaker at the switch gear?

3. In this scenario, do I add up the total Load across each phase to determine total amps drawn? or is each phase rated at 100Amps?

Thanks a bunch....

480sparky

Not sure what you mean by '2 OT feeders', but sounds like you've got a panel that is using a 3-pole 100a breaker that is back-fed. One question: Why do you want to remove it? There can't be too many more loads you need to put on it, considering it's 480/277.

Sounds like a loose connection in the subpanel, either breaker-to-busbar or wire-to-breaker. Or possibly a defective breaker.

Each phase will be rated to carry 100 amps.

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brybedo

1. The breaker in the sub keeps tripping (it is back fed). I was able to get a 3/4 turn on the breaker lug and it hasn't tripped in a few days- I hope that solves the heat issue. The 2 OTT(?) conductors were also stripped f a few strands to fit the breaker lugs.

I understand that each phase of the breaker is rated for 100 AMPs. Can each phase carry up to 100 amps at the same time? or do I add the amperage from the three phases to get my total ampacity?

Thank you very much by the way.

480sparky

No matter how you look at it, it's still only a 100amp panel. Amperage flowing through one phase will be flowing back out through another phase or the neutral. You can't add the 3 100's together and say it's a 300amp panel.

I still don't understand what you mean by 2 OT / OTT.

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brybedo

...actually the panel is rated for 225 Amps and back fed by the 100A breaker. My concern was that each phase is drawing 36 Amps (totalling 108 Amps if you add 'em together). I still have a tough time thinking I could pull Amp readings close to 100 Amps per phase if this panel was maxxed out and still be within tolerable working limits. Could I have for example: 40 Amps on Phase A, 60 on Phase B, and 50 on Phase C and be fine?

Thanks again.

goose134

The closer you are to a perfectly balanced load the better. 36A on each leg sounds great. Now if you start getting numbers that are not even close, then you may have to re-evaluate your loads.

brybedo

Thanks guys.

Roger123

Bry,

If I'm understanding your question right, convert your loads to VA (watts) and then divide by 480 x 1.732 to find your total draw in amperes. Example:
A load of 40amps @ 277V = 11,080VA
60amps @ 480V= 28,800VA
50amps @ 277V= 13,850VA

Total 53,730VA/(480 x 1.732) = 64.65amps each phase.

frmRUwLV

What are 2 OT / OTT?? I am curious also?

LawnGuyLandSparky

Just because the disco in the switchgear reads 100a, and the subpanel reads 225a, doesn't mean that is the size of the service. It's quite possible every store on the strip is connected to a 100a disco, but each store only has it's feeder sized for whatever the original load was going to be.

What is the guage of the actual wire feeding the salon from the main disco?

brian john

Each phase of this panel are sized for 100 amps so this panel can handle 100 amps per phase/leg or a total of 300 amps, this assumes the conductors are sized properly and the loads are not continuous duty.

When you have a heat issue why not check for the source of the heat in lieu of just randomly tightening connections (which generally works but can miss major connection problems).

When there is a heat issue on any electrical conductor and the heat is not an overload issue, it is due to a high resistance connection. Try to locate and isolate the high resistance issue. In order to do this all you need is a decent multimeter, use the millivolt setting and measure across the poles of the equipment in question.

Fall of Potential test (FOP)

With any electrical procedures that involve making measurements with energized conductors care must be taken and proper PPE must be worn

Fall of potential is a measurement of voltage drop (VD), in a circuit that is the voltage lost to heating due to resistance in the circuit. To perform this test one measures voltage from the line side connections to the load side connections, the readings are typically in the millivolts range and require load.

For example with a 3-phase Circuit Breaker that is nuisance tripping, measure current (balance loads are beneficial but not necessary), then measure from the line bus to the load conductors (if bare conductor is exposed), for all three phases. If one phase has a higher that average millivolts reading, then try to isolate this issue. Measure from line bus to the Circuit Breaker bus stab, if all 3-phases have millivolts readings that are the same (or close to the same) measure from the bus stab to the load side conductor termination connector (through the Circuit Breaker), if all these readings are close, next take measurements from the load side termination connector to the load conductor. In this case you can determine if it is a line bus connection issue, bad Circuit Breaker or a load conductor termination issue. Any accessible portion of the device can be tested in this method.

brybedo

Thanks everybody. brian john answered this one beautifully.

My hillbilly slang for 2/0 is 2 OT

480sparky

You do not have 300amps if there's 100amps going through each of the 3 phases. It is still 100 amps.

Take a standard, 120-volt 20amp circuit. You do not have a 40amp circuit if you have 20amps flowing through the ungrounded (hot) and the SAME 20 amps flowing through the grounded (neutral). You are simply measuring the same amount of current, just on a different portion of the circuit.

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brybedo

Once again, the Amp Readings from each phase were: A=36A, B=36A, C=36A giving me 108Amps if I add the amperage from all three phases together. What I'm understanding is that that IS NOT the correct way to determine the total Amps being drawn through my 3P 100A breaker and that the total Amps being drawn on this system are 36.03Amps

What I'm now understanding is that each phase can hypothetically draw up to 100Amps at the same time- as in Amp readings of each phase are A=100A, B=100A, C=100A

Using the Formula Roger presented, I'm left with a total of 100.07Amps.

Im still a wee bit confused though. If I'm hypothetically drawing 100Amps on my A Phase, 100 Amps on my B Phase and 100 Amps on my C Phase laymans logic would tell me I'm drawing 300AMPs through my electrical system.

With this scenario and new found knowledge, am I correct in saying the hypothetical electrical system is drawig a total of 100.07 Amps?

brian john

480 now use simple math for single phase loads 100+100+100=300 amps (total available current), Now this is not how electricians normally think of a 100 amp circuit breaker BUT in reality for single phase loads it is 300 amps.

480sparky

But a three-phase system does not work on 'simple math'. If it did, why not 'add' the neutral amperage into the equation and claim you have 400 amps? The amperage flowing through the 'A' phase will also be flowing back out either the 'B' phase, 'C' phase or neutral.

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brybedo

Got it. thats a sweet little diagram 480. Thanx for all the help guys.

bigredc222

The fact that some of the outer strands are trimmed can cause a heat issue if you where drawing more amps. If your sub is a couple hundred feet away that might be causing some trouble. You could have a bad breaker. It's happened to me with a 100 amp breaker also, it kept tripping. I had a smaller breaker, like a 50 amp. Put it in and it held. I got a new 100 and the problem was solved

brian john

480 you are missing a simple point here...FACT each leg of a 3 phase 4 wire wye system 100 amp circuit breaker can and DOES carry 100 amps, discounting continuous loads, discounting the fact of the capabilities of a circuit breaker to carry more than it's rated current due to the thermal magmnetic design.

On single phase loads can this circuit breaker can carry 100 amps per phase YES or NO?

And why not count the neutral, now you are missing the math, In a 3 phase 4-wire wye distribution system with 100 amps balanced loads the neutral current is????????????????????????????????Um not 100 amps more like ZERO "0"

480sparky

Yes, BUT it is the SAME 100 amps. The 100 amps that flows through the A phase will flow back out through the B & C phases. If it didn't it would flow back through the neutral.

Put it this way: If you have 100amps going through the A phase only, and there are no loads on the B & C phases, there will be 100 amps on the neutral. Do you have a 200 amps of current flowing?

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