I am relocating a 3 1/2 ton A/C compressor to the opposite end of our home. What size cable will I need for a total length of 80 feet? As an alternative, at this distance would there be any advantage in installing a remote sub-panel at say, 50 feet.
My primary concern is to minimize the voltage drop at the compressor, located 80 feet from the breaker panel.
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Replies
What is the name plate rating on the compressor.
Typically going up one size for that distance should be OK.
A sub-panel as such does not help unless you are going to have other loads in that area.
No advantage to a subpanel, though you must have a disconnect box (generally unfused, but check local code) within sight of the unit.
Hi Scott-
Run six guage wire. Even if the tables say you can get away with eight guage, the six will pay for itself in energy savings due to reduction of resistance losses.
How long will the freon lines be running to this new location. Lines run longer than fifty feet will cause quite a hit to your electric bill.
I have the same sort of question. I need to run 8 gauge wire to a garage for workshop power (60'). Do I need to run 8 gauge through the garage to the other end for recepticle or can I switch to 12 gauge once I get power to the garage. Would it be better to put a small sub panel in at the garage wall, or would it be ok just to split circuits in a J box.
thanks for the help
"I have the same sort of question. I need to run 8 gauge wire to a garage for workshop power (60'). Do I need to run 8 gauge through the garage to the other end for recepticle or can I switch to 12 gauge once I get power to the garage. Would it be better to put a small sub panel in at the garage wall, or would it be ok just to split circuits in a J box."WHY do you need to run a #8. The distance is not that far that you need to upsize to a #8 for voltage loss.You are still limted to tha rating of #12, that is 20 amps (or less if derating is needed).Just what are you trying to accomplish?
Bill
thanks for your reply. I am running 220v from the panel for a 3 hp unisaw, and wiring the rest of the shop for 120v power tools. I got the wire size off of a table that said for 20 amps you needed #8 to run up to 100'. I was going to pull three hot wires, a neutral, and a ground to either a sub panel or a junction box where I would split the circuits into the table saw run, and the 120v power tool circuit. I was going to run #8 to the unisaw, and then drop down to #12 for the 120v circuit. Is that kosher?
Thanks again
oside
If the garage is detached then it would met code as you can run only one feeder or circuit.But even if it is attached it is not the best way.Run #8, 2 hots, neutral, and ground to a sub-panel. Use a 50 amp 2 pole breaker at the main.From that you can feed you unisaw, lights, dust collector, etc.
Thanks Bill a dedicated subpanel for the workshop is the best way to deal with electrical needs. I appreciate your help
Oside
"three hot wires, a neutral, and a ground" sounds like more than one circuit to me. Do you have three phae power to your house?
~Peter
Ray
I am moving the A/C coil location from the attic to the basement, which will bring the refrigerant line lengths down from well over 50 feet to around 15 feet. A new air handler, all new duct work etc will complete the work.
I was figuring on 10 ga for the air handler circuit, whiuch is coming in at around 60 feet.
Thanks
Scott
"I am moving the A/C coil location from the attic to the basement, which will bring the refrigerant line lengths down from well over 50 feet to around 15 feet. A new air handler, all new duct work etc will complete the work.I was figuring on 10 ga for the air handler circuit, whiuch is coming in at around 60 feet."The only real way to size the air handler or compressor is based on the equipment REQUIREMENTS.What does the nameplate say.
Bill,
The nameplate says "Minimum Circuit Ampacity: 23.2; Compressor: Rated Load - 17.3, Locked rotor 94.0; Fan Motor Rated Load - 1.6, Locked rotor 3.8;
For a 3% max voltage drop, the manual suggests #10, with a max wire length of 120 ft at 230 V, 1 phase. Not certain if this distance for the total distance or the total conductor length.
Moving up two sizes to #6 has been suggested -any thoughts?
Thanks Scott
What size breaker have they had this on?
The existing breaker is a 35A double pole.
this is also the recommended max from the compressor nameplate
Thanks Scott
Whatever wire you use has got to be sized for the breaker, along with everything else. I don't have a reference at hand, but I would think that 35A would pretty much require #8. That breaker seems a tad large for the load, however. Does the nameplate list the minimum breaker size too?
There is no reference to a minimum breaker size.
My expectation is that I would be able to use #6 with the existing 35A breaker
Any limits on minimum conduit size? I am not certain how hard it will be to pull that length of #6 through 3/4 inch conduit - probably have to go to 1 inch, if that is not already the requirement for #6.
thoughts?
thanks Scott
"Whatever wire you use has got to be sized for the breaker, along with everything else."That is not true.The breaker is designed to protect the wiring, not the load. Now for a GP circuit you can have any combination of loads.But for a dedicated motor or refigerator circuit the size of the load is determined by the load. And the loads have either internal overload protection or overload protection in the motor starter.The wiring has to be sized to running current.Short circuit protection can be as high as 175 % of the full load current or the branch circuit rating, whichever is higher. But if the starting current causes breaker tripping then you can go upto 225%.Now there are a bunch of conditions and options and you are also limited by the max that the manufacture allows, in this case 35 amps.
What I meant is that the ampacity of the wire has to be at least as great as the breaker rating. Based on the chart footnote (I have my old code book here now) a 35A breaker will require #8. So even though #10 could carry the load, #8 must be used or the breaker needs to be reduced to 30A or less.
I was concerned about the size of the breaker because of this.
The manufacturers specify a max breaker size based, most likely, on the UL rating tests. They have to prove to UL that the unit will "contain" a fault up to the max breaker rating (plus tolerance). (Have observed UL tests. They will distroy a unit or two or three in order to prove that it handles overcurrent situations appropriately.)
Of course, another concern is hard starting. For an AC compressor it's never a bad idea to oversize the wiring, especially with a long run. New units are better at hard starting situations than those of 20-30 years ago, but it still doesn't hurt to have a little "insurance".
The mfgr might specify a minimum or "recommended" breaker rating (larger than the rating of the unit) in order to deal with startup load. I'd be surprised if 35 is the recommended size.
What all this says is that #8 is definitely a good idea, and is in fact required if the breaker isn't changed. #6 is a touch of overkill, but certainly won't hurt. The only potential problem would be with making the terminations at the unit, if the terminals can't handle that size wire. But since a disconnect is required anyway, one can switch from #6 to #8 at the disconnect.
"What I meant is that the ampacity of the wire has to be at least as great as the breaker rating. Based on the chart footnote (I have my old code book here now)"No.That foot note referes to 240-3 (99NEC). 240-3 requires to be protected agaisnt overload based on the ampactiy ratings in 310-15 and for small conductors (14,12,10 copper) no mmore than 15, 20, 30 amps respectively.EXCEPT section (g) wiipes all of that out says that you and use the overload protection called in for specific applications and then it list them allong with the appropriate section numbers.Those includ AC and refigeration, motors and welders among others.
Well, my book is '81, so I know it's hopeless to try to look up those references. But my point remains -- the wire must be sized for the rating of the breaker, whether footnotes apply or not.
"But my point remains -- the wire must be sized for the rating of the breaker, whether footnotes apply or not."But that is not correct for motor circuitsIn general that is true, because you don't know what the load is going to be.In a dedicated motor circuit you know what the load is becasue it is fixed by that motor(s). If the motor becomes overload then the internal motor protection or starter turns off the motor. So if the wiring is sized by the motor load it can not be overloaded.The breaker is ONLY used for short circuit protection. Thus is can be sized differently that just using the wiring for sizing.This goes through the sizing for hermeticaly sealed AC units and shows how the nameplate data relates.http://www.selfhelpandmore.com/homewiringusa/2002/motor/hvac/index.htm"SPECIAL NOTE: Notice that we sized the electrical equipment by using the name plate above and installed a 3 horse power rated disconnect using a maximum amp rating of a 45 amp HACR breaker installed serving a 10 awg copper branch circuit conductor. THIS MAY SEEM WILD OR LUDICROUS BUT HANG IN THERE AND READ THE DETAILS HOW WE COME UP WITH THAT 45 AMP BREAKER ON A 10 AWG 30 AMP RATED COPPER CONDUCTOR. MAY BE INTERESTING."" These two subjects both hermetic compressors and standard electric motors are the “when otherwise allowed or mentioned” as referred to in the NEC. Chapter 4 of the NEC which includes both article 430 standard electric motors and article 440 hermetic compressors is that “when otherwise allowed or mentioned in the NEC”. The designs and even conductor ampacity in the NEC table 310-16 changes allowing different from the normally accepted in Chapters 1 through 3 of the NEC. Chapter 4 of the NEC is a different world in the NEC allowing or requiring much different electrical designs than is normally accepted in standard wiring methods found in Chapters 1 through 3 where you must have the branch circuit protective device [branch circuit fuse or breaker] on the beginning [line side] where the branch circuit gets its power from. When you are dealing with motors of almost all kinds the motor’s overload device is the device that protects that branch circuit serving a motor. This overload device is most often found or installed at the end of a branch circuit conductor at or in the motor itself. Seems like the rules tend to reverse from what is required concerning motors compared to all other types of wiring designs. The difference is the breaker or fuse is serving a motor is often times just a short circuit protection device only. The breaker or fuse is commonly serving to trip due to its interrupting rating due to short circuits only. The overloading or overheating of branch circuit conductors or windings of the motors are expected to be protected by the “overload device”"This is a more generic one on motor wiring.http://www.selfhelpandmore.com/homewiringusa/2002/motor/motordesign/motordesignshowall.htmhttp://www.iaei.org/magazine/99_d/simmons.htm
"The concept for protection where a No. 12 copper wire with an ampacity of 25 amperes is permitted to have overcurrent protection of 40 amperes is as follows: 1. The 40-ampere fuse or circuit breaker at the origination of the circuit will protect the conductors from short circuit [ungrounded (hot) conductors which fault together, line-to-line] and ground fault [ungrounded (hot) conductor(s) which fault to the equipment grounding conductor or grounded equipment]. 2. The conductor is protected from overload by the running overcurrent device usually contained in the motor controller.The combination of the two protection elements provides the overcurrent protection for safety. "
Good posts.You have patiently explained the situation, the difference between a dedicated load motor circuit and general use, the difference in roles of circuit protection, overload and short-circuit protection and how these rolls are separated in these circuits. I couldn't have done as well.I found it interesting that 100' is considered a long run and that a general rule-of-thumb has developed to bump up the conductor size. IMHO this is unfortunate. It substitutes a crude WAG for careful design consideration, calculation and knowledge. Given that the extra money to buy the increased conductor, and possibly box and conduit, size is minimal in terms of overall construction costs I have still found that in most situations the return on such investments is extremely slow. Often the AC unit or appliance will fail, often due to rust if not wear, long before the break-even point is reached. There, depending on the individual situation, may be some merit in the case of a AC unit in Florida where it can run nine months out of a year but IMHO it is often far more cost effective to spend the money on weatherstripping or improved insulation. In the Florida case a more efficient AC unit will often pay substantial dividends. Something I would like to see is builders just avoiding the obvious building errors like dark shingles, unsealed ceiling planes and ducts that leak, or soon will. How anyone can worry about saving a nickel a year running heavier lines than the numbers support and ignore the glaring waste is beyond me. I suspect that it has something to do with the builders not being the folks who will be paying the bills. But here again a friend just got himself a H2 that is a lush for gasoline. Which points out the other major factor: Some folks seem to have more money in their wallets than they know what to do with. I suspect that situation might not last.
Thanks for the compliment. I often get too focused in these kind of things and find that you come in later and make it so much clearer.Part of what is happend is that so many people have heard the #14 - 15 amps, #12 -20, #10-30 (after that they need to look at the tables) that they assume that it covers EVERYTHING. And sometimes, because of derating, it can handle less. Others more.One common one is that on tool power cords. They think that there new $200 13 amp router is defective because it has a #16 power cord.PS - Which reminds me of our "friend" Ell (or Mistress ELL as she like to call herself over on the Bob Villa forums). I think that she has left there after she had a meltdown while trying to claim that you can't having lighting on 20 amp circuits."There, depending on the individual situation, may be some merit in the case of a AC unit in Florida where it can run nine months out of a year but IMHO it is often far more cost effective to spend the money on weatherstripping or improved insulation. In the Florida case a more efficient AC unit will often pay substantial dividends."Yes, one of the things that is often overlooked is that in most cases the funds are limited to one extent or another and that funds might very well be more useful spent for something else.Happens all the time in both private and goverment.
Edited 6/12/2005 8:29 am ET by Bill Hartmann
"For a 3% max voltage drop, the manual suggests #10, with a max wire length of 120 ft at 230 V, 1 phase. Not certain if this distance for the total distance or the total conductor length."That is path lenght. Total conductor distance is twice that.#10 will work, but because of the large starting current it would not hurt to go up to #8.But you aren't going to gain much going lower.With a 23 amp load and assuming 20 hours a day run time#10 - 4.4v (drop) * 23*20 = 2.024 kwh/day lost in wire
#8 - 2.8v (drop) * 23*20 = 1.288 ""
#6 - 1.8v (drop) * 23*20 = 0.828 ""Going from 8 to 6 you would save .460 kwH/day. At 7.5 cent/Kwh that would save $1.04/month.And that is proably high in both run time and that fact that it will draw less current when the temp differences are lower.Now I don't know what area you are in so I don't now many months that you would run it.http://www.csgnetwork.com/voltagedropcalc.html
Edited 6/10/2005 8:09 pm ET by Bill Hartmann
Bill,
The voltage drop calculator is terrific - thanks for the reference
Looks as tho #8 is pretty good balance between energy savings and cost/ease of installation.
Is 3/4 conduit OK for #8?
thanks again, Scott
If you use THHN/THWN then 3/4 sch 40 PVC conduit is rated for 5 #8 and 6 if it is EMT.
I'm going to agree with Bill here and say that #8 is the right wire size for the job. You will see a payback in reduced line losses. Use a 30 amp breaker. I think upgrading one wire size is a good line to draw on large loads with significant run times. Refrigerators, HVAC (condensing unit and fan coil unit), ovens, and dehumidifiers, can all pay for the upsizing with reduced consumption. The compressor motors may last longer but that would depend on the incoming line voltage.
"Minimum Circuit Ampacity: 23.2; Compressor: Rated Load - 17.3, Locked rotor 94.0; Fan Motor Rated Load - 1.6, Locked rotor 3.8; "
They get the minimum circuit ampacity by taking 125% of the compressor rated load and adding the fan motor rated load: (17.3x1.25)+1.6=23.2. So, all you need is a 25 amp breaker but the 35 amp breaker is OK, too. However, the 35 amp breaker will require #8 wire (you could use #10 with a 25 or 30 amp breaker.)
If the total length of wire from the breaker to the A/C unit is 80 feet, using the voltage drop calculator at http://www.electrician.com/vd_calculator.html says that #10 wire will have a voltage drop of 1.9%, and #8 wire will have a voltage drop of 1.2%. #6 wire will have even less voltage drop. You want to keep the voltage drop at 3% or less, so any of these are OK.
Like Bill showed you can calculate your kilowatt-hours, look at your electric bill to see what your local utility charges per kW-h, and figure out if the extra expense of the bigger wire is worth the savings on your electric bill.
Stuart,
Thanks for the reference - lots of interesting stuff on "electricians.com"
I was hoping to get started on the conduit tonight, after the sun drops - any thoughts on 3/4 in vs 1 inch for #8?
Conduit is sooooo expensive these days
Thanks Scott
Assuming 4 wires (two hots, a neutral and a ground so you have both 120 and 240 at the A/C unit) you can use 1/2" for #10 wire or 3/4" for #8. I'd use 3/4" in either case, 1/2" is kind of hard to pull through. Remember you can only have 360 degrees of bends in your conduit before you have to put in a junction box - in other words, no more than four 90 degree bends.
If you dig around on that electrician.com website there are several conduit size calculators.
Edited 6/10/2005 9:42 pm ET by Stuart
That's great on the freon line length.
I would run 6 guage to the compressor and 6 guage to the air handler. The 6 guage to the air handler is assuming emergency resistance heat strips. As Bill says, you need to check the installation guide, and then I would upsize one size to reduce your resistance losses unless the runs are short. 60 feet is long enough to justify the larger size.
Hi Ray,
Your response has me pondering that a longer refrigerant line will cause increased electrical expense. When installing refer lines and the connecting equip. I have never seen that when the refer line exceeds 50 ft. that I should increase wire size or breaker size. I assume that with the increase in friction that that would be the case for the increased utility bill. But wouldn't that be the case with a recirculating pump as well. One would have to size up in hp for the pump as well as an increase in the tonage size of the compressor if the lines were beyond a certain point. This is certainly true of a open pumping system. Please teach me a lesson hear as I always like to learn.
Sorry, I spoke about two independant issues. One is the issue of long wire runs and the ability to reduce line losses by upsizing the wire by one guage size. The other issue is that when refrigerant lines exceed 50 feet, the system will use more energy to achieve the same level of cooling and so that situation should be avoided. The second concern is not an issue here because the freon lines, as it turns out will only run 15 feet. I'm sorry that I was not clear.
"But wouldn't that be the case with a recirculating pump as well. One would have to size up in hp for the pump as well as an increase in the tonage size of the compressor if the lines were beyond a certain point. This is certainly true of a open pumping system."That is true to a certain extent.Howver, most of the energy in a compressor is used for compression the gas and only a minor amount (but I don't know how minor) is used for trasporting the gas/liquid.However, there are sizing charts and they will upsize the lines for longer runs to reduce the friction head losses.Also a longer run allows more ambinet heat to warm up the lines, even with insulation.But since you are talking about relatively large steps in compressor size only in the very extreme cases would it ever affect the sizing.
Bill,
I wanted to find out how much of an increase in amp draw or energy costs the line set over fifty ft. might cost some one. Would be interesting to find out.
Increase in line set size no doubt, however, that should negate the increase in energy useage do to friction. Leaving the heat gain for the additional energy costs. Would love to find out what the btu gain would be. In other words; put the condenser in the side yard out of sight or is it worth the savings putting it a shorter distance and have to look at it or its fence.
You might try asking over at http://www.hvac-talk.com
Bill,
Thanks I didn't know it existed.
Remember, if the wire is inside the conditioned space, and it's used for an AC, you pay for the heat loss twice. If, on the other hand, the wire were being used to power resistive heating units, you wouldn't pay for the heat loss at all, if it's in the conditioned space.The equation gets more complicated when you're dealing with motors, though, since reduced voltage at the motor reduces it's efficiency.But the real issue with an AC is starting current. Voltage drop during starting will be 2-4 times what it is during normal running, and this can severely reduce starting torque. Add a "brownout" situation and an increase in wire size (or decrease in length) may make the difference between an AC that works and one that doesn't. Also, the difference in wire size may make a difference in the operating life of the unit, since the lower voltage drop will allow the unit to run cooler and allow an aging unit to run longer before replacement.