HVAC drawing power on parallel wires of different sizes?????

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rt66electric

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Oklahoma
Went on a trouble call.

200 amp main tripping on a all electric house.

Checked for the standard loose connections and signs of overheating.

After grilling the customer about usage and timing of tripping, I decided that it was probably a weak breaker? or overtaxed breaker tripped by (2) hot water tanks, well pump, well pump, well pump-heater, bath HVL, aircompressor, AND.. everything in the house on AND... 70 amps of electric heat,.

I took amp readings and found ........

The outdoor heatpump/resistance backup heat package unit is fed by a 2P 60 amp breaker and a 2P 30 amp breaker.

When the unit is on... the 2p 60 draws 45 amp ......and........ the 2p 30 draws 25 amp for a total of 70 amps....

THE BIG KICKER.... If I turn off the 2P 60 the 2p30 jumps to ---70 AMPS---

If I reverse and turn the 2p 60 on and the 2p30 off the load jumps to ----70 amps----

It is as if the two circuits are combined inside the unit and draws upon parallel wires of different sizes. This does not seem right to me.

I related this info and he understood the conflict but had no basis for reference to make a decision. He called the HVAC company and relayed the info.

The HVAC replied that this was Normal. and the panel box breakes are designed to trip if necessary..IF anything goes wrong.

I allways thought the circuits were independant from each other. The compressor/heat pump was on the 2p 30 and the dry resistance heat was on a seperate 60 amp relay.

Anybody know the proper set-up? :blink: :?
 
When you have one of the breakers open, do you still have voltage on the load side of it?
 
Sometimes those units can be set up with a supply side jumper that allows for a single feed to the unit, or you have the option of removing the jumper and running two circuits.

But if the jumper is in place and the two circuits are in parallel I find it a little hard to believe the amps pulled by each set is not divided closer to 50-50. The larger conductor should carry more because it should have less resistance, but unless a really long run probably not so much less that your amps are not somewhere near 50-50 on each conductor.
 
Is the service sized correctly? Around here anytime I see two water heaters, doubled up HVAC etc, I know I'm in a Toll Bros. Mc Mansion with a 320A service. I don't think you can do it on less.
 
kwired said:
Sometimes those units can be set up with a supply side jumper that allows for a single feed to the unit, or you have the option of removing the jumper and running two circuits.

But if the jumper is in place and the two circuits are in parallel I find it a little hard to believe the amps pulled by each set is not divided closer to 50-50. The larger conductor should carry more because it should have less resistance, but unless a really long run probably not so much less that your amps are not somewhere near 50-50 on each conductor.
Click to expand...
If the two paths have a common point at the panel bus and a common point (through a solid jumper or connection to a common terminal, the difference in current between the two paths will be ratio of the resistance in the wires (however short) and the resistance across the thermal element in the breaker.
For what it is worth, I would expect that the resistance of the thermal element will be greater in the lower current breaker, and the resistance of equally conservatively selected wires should be inversely proportional to the expected or rated current.
I am surprised that the division is as precise as it is, but not to the point of disbelief.


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GoldDigger said:
I am surprised that the division is as precise as it is, but not to the point of disbelief.
Click to expand...
That is what has got my attention is just how precise the division is, nearly what it would be if there was no parallel connection. We are probably looking at 6AWG and 10 AWG conductors. We don't know how long they are, and I guess that shouldn't matter as long as they are both about same length. But I guess after looking at Table 8 DC resistance of those two sizes there is more difference than I maybe realized at first in the resistance between those two conductor sizes. And the division is closer to 50-50 than I first thought, I first thought maybe around 40-60 wouldn't be out of the question but didn't run any figures, turns out we have a 36-64 division of current. Change the 10 AWG to 8 AWG and we probably will go over 40% on the smaller conductor.
 
more details

more details

The 60amp breaker has #6 wire
the 30 amp breaker has #10 wire

The HVAC unit is close to the panel maybe 25-30 ft total lenght of wire.
The pre-fab house ? actually has emt and thhn wiring methods.
The house is approx 30 years old
Main panel G.E. completely stuffed thin-twined 20/40
Main breaker G.E. type CC??( same as a T.E.D. 3pole, but only 2 pole)

Any thing else??
 
rt66electric said:
The 60amp breaker has #6 wire
the 30 amp breaker has #10 wire

The HVAC unit is close to the panel maybe 25-30 ft total lenght of wire.
The pre-fab house ? actually has emt and thhn wiring methods.
The house is approx 30 years old
Main panel G.E. completely stuffed thin-twined 20/40
Main breaker G.E. type CC??( same as a T.E.D. 3pole, but only 2 pole)

Any thing else??
Click to expand...
As my number one suspicion, I'm sticking with check the HVAC end of the circuit(s) and make sure there is no jumper between the two circuits, especially a factory jumper on the supply side of unit breakers. Next step may be to make sure some service person didn't modify anything and create a path between both circuits

Turn both circuits off as well as breakers in the unit. You shouldn't have any continuity between the two circuits, if you do you need to find out where this continuity is coming from.
 
kwired said:
As my number one suspicion, I'm sticking with check the HVAC end of the circuit(s) and make sure there is no jumper between the two circuits, especially a factory jumper on the supply side of unit breakers. Next step may be to make sure some service person didn't modify anything and create a path between both circuits

Turn both circuits off as well as breakers in the unit. You shouldn't have any continuity between the two circuits, if you do you need to find out where this continuity is coming from.
Click to expand...



Wouldn't it be easier to open one breaker and see if there's line voltage on the load side?
 
Electric Fan Coil

Electric Fan Coil

Based on the readings changing when you are disengaging the heat pack breakers, that unit is raising red flags if all sequencers are calling when this happens.
That is FULL HEAT call W1. W2 from control.
The way those systems are set up typically say by Carrier is that the fan drive and control transformer will run off one of the feeds which supplies your control power of course to ALL heat sequencers, or in older units a black box type sequencer which is what brings the elements on line
It sounds like the on board wiring is compromised, likely from an improper service procedure in the past.
I have seen all manner of wiring being messed up on electric heat units, one with an element that was running on the off cycle and cycling on high limit without burning out. The customer had a power bill over $300.00.
If you have a wiring diagram, or even without they are pretty straight forward to put right. 24V at any sequencer brings that element on the line.
The fan motor should have its own sequencer contact or relay.

All the best.
 
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