Max Circuit Load?

[Jawgarage]

Im trying to figure out the sizing of my sub panel for the 100amp service coming out to my detached garage. Specifically I am questioning how many heaters I can have on a single circuit. Here are the details:

100 Amp sub panel in detached garage 60' from main house panel

Each Marley/Qmark hydronic baseboard heater is 240v and 2,000 watts at 8.6 amps

Instructions say you can hook two of these together in series

I have 6 of these to install across the two separated 25x32 double garage halves, and the two second floor independent 20x20 workshop/storage "rooms"

My plan was to run 12ga copper to two of them off of a dedicated 20amp breaker

Now I am hearing that a 12ga run with 20amp breaker has a max wattage rating of 3,500 watts and 16 amp heater draw?

Help.....how many circuits of what size do I need in the sub panel to operate the Six units? One dedicated 20amp breaker for EACH unit?

 

[mm08822]

To be code compliant, to put 2 on a ckt, it must be a 30 ckt. Two of these heaters put you over 16a (17.6a) which is the max permissible continuous load for a 20a ckt. Heaters are considered a continuous load. And 3 is too many for a 30a ckt.

You don't mean wired in series but rather ganged in pairs. The two limit is probably a pass-through internal heater wiring load limit.

 

[Jawgarage]

To be code compliant, to put 2 on a ckt, it must be a 30 ckt. Two of these heaters put you over 16a (17.6a) which is the max permissible continuous load for a 20a ckt. Heaters are considered a continuous load. And 3 is too many for a 30a ckt.

You don't mean wired in series but rather ganged in pairs. The two limit is probably a pass-through internal heater wiring load limit.

Thank you for the quick reply. So if I use a 30a ckt can I stick with the #12 copper wire dedicated runs to two heaters per circuit?

 

[mm08822]

Nope, 30a requires #10. I assume you are planning to use romex and not run conduit.

 

[Jawgarage]

Aarrgh! Seems like overkill for two baseboard units with no startup load. What about simply then running each unit on its own 15amp ckt connected by #12? That gives me more load balance control also even if it may require a sub panel with more slots.

 

[mm08822]

If you power these seperately, then #14 is good enough to each. That is then probably 2x the cable and 2x the labor. Consider that as well.
Thats a lot of panel space to use up also. Don't cheap out on the panel ckt size anyway.

 

[pattenp]

Nope, 30a requires #10. I assume you are planning to use romex and not run conduit.

Ampacity wise, running THHN in conduit for this type of circuit will make no difference vs. Romex. #12 is limited to 20A. (NEC 240.4 (D) (5)

 

[Jawgarage]

If you power these seperately, then #14 is good enough to each. That is then probably 2x the cable and 2x the labor. Consider that as well.
Thats a lot of panel space to use up also. Don't cheap out on the panel ckt size anyway.

I think I like single #14 to a 15amp ckt per unit better than pulling that much #10 and running to 30amp ckts to gang two units together. I did not realize that #14 was to code for this 240v application and since I am doing the pulling over an extended period of time I dont mind the added runs of #14.

This has been lots of help. Thanks!

 

[mm08822]

Ampacity wise, running THHN in conduit for this type of circuit will make no difference vs. Romex. #12 is limited to 20A. (NEC 240.4 (D) (5)

I was thinking **values gave room on the 80% value also, but it is only for fill and temp derate usage now that I have my book in front of me.
Agreed, 20a was max ocp allowed anyways.

 

[KenC]

I think I like single #14 to a 15amp ckt per unit better than pulling that much #10 and running to 30amp ckts to gang two units together. I did not realize that #14 was to code for this 240v application and since I am doing the pulling over an extended period of time I dont mind the added runs of #14.

This has been lots of help. Thanks!

If you power these seperately, then #14 is good enough to each. That is then probably 2x the cable and 2x the labor. Consider that as well.
Thats a lot of panel space to use up also. Don't cheap out on the panel ckt size anyway.

If your panel allows them, this would me a perfect usage of 15A tandems IMO. No more slots used than your original plan, load way under the capacity provided and you can use 14Ga.

 

[nehog]

...

Instructions say you can hook two of these together in series
...

Uh, not series, parallel.

 

[theoldwizard1]

uh, not series, parallel.

concur !

 

[theoldwizard1]

I did not realize that #14 was to code for this 240v application ...

Wire gauge is related to the current (amps) and the distance. Insulation is what you have to be concerned about with voltage. Most common electric wire (THHN, THWN or NM-B) insulation is rated for at least 300V or 600V.

 

[sberry]

Using a 10 cable would save a lot of cost and work. From an ultra efficiency standpoint its even better, like most branch and trunk designs when only 1 unit is on it can serve the load better than a home run. 2 units on about the same.

 

[Jawgarage]

Using a 10 cable would save a lot of cost and work. From an ultra efficiency standpoint its even better, like most branch and trunk designs when only 1 unit is on it can serve the load better than a home run. 2 units on about the same.

Sorry, I am not following....use a 10 cable for what part(s)?

 

[sberry]

Putting 2 heaters on 1 number 10 wire is better than a wire of 14 for each when only 1 unit is running at a time. The difference would depend on some factors including length and duty. This is somewhat theoretical, real world difference would be minor except for possible cost savings on installation.

 

[sberry]

I would probably buy a roll of 10 and use it for everything I could 240 in a hobby garage. Buy a box of 30A breakers. All the same if it was legal and practical.

 

[wyliesdiesels]

Sorry, I am not following....use a 10 cable for what part(s)?

Hes saying to use #10 with a 30a breaker for 2 heaters because its theoretically less wire to run.

However we dont know the layout of the shop and how far apart the heaters are so #10 might not make sense. Only u can tell us if that is true.

 

[Jawgarage]

OK got it. Based on the layout I sort of like the idea of each on it's own breaker. What I may do is simply use the 15amp tandem breaker for each and run #12 copper NMB. A little more money than the #14 but cheaper than #10. This way I can turn each one off independently too.

 

[mm08822]

What is the extra value of #12 if each on own ckt?

 

[Jawgarage]

What is the extra value of #12 if each on own ckt?

Right now it is pure overkill - which I don't mind

But I also like the idea of having the 12 run in case I want to upgrade to something more powerful down the road for one or more of these units. At least then I already have the 12 run and I'd just need to swap the breaker to a 20amp.

 

[mm08822]

You could even put in a 20a 2p now with the 12 instead of later.

 

[sberry]

You could use 20, its designed for up to 30

 

[Jawgarage]

I forgot that since the heaters are 240v I have to use dual pole breakers which knocks out the idea of using tandems. So if I am running 12 to each unit, and I have to use dual pole breakers, I like the idea of also upgrading the breakers to 20amp. BUT I thought the breaker is supposed to match the load on the line more or less? So if the each heater unit is 240v, 8.6amps, 2000watts, and I am running one line per unit, is it still OK to use a 20amp dual pole breaker, or is the 15amp better suited? I'd really like to run two units off of each ckt and thought that the #12 would be sufficient with a dual pole 20amp since this is how I've seen another garage wired.

So if I am reading this all correctly I have three options per the code:

1. Run each unit on it's own 15amp dual pole breaker with #14 copper NMB
2. Run each unit on it's own 20amp dual pole breaker with #12 copper NMB
3. Run two units in parallel on one 20amp dual pole breaker with #10 copper NMB?

Is this correct?

 

[wyliesdiesels]

That old story about matching the breaker to the load is a complete myth.

If you think about it logically, does the breaker that feeds the outlet for your table lamp match the load? NOPE And it would be impossible to do so.

 

[Falcon67]

I would use #12, mostly because you'll use that for most other runs and it comes in 250' rolls. Unless you plan to run a lot of lighting circuits using #14, then you'll likely have a 250' roll of each anyway because it's more economical. 250' of 12 at HD is around $52, I just bought 50' of 10-3 for the lift and it was over $60.

 

[Jawgarage]

That old story about matching the breaker to the load is a complete myth.

If you think about it logically, does the breaker that feeds the outlet for your table lamp match the load? NOPE And it would be impossible to do so.

Now that's ironic wyliediesels as that is exactly what I was thinking as I typed out my post earlier regarding matching the breaker to a load.

Settled then.....#12 with dual pole 20amp breaker for each unit. Definitely going to need the larger sub-panel box to handle this many dual pole breakers and the other needs. How does this sub-panel look to you guys?

http://www.homedepot.com/p/Square-D-Homeline-100-Amp-30-Space-60-Circuit-Indoor-Main-Plug-On-Neutral-Breaker-Load-Center-with-Cover-HOM3060M100PC/204836371

http://www.homedepot.com/p/Square-D...oad-Center-with-Cover-HOM3060M100PC/204836371

 

[mm08822]

I forgot that since the heaters are 240v I have to use dual pole breakers which knocks out the idea of using tandems. So if I am running 12 to each unit, and I have to use dual pole breakers, I like the idea of also upgrading the breakers to 20amp. BUT I thought the breaker is supposed to match the load on the line more or less? So if the each heater unit is 240v, 8.6amps, 2000watts, and I am running one line per unit, is it still OK to use a 20amp dual pole breaker, or is the 15amp better suited? I'd really like to run two units off of each ckt and thought that the #12 would be sufficient with a dual pole 20amp since this is how I've seen another garage wired.

So if I am reading this all correctly I have three options per the code:

1. Run each unit on it's own 15amp dual pole breaker with #14 copper NMB Yes
2. Run each unit on it's own 20amp dual pole breaker with #12 copper NMB Yes
3. Run two units in parallel on one 20amp dual pole breaker with #10 copper NMB? No - needs to be 30amp 2 pole

Is this correct?

Two heaters per ckt require #10 and 30A cb

 

[mm08822]

I would go with a 40 space - only a few $ more - good insurance.

 

[Jawgarage]

I would go with a 40 space - only a few $ more - good insurance.

Who makes/sells a 40 space 100amp sub panel? I'd go for it but the largest I could find was the 30 space one I linked to.....

Jeff

 

[Jawgarage]

Two heaters per ckt require #10 and 30A cb

Ah yes.....forgot that about the #10 and 30A cb.......sticking with the #12 and 20A's then.......

#12 is relatively inexpensive in comaparison to 10 and easy(er) to work with

 

[mm08822]

Who makes/sells a 40 space 100amp sub panel? I'd go for it but the largest I could find was the 30 space one I linked to.....

Jeff

You can use a 150a or 200a rated panel - easy to find. Main CB is only for disconnect purposes. CB in house panel protects feeder.

 

[sberry]

Common 100A panels are up to 20 space and some 24 if you look. This is why some conservation is in order. You can run 2 heaters on a 10/30 circuit. Its designed just for this. You essentially tap this circuit, its maximum short circuit or breaker is 30, you can tap each one using a 14 wire basically. You need a 10 wire so both 14 cant overheat it. In theory each 14 could run back where they both were under a 30 breaker. Wouldn't need any 10 wire. Could actually hook one 14 to 30A in some respect if connected to this heater.

 

[sberry]

In a theoretical world could run 14 from one to the other and 10 to the breaker but the deal here is to run a 10 cable to 2 units with 30. Each has its on control, the breaker is here basically for fault protection and service.
This is basically what an electric range is, a 6 cord with a bunch of number 12 taps for each element with a control on it.

 

[sberry]

The circuit and the cable in the case of the range thermal protected by the breaker but all the internals by applied load and the breaker merely provides for short protection beyond cord. Basically doesn't have extra internal protection except for the clock in old models but today may have some in electronic controls.
In an AC welder the only protection for the whole thing is the branch circuit breaker. Smaller wire feeders have it following the switch so their internals may be connected to a 50A circuit.

 

[sberry]

I have seen the wonder in older 4 lamp fixtures come out of offices, they had their own fuses as they were often connected to circuits above 20A. If you see them with tap wire of 14 wire connected to 30 and SD actually does make a 50A 120V breaker for commercial lights, every fixture or tap would need to be 12 to be on this circuit and may require additional unit fuses, likely require?
I dont believe I have ever seen a single pole above 30 in a box store.