Temporary power for sub panel

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Old 09-18-12, 07:06 AM
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Temporary power for sub panel

Hello!

I have a vending trailer that has a professionally installed and wired 100 amp panel. It's a typical panel that you would see installed in older residential applications before 200 amp services became the norm. (Although this panel is new) It was wired with 100 amps with a couple 30 amp double pulls for the possibility of using electrical ranges, ovens, etc. the rest of the circuits are single pull 20 amp breakers that run the interior lighting, and electrical outlets for fridges, freezers, microwaves, and other countertop appliances. We do not use the double pull 30 amp circuits as our major cooking appliances are LP gas powered. We only utilize the 20 amp breakers for the aforementioned freezer, fridge, lighting, and small countertop appliances. The Panel is supplied by an 80 foot long,4 wire 10 AWG, water resistant cable with pigtail that the host venue's electrician will wire into their grid at the time of the event.

My question is this:
For testing purposes at my home, can I hook my mobile kitchen panel (sub panel in this case) in to my residential 100 amp panel using the double pull 50 amp breaker that is designated for my resident range breaker? My thought is that if I remove the black and red leads going to my home range and replace them with the hot leads to my sub panel then tie my neutral and ground leads into my residential ground/neutral bar things should work just fine. I realize that it doesn't make much sense to power a 100 amp panel with a double pull 50 amp breaker, but considering we are using nowhere near 50 amps in this trailer and I'm just trying to use it to check light bulbs and working condition of my smaller appliances.... It should be a convenient and temporary way to work on my trailer.

Will this be ok? Or am I missing something important here?

Thanks so much for your input!
 
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Old 09-18-12, 08:57 AM
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You could connect the trailer feed to an existing double breaker of any size in the panel but that would be cumbersome if that breaker pair is shared with something else.

For test purposes you could also connect the trailer feed to a junction box from which a 20 or 30 amp cable emerges with 120/240 volt plug to fit a 120/240 volt stove or air conditioner or dryer receptacle.
 
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Old 09-18-12, 09:43 AM
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For testing purposes at my home, can I hook my mobile kitchen panel (sub panel in this case) in to my residential 100 amp panel using the double pull 50 amp breaker that is designated for my resident range breaker?... Will this be ok? Or am I missing something important here?
I think so. It sounds like you're thinking about opening your live panel, killing the breaker and disconnecting the branch feeder wires for your kitchen range, connecting the wires in the cable for your trailer to that breaker and neutral and ground, and turning that breaker back on and going out to work on the trailer, with the panel live and uncovered. If so, there are multiple code violations in what you're planning, not to mention the abrasion to the conductors for the range, every time you connect or disconnect them.

To safely connect your trailer to your residential service for testing and maintenance, you should install a new, separate 2-pole breaker of the appropriate size and wire it, with appropriately-sized conductors, to a receptacle rated for the overcurrent protection and the load, mounted in a box outside the panel. Then close the panel and leave it closed. Yes, that means that you'll have to add a plug to the cable whenever you want to do some work on the electrical system in the trailer, but it's safe and code-compliant, and all of the potential conductor damage is limited to the end of the cable, which is trimmable.

I would just leave the plug on the end of the cable until and unless the venue's elecrtician told me he didn't intend to use it.

Tech note: A temporary power connection is subject to the provisions of the National Electrical Code, just as a permanent power connection is. In fact, there are sections in the code that specifically address temporary services.
 
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Old 09-18-12, 10:07 AM
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Thanks do the feedback!

Thanks guys!

I, in fact, was considering adding an additional double pull breaker and installing a box with appropriate plug. I have to do some calculating of maximum potential load and am going through the ratings of my appliances now. I was hoping to avoid installing a plug on the supply pigtail every time I service the vehicle, but I agree that it would be the safest scenario available. Thanks again!
 
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Old 09-18-12, 10:13 AM
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your 10awg wire is rated for a maximum of 30 amps! tho this may work on a temporary site, it cannot be connected to anything larger than a 30 amp breaker
 
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Old 09-18-12, 11:26 AM
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I realize that it doesn't make much sense to power a 100 amp panel with a double pull 50 amp breaker
And is dangerous as Breather said because your cord is only good for 30 amps. In fact I seriously doubt the cord is suitable even for temp use at a venue.Have you done a load calculation? In this case add up all the loads that will be used at one time.

If it were me I would have installed an inlet instead of a cord. You could have then use one of two cords depending on home or venue. One that had a male plug and female plug for at home and one with only a female plug on one end for direct wiring. On the second cord I might suggest tinning the leads on the end with no plug with high temperature silver solder so they didn't "frazzle" from being placed in breaker lugs.

Final thought if power is really going to be 80 feet away at the venue then a #10 cord may not be enough for even a marginally less then 30 amp load because of voltage drop.
 
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Old 09-18-12, 12:03 PM
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your 10awg wire is rated for a maximum of 30 amps!
I looked and looked for the wire size in the original post, and missed it! That's why my post was full of all that "appropriately-sized" language. Thanks, Braether.
 
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Old 09-18-12, 12:40 PM
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The cable issue is very concerning. I wasn't involved when the service was contracted by a licensed electrician.... But I can't imagine that they would, in good conscience, hook up a max 30 amp cable to a 100 amp service. Maybe I read something wrong? I will have to check into this further. When I get back to the trailer I will post the exact specs off the cable to make sure I haven't provided incorrect information.

Thanks again for the feedback!
 
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Old 09-18-12, 01:05 PM
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But I can't imagine that they would, in good conscience, hook up a max 30 amp cable to a 100 amp service
Actually it is perfectly okay to use a #10 with a 100 amp panel as long as the #10 is protected by a 30 amp breaker. This is often done to have enough breaker spaces in a panel. In that case the main breaker in the subpanel is strictly a disconnect switch. The problem is if the total loads on at any one time exceeds the 30 amp limit of the cable. You description of what may be on at one time seems to be close to that.

To be clear it is actual load not a sum of the amp ratings of the breakers installed. It is okay for the sum of the breakers installed to exceed 30 amps on the assumption most will not be used to their maximum capacity at any one time.

But I can't imagine that they would, in good conscience, hook up a max 30 amp cable to a 100 amp service
You would not need a 100 amp service. 100 amp is simply the rating for the box. See above explanation. But lets say they were aiming at a 50 amp service, common for a 4-wire RV plug. That would be 6-4 cable at a cost of ~ $275 for 80 feet.

30 amp may be fine for your use but you won't know till you run the figures so lets not get ahead of ourselves.
 
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Old 09-18-12, 06:10 PM
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I'd install a L14-30 male plug on the trailer cord and leave it there permanently. On the house, I'd install a L14-30 receptacle, and for venues, I'd either plug into a matching receptacle, or have a pigtail adapter with a L14-30 female cord end, and a length of SO cable.


Don't fairs use Cam-Loks??? I know that's how half the vendors at my local fair are connected.
 
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Old 09-19-12, 08:17 AM
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The Panel is supplied by an 80 foot long,4 wire 10 AWG, water resistant cable with pigtail that the host venue's electrician will wire into their grid at the time of the event.
How is the cable attached to the trailer and panel? Does the panel have a main breaker?
 
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Old 09-19-12, 08:39 AM
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Yes, the cable comes in through the floor, up the wall and down through the top of the panel where it hits a 100 amp main breaker.

The exact specs on the cable are as follows:

CAROL 4/C 10 AWG (5.26mm2) 90 degrees C (UL) Water Resistant SJOOW CSA (-40 degrees C) FT-2 P-7K 123033 MSHA 300 VOLT RoHS MADE IN USA
 
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Old 09-19-12, 09:27 AM
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the cable comes in through the floor, up the wall and down through the top of the panel
That is not the standard way that power is connected to a trailer or RV. You should install an inlet to feed the panel, as Ray suggested in post #6, and use the female end of a cord to feed that.

the panel [has] a 100 amp main breaker.
That 100A breaker is a means of disconnect. It doesn't really mean anything in terms of overcurrent protection if the wiring supplying it can't safely conduct 100A. The 10 AWG conductors in the cable can only conduct a maximum of 30A. I would do a full load calculation for everything in the trailer, and size the wiring, inlet, supply cable and a new main breaker to serve that. I would also make the new 2-pole breaker in your home panel the same size.
 
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