Twinning Carrier furnaces--electrical question
Electrical question about twinned Carrier gas furnaces with A/C coils at my church. Furnaces were installed some years ago, with Carrier twinning kit. Twinning kit was installed correctly as far as I can tell, but the furnaces were not given separate 15 amp circuits as required. They were tied together on one 15 amp circuit, and the breaker is tripping, of course. So we need to provide another 15 amp circuit and give each furnace its own circuit.
The twinning kit instructions say both circuits must be on the same phase leg so that the 24VAC transformers in both furnaces are in phase. I understand this. My question comes from the fact that the new 15 amp circuit we're planning to install has to come from a different sub-panel. I know we can get the same phase leg in this other panel so that both circuits are in phase, but I don't know if it's OK to have the power supplies coming from different panels. The actual text of the twinning kit manual says "each breaker should be located directly across from each other in the service panel in order to be on the same phase leg."
So I understand the need to be on the same phase leg. But do the breakers actually have to be in the same panel as well, and if so, why? Is there some slight voltage difference possible between panels that will cause some sort of problem between the control boards in the furnaces?
Thanks.
Comments
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Can you move a circuit into the other panel to get the space you need?
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@ratio, it would be difficult. If both breakers have to be on the same panel, we'll probably have to abandon the existing 15 amp furnace circuit and run two new circuits off the other panel, which is actually closer to the furnace room and will have shorter wiring runs with easier access.
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Good heavens. Then do it that way There is no really good reason not to run the two circuits off separate subpanels — provided you do in fact get the same phase leg from both panels.
When you install the circuits this summer.
But — how do you ensure that in some subsequent work someone doesn't swtich the phase legs on one or the other subpanel? Without realising that that will affect the furnaces? You can't. Simple as that. And Murphy says that someone will at some point.
So. Do it the easy way — which is also the correct way — and abandon that existing circuit and run two new circuits off the other panel.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England0 -
@Jamie Hall, thanks for your thoughts. Let me make sure I understand what you said.
" There is no really good reason not to run the two circuits off separate subpanels — provided you do in fact get the same phase leg from both panels."
So you are confirming that it's OK to run these furnaces on separate sub panels? Because the Carrier instructions are written assuming (requiring?) the breakers to be in the same panel, as well as on the same phase leg. I can imagine a scenario in which (say because of different different distances from the main panel and different loads in the sub panels) that the voltages in the furnace circuits are slightly different, and could this cause a problem between the two control boards, where the two 24VAC control boards are tied together?
I'd prefer to do it that way because of cost. We're a poor church paying for a licensed electrician to do the work, so it's not my free labor. So we'd rather only have to pay for one new circuit vs two.
I understand your point that, in future, that "two panel" system could get screwed up if someone changed the phase legs in one panel. OTOH, if you saw what this 100-year-old church wiring looked like a few years ago, you'd need a stiff drink. There was a 100-year-old porcelain fuse panel with both the hot and neutrals fused, which is no longer allowed because if the neutral fuse blew, the hot was still hot, and unwitting people would get shocked/killed working on circuits they thought were no longer hot. (That relic got replaced with a proper breaker panel last fall).
Point being that the possibility of future problems if a phase leg got switched pales in comparison to some of the other possible problems that still are on our "to fix" list…with a very limited budget.
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They should come out of the same panel. Just re arrange a circuit or two so you can get the two breakers opposite each other.
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If the wire on the existing 15 amp CB is #12 gauge then you could install a 20 amp CB in the existing panel .
This probably worked for the first few years but the CB may have gotten weak from 2 blower motors starting current draw.
If the furnace must absolutely have 15 amp protection, then each could have a "SSU" with 15 amp fustat installed. But still on your now 20 amp circuit.
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@EBEBRATT-Ed OK, thanks Ed. Am I right in guessing that the concern is the potential for slightly different voltages that could cause (linked) control board issues if the power comes from different panels?
@JUGHNE , It was worse than that. The circuit has old 14 gauge wire that until last fall was powered from a 100-year-old porcelain fuse panel. It probably had a 15 amp fuse originally, but someone had the bright idea to up it to a 20 amp fuse. Even the 20 amp fuse blew occasionally. I just put a clamp ammeter on the circuit and found both furnaces are drawing 22 amps combined. Fortunately the building did not catch fire before we replaced the old fuse panel with a new breaker box last fall. At that point a 15 amp breaker was installed, but the old wiring remained. So the electrician (correctly) will not install a 20 amp breaker there. So if we have to have both furnaces on the same panel, we'll just run two new 15 amp circuits from the other panel that's closer and easier to access.
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what about a twin mini 15,
takes the space of the single pole breaker, but is divided into 2 switched/protected poles, on the same phase, in the same single pole space,
known to beat dead horses0 -
The twinned mini is a very good idea — and is inherently all on the same phase.
However. If what I am reading between the lines here… you may be a church with very little money. So is the one I care for, so I know what you are talking about. However, if something electrical goes wrong, you will not only be a church with very little money, but a church with no church.
Now. There is nothing wrong with "porcelain" fuses in and of themselves. If the fuse boxes have been retrofitted with restrictors so that only the correct fuse rating can be installed, then they are very good protection indeed. What does need to be checked is the quality of the wires. Sometimes hundred year old wire is sound, if it was high quality insulation and steel BX and hasn't been allowed to get wet. However, if it is in conduit, or has paper or cloth insulation, you should either replace it — or install a circuit breaker box with arc fault protection breakers.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England0 -
The problem isn't lack of space in the panel that now has the one 15 amp circuit running both furnaces. The problem is that the panel is on a different floor (upstairs vs basement) in a very old building where it will be very difficult to fish a new wire from the furnaces to that panel. So if both breakers need to be on the same panel, it will be easier/faster to run two new circuits (instead of just one new circuit) from a different panel, which happens to be in the adjacent room to the furnaces and has plenty of space for two new breakers.
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Thanks, Jamie. The old fuse panel has been replaced with a new breaker panel, so the only "legacy" part is the old wiring, which still seems in good condition, and our (very competent) electrician had no concern reusing it when installing the new panel last fall (yes, it's the old cloth-type insulation in BX).
But I agree safety is job #1. When I inspected the wiring, I found that the newer 14 AWG wiring near the furnaces had been wire-nutted to the "old" wiring coming from the breaker panel, and one of those wire-nut junctions had overheated at some point (the white neutral wire had been toasted to black). When I undid the wire nut, I found a lot of corrosion on the copper wire ends, so high resistance due to corrosion probably heated that junction badly at some point. I cleaned both wire ends and installed a new wire nut for now, but the breaker is turned off and will stay off until this whole mess gets fixed by the electrician.
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L1 coming into the building is L1 no mater what sub panel!
Your volt meter can confirm.
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I understand phasing. But this is not about phasing. The Carrier instructions say the breakers should be as near as possible to each other on the same panel, and I'm trying to understand if this is a hard requirement or not. For example, I can imagine different loads on different panels causing mismatched voltage transients that could cause the 24VAC transformers to have different output transients, possibly making problems between the twinned control boards, and no one has yet definitively told me that this is or is not a potential issue. @EBEBRATT-Ed has come closest by saying the breakers "should" be on the same panel. But does that mean they "must" be on the same panel, and what are the possible issues if they aren't?
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it doesn’t matter.
How much of a voltage difference are you going to get from one panel to the other? Even if they’re hundreds of feet apart, you couldn’t measure the difference.0 -
Then why is Ed saying they "should" be on the same panel?
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SHOULD
Doesn’t have to be!
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Then why the "should?" I'm not trying to be argumentative. I'm trying to understand the fundamental principle that drives the "should," and no one seems to know.
Maybe Jamie's reasoning is the answer, is that if on different panels, some future change in phase legs on one panel but not the other could screw the phasing up.
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As far as someone changing phases in the future, that could easy happen even if both circuits are in the same panel. Both should be distinctly marked for future reference. Ideally both furnace breakers ought to be in same panel, simply for service and clarity but mechanically they will run fine out of different panels.
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Will the existing 15 amp circuit wire handle 20 amps? Is it 14 gauge wire or 12 gauge wire?
If you can use the same wire with a 20 amp circuit breaker in the panel, then the problem is easy peazy.
Split the 20 amp circuit in the furnace room, with a sub panel that has two 15 amp circuits, one to each furnace.
If you can't use the same wire, then you will need to run a second wire anyway. Then the 15 amp mini breaker is the answer.
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
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@jesmed1 said: The problem isn't lack of space in the panel that now has the one 15 amp circuit running both furnaces. The problem is that the panel is on a different floor (upstairs vs basement) in a very old building where it will be very difficult to fish a new wire from the furnaces to that panel. So if both breakers need to be on the same panel, it will be easier/faster to run two new circuits (instead of just one new circuit) from a different panel, which happens to be in the adjacent room to the furnaces and has plenty of space for two new breakers.
This sounds like the easy answer, but I would still use the mini breaker to ensure the same leg is used. Also a notice instructing future electricians that "These two circuits MUST be on the same phase. Do not separate from this Circuit breaker".
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
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@EdTheHeaterMan Hi Ed, thanks for your thoughts. I haven't checked the gauge of the old wiring on that circuit, but our electrician is conservative and doesn't want to put more than a 15 amp breaker on it. In reality, that old wire has been carrying 22 amps since the twin furnaces were installed in 2010 (?) with no apparent ill effects. But it's his license and I can't force him to do anything against his better judgment.
I do think your idea is a good one and would probably work. I've measured the draw of both furnaces at 22 amps total with a clamp ammeter, and that might be low enough that a 20 amp breaker wouldn't trip. The hangup is getting him to agree to put a 20 amp breaker on that wire. Maybe I can caliper that old wire gauge, show him the measurement and convince him it's still OK for 22 amps. Will give it a try, thanks.
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measures are taken to insure that Doesent happen. .
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breakers Witt trip within 80% of rated amperage.
get that breaker changed and 2 circuits run0 -
If you're going to have to run one circuit from the new panel, it's a small additional cost to run another at the same time and get it done right.
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Can we please make sure that whatever is done is done to code? Whether it's the code which prevailed when the installation was done, or the current code, one thing hasn't changed: a 14 gauge wire takes a 15 ampere breaker or fuse. Now, if it's a problem with drawing excess current for a short duration, it is permissible to provide a slow blow fuse — or, assuming you could find such an animal in that size, a slow blow breaker. The slow blow fuse is easy to get.
However, it's still 14 gauge, 15 amp protection. No exceptions.
Your comment above raises more concerns for me, while we are at it. If that was in fact a 15 amp breaker on that line, and it didn't trip at 22 amps, the breaker is not behaving correctly and must be replaced. Worse, if that 22 amps was for any length of time — not just a motor starting peak — you have been consistently overheating that wire. That damages the insulation which, over time, will break down — it's just a matter of when, not if — and the damage has already been done (if it's a motor starting peak only, then the breaker is still junk, but the wire may be OK if the running current is less than the rating) and the entire circuit MUST be replaced.
You say your electrician is conservative. He's also right. So are the code ratings. For a reason: they are there to protect you and your property.
So as I said… please do it right and to code.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England0 -
@Jamie Hall Yes, we use probably the most-respected electrical company in our Boston suburb, and they do excellent work, always to code. I would never advocate doing any electrical work not to code, and they certainly would not listen to me even if I did so unwittingly. And I've already pointed out to them the deficiencies in the existing wiring, including a burnt wire nut junction.
Re your comment about circuit breaker behavior, that is another rabbit hole. In researching this particular Siemens 15 amp breaker, I was surprised to find that the time-current curve (TCC) of the breaker shows that it will allow 22 amps for about 5 minutes before it trips thermally. Which agrees closely with observed behavior of our breaker in this case. So the breaker is behaving as it should and tripping after about 5 minutes. I learned a lot about breaker behavior from the first answer on this Quora thread. You may already know all this, but I'll post the link for anyone interested.
https://www.quora.com/How-many-amps-does-it-take-to-trip-a-15-amp-breaker
The reason the furnaces worked (most of the time) in the past is that previously the circuit had been on an old fuse panel and (improperly) fused at 20 amps before I became involved. But when we upgraded to a new breaker panel, the new 15 amp breaker began (correctly) tripping after about 5 minutes. I was then asked to investigate, and found our two 15-amp furnaces on one 15-amp circuit.
In any event, as I've said, the electrician doesn't want to go above 15 amps on this old wire, and that will likely force us to add at least one new circuit. To code.
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I have a question! That existing wire has handled 22 amps for years according to @jesmed1.
"Furnaces were installed some years ago" and "I've measured the draw of both furnaces at 22 amps total with a clamp ammeter"
My question is about operating 2 furnaces simultaneously for years: Is that 22 amps of a constant nature or is that a spike of 22 amps for a short time (perhaps on start up)? Because 22 Amps of constant operation may not trip a 15 amp breaker in the beginning months of operation (although it should), but that over-amperage will eventually trip that breaker after time has taken its toll on the tripping over the years.
I believe that something has changed on those furnaces. Has there been any adjustments in the blower speed or any parts replaced? You can not have operated that system on a 15 amp breaker for "Years" at 22 amps. Something is different than it was "Years" ago.
If there is a panel nearby, then I would just start there with 12 gauge wire off of a mini double 15+15 breaker off of the same leg of the panel.
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
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@EdTheHeaterMan Yes, the 22 amp draw is a very stable steady-state condition measured over several minutes, with the blowers running at constant speed in cooling mode. I wasn't able to measure the startup transient because I had to go upstairs to reset the thermostat in order to start the furnaces running on a cooling cycle, but evidently the startup transient isn't enough to trip a 15 amp breaker. The breaker trips thermally after about 5 minutes of the 22 amp draw, and as I mentioned, that seems to be the correct trip time for 22 amps on this particular breaker.
What has changed is that until last fall, the furnace circuit was on an old fuse panel that someone had improperly installed a 20 amp fuse on. That 20 amp fuse was just enough to allow the furnaces to run with only an occasional fuse blow. (Again, this was before I got involved, so I had nothing to do with that over-fusing. And I was the one who told church management the fuse panel was dangerous and needed to be replaced with a modern breaker panel.)
So last fall the old fuse panel got replaced with a breaker panel, at which time the 20 amp fuse was replaced with a 15 amp breaker. Then during this past heating season, the furnaces ran their heating cycles with only an occasional breaker trip. Since the church members had already been accustomed to changing the previous 20 amp fuse occasionally, they shrugged off the occasional 15 amp breaker trip this past winter as "business as usual."
However, when the cooling season began a week or two ago, the breaker tripped repeatedly within a few minutes of furnace (blower for the AC coil) startup, and the furnaces would never run long enough even to begin cooling the building. That's the point when I was asked to investigate, and found both furnaces wired onto one circuit.
I assume the furnaces worked this past winter in heating mode with only a few breaker trips because the furnaces were drawing fewer amps in heating mode than in cooling mode. Maybe the controller maxes out the blower speed in cooling mode, drawing more amps than in heating mode.
Whatever the reason, the fact that the breaker rarely tripped this past winter in heating mode leads me to conclude that the steady-state 22 amps I measured in cooling mode, which causes the breaker to trip repeatably in about 5 minutes, is the "worst-case" steady state draw under normal operating conditions (excluding startup transient which doesn't seem to be a problem.) Also, I did check the blower squirrel cages and they spin freely, and there are no clogged filters, etc, that would cause the blower motors to draw more current than normal. So I am confident the 22 amps is normal.
Yes, there is a panel nearby and I will suggest the single 12-gauge wire with a mini double 15+15 to the electrician. Thanks for that good idea.
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Makes sense now. The cooling speed on the blower motors are drawing more amperage than the heating speed.
Still say that your best option is the mini 15+15 breaker from the closer panel is your permanent fix.
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
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Should (in this case) is just common sense. You have two furnaces twinned together. Twinned together means they have some wiring in common on the low voltage side at least.
Some may mistakenly treat both furnaces as one appliance.
It just makes sense to have two circuit breakers in the same location and clearly marked. I don't believe it is a code issue you can probably feed one furnace from a panel in a basement and the other from a panel on the third floor as long as everything is marked.
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@EBEBRATT-Ed Thanks for the clarification. I discussed this at church today with "management," and they asked me to have the electrician price the job both ways. If it's not that much more costly to install two new circuits vs one, that's probably what they'll opt for.
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One thing that could have changed in current draw for the furnaces is if the run capacitors are dropping off in their microfarad values they may increase current draw.
Even if low values they will still let the motor run, but at higher draw.
These should be tested whenever there is any HVAC service done….easy to overlook.
IIRC, code requires a GFCI outlet at or near the furnaces. I just changed out my church twins that had been in the attic since 1969. Yes, hole in heat exchanger. Very heavy Lennox modules.
I was glad to have the newer version of low voltage twining control, although had to move CB to get the phasing matching.
There should be labeling on both the panel and furnaces indicating that: there could be 2 sources of power for the furnaces and to keep the furnace home runs on the same phase.
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@JUGHNE Yes, we have a GFCI near the furnaces, and I wondered why. I guess "because code."
I am not impressed with our HVAC company. These are the guys who installed our Sunday School furnace with an undersized return plenum, then failed to measure static, then couldn't figure out which safety switch was tripping, then couldn't figure out why it was overheating, then said our ductwork was to blame, then finally admitted that it was their undersized return plenum that was at fault after I measured the static at different points in the system and proved it was their plenum, then wanted to charge us to fix their mistake.
That's a long way of saying I'd be totally surprised if they tested the run capacitors. But with each furnace drawing 11 amps while being rated at 15 amps, that seems OK to me.
We will label everything clearly as you say for no surprises.
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