Running a 36amp load on a 40amp breaker

[Homeownerinburb]

I have wired up a Jacuzzzi that it turns out is set to draw 36amps. I used 8ga NM for part of the run, which for no reason that I can comprehend, is rated at 40 amp. I propose to use a 40amp breaker to protect the circuit.

I am not troubled by running the wire steadily at 36amp, I really think that it can take a fair bit more than this.

But how will the breaker perform? I know that one traditionally runs a circuit at 80% of the rating of the breaker, and can make out with ease that 36 is 90% of 40.

What do you think? I don't want to reconfigure the Jacuzzi (apparently it can be set to run under a 30 amp breaker but this precludes the heater running at the same time as the two pumps are running at full speed.)

 

[Jadnashua]

The 80% rule, as I understand it, depends on how long the load is active (and some specific things are called out in the code). Second part of this is you must follow what the manufacturer calls for in their installation instructions. My guess is that they specify a larger circuit than 40A if you want to have everything on at the same time.

 

[Sponsor]

 

[Homeownerinburb]

The load would only be at 36 amps when the heater is on and both the pumps are running, with one pump running at slow (the other pump only runs fast) the heater cuts out when both pumps are running at full speed. The manufacturer calls for a 50 amp breaker under this regime.

There IS a setting that calls for a 30 amp breaker that does not heat if the pumps are running at all. But I don't know how to reconfigure it to be at that setting.

And I don't want to back my friend into that limited a configuration.

I certainly don't want to replace the 8ga with 6ga, it is a nasty crawl under the house.

 

[Jadnashua]

When you do not follow the manufacturer's instructions, the inspector should not approve the installation. Whether it will work, it probably will, but you could run into problems if there ever was a problem, things could get messy and liability issues could erupt. That could have insurance issues, too.

 

[WorthFlorida]

Here you go again doing something that can get you in trouble with this tub as you started a thread in July and I stated back then it must be a 50 amp with 6 ga connection. No less. (https://terrylove.com/forums/index.php?threads/jacuzzi-and-gfi.66387/page-2).

If you read the third pinned post in this forum it states:
Code compliance is also important as the NEC (National Electrical Code) is a minimum safety standard. Compliance with the NEC results in an installation that is essentially free from hazard to both persons and property.
Any non-compliant advice will be addressed in a respectful manner. There will always be room for debate concerning NEC rules but any and all disrespectful or degrading remarks concerning the NEC will be deleted.

You're looking to beat proper installation and safety concerns buy asking questions to get around the required manufactures standards and electrical code. Should something bad happen such as a fire or electrical shock, you will be responsible. Follow Judnashua suggestions, get smart, don't get cheap. No one here can or will recommend anything that could be a safety hazard.

 

[DonL]

I certainly don't want to replace the 8ga with 6ga, it is a nasty crawl under the house.

So you would rather do it half fast ?

 

[Jadnashua]

The manufacturer gives you an option to run things on that 40A breaker...you can be compliant if you configure it that way. If you want the full capacity of the system for heat and pumps, you must run it using a 50A circuit.

 

[WorthFlorida]

The manufacturer gives you an option to run things on that 40A breaker...you can be compliant if you configure it that way. If you want the full capacity of the system for heat and pumps, you must run it using a 50A circuit.

Your right but this guy is trying to get the most for nothing. But still, he has a 5o amp on a 8 gauge wire. He also stated in the previous thread that he is turning things on with no water in the tub and it trips the GFCI.

 

[Homeownerinburb]

But still, he has a 5o amp on a 8 gauge wire. He also stated in the previous thread that he is turning things on with no water in the tub and it trips the GFCI.

Turns out the tripping breaker was due to a frozen pump, which has been replaced and the breaker carries the load ok.

It is not necessary to run 6ga. 8ga thwn is rated to 55amp. It is just the NM that is derated, but I can't figure why. 14ga NM will carry 15 amp. Using the rule that wire ampacity doubles with each 3 in ga, then 11ga is good for 30 amps and subtract 3 again to 8ga and you double to 60 amp. I assume that 8ga NM has been stress tested by the appropriate authorities and it has begun to fail at something a bit more than 40 amps, but then so much for the rule of doubling with each 3 of the ga.

I still fail to see what disaster will happen if I run the system with a 40 amp breaker. The wire will not see more than 40 amp, so where is the problem? Unless for some reason a 40 amp breaker deteriorates if run for an hour at 36 amp, (90%).

Remember: the system will not see 36 amps except when the jet pumps and heater are running. How long do people actually sit in a hot tub anyway? Five or six hours? They don't.

I'll reconfigure the spa to not heat while the pumps are running at all before I run new wire, the access was a perfect pig.

 

[DonL]

I'll reconfigure the spa to not heat while the pumps are running at all before I run new wire, the access was a perfect pig.

Won't that be a bit hard on the heating element ?

What is the temperature rating of your wire ?

 

[Homeownerinburb]

Won't that be a bit hard on the heating element ?

What is the temperature rating of your wire ?

That is how it is designed.

THWN is rated at 90 degrees Celsius, if memory serves. NM obviously is rated a good deal less.

 

[Reach4]

I still fail to see what disaster will happen if I run the system with a 40 amp breaker. The wire will not see more than 40 amp, so where is the problem? Unless for some reason a 40 amp breaker deteriorates if run for an hour at 36 amp, (90%).

You are trying to discuss logically.

 

[Jadnashua]

There is some logic behind the codes. I do not claim to understand all of the nuances, but ignoring them, and the manufacturer's specific instructions, plainly puts you at fault for any problem, and if you have a competent inspector, should get things red-flagged.

Electricity and NG are probably two of the areas the average person should avoid and have done by a professional. Certainly, some stuff can safely be done, and I have done some of those things, but have always done things per code. One reason why codes sometimes change is from lessons learned. Nobody knows or needs to know all of the reasons behind them, only that history has shown there is justification for change and to improve the safety margins to prevent injury or property damage. It's all of those little 'what-ifs' that could create problems. You may never see that set of conditions, but what-if you do? The codes try to prevent that from ever becoming an issue.

If you ever had a big party, you might find that your hot tub was running for many hours on end as people swapped places...this is one of those 'what-ifs'. You go to bed and both you and your partner think the other turned things off and it stays on all night. The list could go on and on.

 

[Speedy Petey]

It is not necessary to run 6ga. 8ga thwn is rated to 55amp. It is just the NM that is derated, but I can't figure why. 14ga NM will carry 15 amp. Using the rule that wire ampacity doubles with each 3 in ga, then 11ga is good for 30 amps and subtract 3 again to 8ga and you double to 60 amp. I assume that 8ga NM has been stress tested by the appropriate authorities and it has begun to fail at something a bit more than 40 amps, but then so much for the rule of doubling with each 3 of the ga.

Just what "rule" is this??
Sorry, but you have no idea what you're talking about.

 

[Homeownerinburb]

I don't remember where I heard it before, decades ago, it possibly only pertains to electronics, not NEC. Well obviously not NEC when 14, 12, and 10 ga are rated the same in NM and in THWN, but for some reason (the insulation can't take the heat) 8ga is different for NM and for THWN.

Go figure.

 

[Reach4]

Just what "rule" is this??

In AWG wire sizes, a difference 3 AGW numbers corresponds to very close to twice/half of the cross sectional area of the wire. https://en.wikipedia.org/wiki/American_wire_gauge#Rules_of_thumb has the details.

 

[Bluebinky]

How are you planning on running the heater without the low-speed pump? The flow switch will prevent it, and even if it didn't, how would the heated water make it into the tub?

 

[Homeownerinburb]

In AWG wire sizes, a difference 3 AGW numbers corresponds to very close to twice/half of the cross sectional area of the wire. https://en.wikipedia.org/wiki/American_wire_gauge#Rules_of_thumb has the details.

Well, don't tell Pete, who is under the impression that I am an imbecile. Obviously the insulation makes all the difference.

Just makes you wonder why NM is not jacketed with the same plastics as THWN.

 

[Homeownerinburb]

Bluebinkiy:

I'm not sure I am planning much of anything now!

The lowest amperage setting, which requires an 30amp service, allows the low speed of the two speed pump to run while the heater is running.

So the hot water does get moved about.

 

[Homeownerinburb]

Thanks Reach:

Rules of thumb[edit]
The sixth power of 92 39 {\displaystyle {\sqrt[{39}]{92}}}

is very close to 2,[5] which leads to the following rules of thumb:

• When the cross-sectional area of a wire is doubled, the AWG will decrease by 3. (e.g., Two No. 14 AWG wires have about the same cross-sectional area as a single No. 11 AWG wire.) This doubles the ampacity.
• When the diameter of a wire is doubled, the AWG will decrease by 6. (e.g., No. 2 AWG is about twice the diameter of No. 8 AWG.) This quadruples the cross-sectional area and the ampacity.
• A decrease of ten gauge numbers, for example from No. 10 to 1/0, multiplies the area and weight by approximately 10, and reduces the electrical resistance (and increases the conductance) by a factor of approximately 10.
• For the same cross section, aluminum wire has a conductivity of approximately 61% of copper, so an aluminum wire has nearly the same resistance as a copper wire 2 AWG sizes smaller, which has 62.9% of the area.