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I am considering adding a hot tub to my backyard. Most of the ones I've seen require a 240v 50A breaker, presumably with #6 4-wire.

It would be most convenient to run the line for this from my subpanel in my detached garage. I had an electrician put in the subpanel just a year ago (so it is up to modern standards; previously it was knob-and-tube! house built in 1927) and he set it up as a 60A subpanel. This consists of a 60A breaker at the main panel in the house, run underground to the subpanel in the garage, which has room for eight GE Q-line miniature breakers. Due to the number of available spaces, I'm guessing the breaker panel could handle more than 60A, but it's just limited by the breaker at the main panel and by the amp rating of the wire going between the panels.

There isn't a lot of load in the garage usually, just a couple lights and outlets. I think we could have gotten away with using two 20A circuits, but he wired up two 20A circuits and two 15A circuits, using 4 of the breaker spaces. This adds up to 75A, but from what I understand this is okay since once it hits 60A it would trip the breaker at the main panel and shut off the entire garage. Occasionally we will run a 12A electric lawn mower or other similar electric lawn tools from the outlets. The lights are motion lights on the outside, and switched fluorescent shop lamps on the inside.

Okay, now for the the question: Would it be okay and up to code to run a 50A 2-pole breaker in two of the remaining 4 slots in the panel and hook the hot tub up to it?

I'm assuming that the hot tub isn't going to be drawing close to 50A under normal conditions otherwise it'd regularly trip the breaker. But even if someone is sitting in it with the jets on and say it draws 40A, it seems like I could still run the lawn mower at 12A and have a couple lights on and not trip the 60A breaker at the house. (Related: does anyone know how many amps a typical hot tub would draw at peak and during regular operation?) I assume this line of thinking is why the electrician was able to put 75A worth of breakers on that 60A panel.

But, if any of this screams "could cause a fire" or "not up to code", please let me know! Thanks in advance!

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Hot tubs have a pump and a heater, both of which will draw a fair amount of current. You may find that the breaker trips when the hot tub is on, and you try to run other devices. You should be able to determine the "actual" current draw, by checking the nameplates of the motor and heater. – Tester101 Apr 28 '14 at 23:21
Note also that the load from the pumps is likely to be inductive. Think of inductive loads as being like adding a water balloon into the middle of a hose: as it expands the water at the source is not making it to the business end, and when the hose goes off, the water keeps flowing as the balloon empties. Inductive loads can draw extra current when they start up and produce voltage spikes when they shut down, and that can sometimes cause breakers to trip unexpectedly. – Eric Lippert Apr 29 '14 at 17:47
up vote 2 down vote accepted

Regardless of what all the individual breakers add up to, the maximum you can draw through that subpanel is 60amps. Realistically you want to be a bit lower then that to allow for some spikes.

To determine if you can support the tub, you need to take a measurement of the current (amps) on the circuit with some or all of the existing devices in use (i.e. normal use). Ideally you would use a clamp meter around the hot wires in order to measure this, but alternatively you could add up the loads on the circuit. If you subtract this value from 60, and you have enough capacity for the tub, then it should be OK to run from that sub-panel.

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I don't think I've ever seen something that when the manufacture provides you with the circuit size to connect it to, either literally or implied by the factory installed plug type, it isn't at least 125% of the most it will draw. Often times if the device, appliance, machine or whatever it is, only slightly exceeds 80% of the next smaller circuit size, in this case 40 amps, the manufacture will rate it for the next higher. My guess is that your hot tub will draw between 33 and 40 amps at 240 volts with everything on heater, pump and whatever else it has.

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@Steven is essentially right. What has been explained many times on this site is that there is NOTHING dangerous about what you propose. The 50A breaker will be protecting the wiring to the tub (and internals of the tub). The 60A breaker is protecting the wiring to the subpanel (and its internals). You can do this if you want and you will be perfectly fine. BUT, if the tub really does pull 50A and you have it on at the same time as other devices in the garage which at taking more than 10A then the 60A breaker will trip.

Bottom line: You can have as many devices breakers as you want totaling more than their main/subpanel breaker. It is safe. It just will trip if you actually consume more than that at any one time. You have to figure out whether that situation will ever occur.

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"if the tub really does pull 50A and you have it on at the same time as other devices in the garage which at taking more than 10A then the 60A breaker will trip." Should say "if the tub really does pull 50A, and you have it on at the same time as other devices in the garage which are taking more than 10A for long enough then the 60A breaker will trip." Breakers trip based on their trip curves. Momentarily drawing more current than the breaker is rated for, may not instantly trip the breaker. – Tester101 Sep 4 '14 at 16:25
@Tester101: Maybe technically true, but beside the point here. The point is to illustrate how breaker protect circuits, not the subtleties of how they work. – ThePopMachine Sep 4 '14 at 19:42

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