#6 Ground Wire? Spa wiring requirements

  
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5030tinkerer

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Correct -- the ampacity ratings are based on material, insulation type, bundle/conduit type, etc, such that the wire can handle a certain number of amps and not get hot or melt. The ampacity is also affected by numbers of wires in proximity. So there are often limits to how many wires can be in a conduit or bundled together in a jacket. I have even seen some underground/wet rated wire that will have a different ampacity whether direct buried or in conduit. The end focus always comes down to keeping the wire within the temperature rating of its insulator and conductor materials.
Interesting. Something I've always questioned is my plumber's installation of a hot water heater in my mechanical room. The wire length is 20', but it is on a two pole 30 amp circuit with #14 wire. The wire has never felt warm even after the water heater was on for nearly an hour following back to back to back to back showers. This is an easy one to correct, though, as the wiring is all exposed (not in conduit and not behind sheetrock) romex. Nevertheless, I've also found it interesting that the wire wasn't perceptibly any warmer when under longer-term amperage draw. Maybe I'll shoot it with my IR Fluke camera to determine how warm it is or is not getting for sure rather than gut-checking the 'feel', but I've felt warm wires before and these aren't that. Regardless, I'll change out the wire since it's easy and I know for certain not 'to code' (the ONLY thing in the house that isn't).
 

Rock knocker

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Interesting. Something I've always questioned is my plumber's installation of a hot water heater in my mechanical room. The wire length is 20', but it is on a two pole 30 amp circuit with #14 wire

That you should change out!
 

CobyRupert

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In this application, the 70 amp spa panel is fed from a 200 amp main panel that is in turn fed by a meter panel with a 200 amp breaker. A grounding rod is connected to the main panel, but I don't recall the wire size connected to it - it's something like a multi-strand cable with maybe 5-7 #10 wires IIRC.

s219 - You said that wire length has nothing to do with 'allowable' ampacity. Do I correctly understand that then that a #14 wire, for example, that is rated for 15 amps of current, but happens to be at, say, 20' in length, simply cannot handle a, say, 30 amp draw - that it'll turn into a fuse, or minimally run hot and be a fire hazard?

CobyRupert - You say that #10 is good for up to a 60 amp circuit. In this case, a singular ground wire is handling a 70 amp PANEL, but one with a 30 amp GFCI, a 20 amp GFCI, and a second 20 amp GFCI breaker. It would seem logical, that any fault would impact only one of these breakers at a time, meaning that the #10 is actually MORE THAN plenty - for the same reason that the #4 wire evidently commonly used as ground in a 200 amp panel,is sufficient (surmising that this basis has to do with the max supported breaker size of 100 amps in that panel).
I assume you have a 70 Amp breaker in your 200 Amp main panel? Again, NEC (The Code) would tell you to use a #10 on a 60 amp breaker. So here's an interesting point: During a ground fault-short circuit, the current that shoots up to maybe 2000 amps doesn't care if the breaker is a 60 amp or a 70 amp (or 15 amps or 200 amps), the fault current will be pretty much the same, so why does the code make you vary the ground conductor size based on the breaker size? I think the answer is "Because." (Basically: "We had good luck with this, but there's no "reason" ) My point being: If by code, a #10 is good to carry the ground fault current (+/-2000 amps) for a 60 amp breaker for a few milliseconds, realistically it's going to do the job for a 70 amp circuit.
Realistically the GFCI's will trip when the ground current is around 3 to 5 milliamps. Miniscule ground current compared to a #10.


CobyRupert#2 - Thanks a ton for the grounding explanation. You said that 'low resistance allows (Ohm's Law) a large enough short circuit to flow so that the circuit breaker trips'. Doesn't resistance also increase with wire length? IOW, if the instruction manual calls out #6 wire for a run up to 100' and my run is only about 34', wouldn't the resistance on that length be less - as indicated actually by the voltage drop calculation?
Yes, 34' is nothing for voltage drop (or rise in the case of grounding)
!

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Tractor Seabee

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I am surprised that all our electricians have not weighed in on all this disinformation being passed. Electrical codes are all about safety. Electrician I am not; but have done a lot of it and even so,e inspection work in the past. Neutral, ground, and bond are the things that baffle DIY practitioners. You electricians are welcome to correct me on this as I am always learning too.

First off your Spa requires more ampacity than your existing circuit can supply, per code. You are feeding a 70A sub-panel with #6 which is rated at 50A. Your connected load is 56 A which is why the 70A panel. You need to feed your Spa panel with #4 wire. The Neutral has to be #6 also as all the heater elements are 120 V. #10 ground is minimal for this, the actual requirement is for #8 from 50-100A.

For good midnight reading instead of counting sheep get a copy of SOARES Book on Grounding and Bonding. It is one of the text books for Electrician Apprentice classes.

Will your existing feeder going to the SPA work? Probably, if it does not trip the 50 A main panel breaker. Is it safe? Probably, if all connections are made properly. A loose connection-LOOK-OUT!

Would I use it? Yes, I would try it before running all new wire. New wire will require conduit; as #6 is the largest Non-Metallic cable made for open building installation.
Check actual amps with cold water, and then with hot water, check temp of conductors, check the GF function. You may be good to go.

I have done couple of these in the past from scratch. Always used #4 for the 240V heater and # 10 Neutral for the 120V pump/aerator and controls, and #8 for the ground/bond. (the ones I have done had 240V heaters instead of the 120V you have. I used 1" conduit. 3 #4s and 1 #8 will still fit into 1".) Crawling under a house to do this in conduit is no picnic.

You will find once the cold water is brought to operating temp the Amp draw from then on is quite intermittent.

Ron
 

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Actually couldn't the neutral be down-rated because it is only picking up the unbalanced portion of the load?

No, all the heaters are 120V so they all put load on the neutral same as the two power conductors. Down rating only works where the 120V loads are incidental. Look at his wiring diagram, all the heaters are connected to the neutral buss. When the water is hot there will be time of unbalanced load across the 2 phases.

Ron

Ron
 

CobyRupert

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A copper #6 cable is good for 65 amps (Assuming insulation and terminals are rated for 75 deg C. (If terminations temp ratings are unknown, one is suppose to, by Code, assume they are the lower 65 deg C rating, and the #6 circuits' ampacity is limited to 55 amps, because of the heat generated at the terminations (note however, for all practical purposes this heat would not be a concern on overheating the insulation on the 65 amp conductors in the wall. (Follow?)

In fact, if the cable insulation and terminations were rated for 90 deg C, the ampacity would be 75 amps. (so we have a #6 cable that may have an applied ampacity of 55, 65 or 75 amps! Simple ehh?)

The diagram doesn't show that branch circuit loads are 120V. Given that the terminals are "L1" & "L2", I'D assumed they are 240v

But let's start making MORE assumptions:
1. We are basing amapacity on 65 degC cable (i.e. #6 AWG =65 amps),
2. Loads are continuous (i.e. may operate for over 3 hours). Thus we base our feeder amp requirements on 125% of largest loads plus 100% of others. (Assume the 3 GFCI all minimally sized (at 125%) for their loads, calculating backwards the loads would be 16A, 16A & 24A = 56A, as mentioned above). Thus 125% of the largest (24A x 125% =30A) plus 100% of the remaining (16A + 16A) = a cable that needs to be sized for a calculated load of 62 Amps.)
3. That is, a # 6 awg ampacity's (65 amps) exceeds the calculated load.
We can then protect this 65 amp cable with the next highest (standard) rated circuit breaker(= 70amp) in the main panel (NEC 240.4(B)).
 
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Rock knocker

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No, all the heaters are 120V so they all put load on the neutral same as the two power conductors. Down rating only works where the 120V loads are incidental. Look at his wiring diagram, all the heaters are connected to the neutral buss. When the water is hot there will be time of unbalanced load across the 2 phases.

I don't think you have that right
 
 
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