Sizing a feeder for a pool heater and pump

[mark32]

Hello,

I am wiring an above ground pool for a friend, the nameplate on the pump is 10.5a @ 120v and the heater is 42a @ 240v. So to calculate: (120*10.5 = 1260w) + (240*42 = 10080w) = 11340w. Both would be considered a continuous load so: 11340*1.25 = 14175w. Then 14175/240 = 59a. At this point I figured I would protect this feeder with a 60a breaker and run 6/3 mc across my friend's unfinished basement, then change over to #6 thhn before going outside and underground out to a pool panel. However, I will also run the convenience receptacle out of this pool panel. Would that force me to go up to a 70a breaker and feeder, 4/3 romex, and #4 thhn?

 

[ptonsparky]

My calculations would have been 1.25 times the largest plus the sum of the others.
(42*1.25)+10.5=63. Already over a #6.

 

[Dennis Alwon]

Mark I am not sure why you converted to watts but you converted a 120v load and a 240v load and then divided all of it by 240v. Tom has it correct in the post above

 

[mark32]

Thank you ptonsparky and Dennis for the replies. I actually did my first calculation in a similar manner as to what ptonsparky posted, I just added 25% to the total, so 42+10.5 = 52.5*1.25 = 65.6a. (I guess only one continuous load needs to be 125%?). Anyway, I then figured that since the motor is 120v, it would be necessary/more accurate to convert the whole calc to watts, then divide by the feeder voltage of 240v. Like, if the motor was 10.5a @ 240v, the calculation would still be written in the same way as shown by ptonsparky.

 

[retirede]

If the pump motor is dual voltage, you could wire it at 230 and get back under 60A.

 

[5050boob]

Can you run separate circuits,one for pump,one for the heater,then figure your loads. And remember pool grounding is essential with a continuous ground to all metal and otherwise parts of the pool itself. Thank you

 

[suemarkp]

What nameplate are you reading on the heater? With an amp rating of 42A, I'm thinking its a heat pump. If you're reading the MCA value on its nameplate, that's what you use as it already includes the 125% of its largest motor. I'd leave some headroom in the feeder for the convenience outlet (you never know what they will run off of that), or they may use a larger pump in the future.

A 60A feeder may be enough if the pump is 120V and the convenience receptacles use the other leg.

 

[mark32]

Thanks guys for the continued replies. Since all of the varying calculations put me very close to 60a and above, I just ended up running 4/3 nm and switching over to #4 thhn for outside (Although I went down to a #6 for the neutral). Dang, that 4/3 was like $4.50 a ft! Anyway, I'm still not sure of the proper way to perform this feeder calculation. As previously mentioned, I initially used the same method ptonsparky used, but afterwards, I thought that maybe I need to change the loads to watts. Let's look at ptonsparky's calculation, (42*1.25)+10.5=63. Okay, to me it looks good at first, but, again, the filter pump is 120v, the heat pump (Thank you suemarkp) is 240v. When we do a service calculation, we do it in watts (VA), yes? Probably because of the mix of 120v and 240v loads. I look at this calculation the same way. Back to this (42*1.25)+10.5=63, to me, this looks like both loads are of the same voltage, but they are not.

 

[ptonsparky]

One leg has both amperage. You size for that max. Your additional receptacle could be put on the least loaded leg.

 

[infinity]

If you calculated load is 65 amps or less your original plan of #6 MC and THWN could be used with the next size up rule or a 70 amp OCPD. Feeders are calculated at 125% of the total continuous load not just the largest one.

 

[mark32]

Thanks fellows. Ptonsparky, what you wrote makes sense, one leg will carry both loads, I didn't look at it from that angle, thanks!!

Infinity, Doh! Oh boy, I was under the impression that rule/exception was only for individual loads and not for stuff like sub panels.

 

[LarryFine]

Yes, you have to size a feeder for the most heavily-loaded line.

 

[mark32]

Thanks Larry!

Not to derail my own thread, although I have found my answer to it, I want to quickly ask a question relating to Infinity's post about going up a size in ocpd. Let's say you run 6/3 nm cu to a sub panel. Considering it's ampacity rating of 55a, I always protected it at 50a, because how can one calculate the load accurately in such a scenario right??

 

[nizak]

Thanks Larry!

Not to derail my own thread, although I have found my answer to it, I want to quickly ask a question relating to Infinity's post about going up a size in ocpd. Let's say you run 6/3 nm cu to a sub panel. Considering it's ampacity rating of 55a, I always protected it at 50a, because how can one calculate the load accurately in such a scenario right??

Where did you find 4/3 Nm with an insulated equipment ground for the feeder?

 

[kwired]

Thanks Larry!

Not to derail my own thread, although I have found my answer to it, I want to quickly ask a question relating to Infinity's post about going up a size in ocpd. Let's say you run 6/3 nm cu to a sub panel. Considering it's ampacity rating of 55a, I always protected it at 50a, because how can one calculate the load accurately in such a scenario right??

It can be used for up to 55 amps, but can be protected at next standard size up overcurrent device which is 60 amps.

You can not connect a 56 amp load to it even though 60 amps is still the next size up device.

We do this sort of thing all the time for many circuits 800 amps or less. Over 800 amps you can no longer use next size up rule for overcurrent protection, though specific applications like motors throws a wrench into the gears a little.

 

[nizak]

I'm sorry. I misread the post and thought you were running the NM cable feeder directly to the pool house.