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Old 02-22-2021, 03:56 PM   #1
KAK
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Trust 15A subpanel breaker to protect SMB from a 20A or 30A service?

Hello,

I've been having lot's of fun with my 1990 SMB. It has a stock subpanel with a 15A GFCI breaker (and switch) that governs when shore power is flowing to the rest of the van.

The shore power connector on the outside of the van is the standard 30a rv receptacle (although SMB added a sticker informing to only use 15A service, assumedly because of the subpanel and stock wiring)

Question: Can I trust that subpanel's 15A breaker to protect the van if I plugged into a 20A/30A (or even a 50A) service?

I should note: The subpanel's 15A GFCI does work. While plugged into shore power, my van's converter was charging up the house battery bank (350w draw). I plugged in a ceramic space heater to keep me cozy while puttering around, using the low setting of 900w. All was good. Out of curiosity, I turned the space heater to the high setting which has a 1500w draw. Sure enough, math does work. The converter's 350 watts plus the high-setting space heater's 1500 watts equaled 1850 watts, which tripped my 15A (1800 watts = 15ax120) subpanel's fuse. Everything reset just fine.

Also, I should note, I am guilty of plugging into a standard 20A outdoor receptacle at my home from time to time. That has been fine so far, but I'm wondering if I should be concerned, and I'm curious whether I should be even more concerned to use a larger plug in?

Thank you in advance for any insight,

KAK
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Old 02-22-2021, 06:31 PM   #2
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I don't see a problem plugging into 20A, 30A or 50A shore power with appropriate 30A and 50A adapters. It's the load that's the issue, not the supply. Inside your house, you're only protected by a 15A or 20A breaker for each circuit while your supply is typically 100A or more.

The only downside is that you'll only have one line of defense, your 15A breaker. Were you to plug in to only 15A shore power per SMB's sticker, you'd be doubly protected, by both your subpanel breaker and the shore power breaker.
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Old 02-22-2021, 06:49 PM   #3
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I agree, a higher current supply is fine as long as you're not actually using more than 15A -- and that's what that breaker is for, to make sure you don't. The breaker just has to be sized for the wiring downstream of it.

This kind of thing is super common, which is why 20A outlets are commonly designed to fit either a 20A plug or a 15A one.

Given that the outlets in campgrounds are often corroded and worn, and struggle to supply their rated current, you'll probably get more stable voltage and less plug heating using an adapter from 30A to 15A than you would just plugging into a 15A receptacle.


By the way, I recommend getting an inexpensive outlet tester to use when plugging into campground power feeds. It will warn you of floating grounds or miswired outlets, which can be dangerous. I leave mine plugged in inside the camper when I'm not using it, which has the added advantage of letting me see whether I have power or not.
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Old 02-22-2021, 07:00 PM   #4
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Originally Posted by Orv View Post
I leave mine plugged in inside the camper when I'm not using it, which has the added advantage of letting me see whether I have power or not.
Good idea, I never thought of that.
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Old 02-22-2021, 09:03 PM   #5
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Thank you! Thank you!
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Old 02-25-2021, 09:24 PM   #6
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So long as you are using a 20 oe 30a shore power cord. Circuit breakers are sized to protect the wire they feed. A short in a 15a cord plugged into a 30a source could cause a fire. Once plug into The van the 15a breaker will protect the 15a wiring from being overload
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Old 02-25-2021, 10:06 PM   #7
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Originally Posted by arctictraveller View Post
So long as you are using a 20 oe 30a shore power cord. Circuit breakers are sized to protect the wire they feed. A short in a 15a cord plugged into a 30a source could cause a fire. Once plug into The van the 15a breaker will protect the 15a wiring from being overload
I would add to check the size of the wire from Shore Power inlet to the breaker, that is actually probably inside the walls. The fact that they have a warning label tells me it is probably sized for 15 amps.


-greg
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Old 02-26-2021, 11:07 AM   #8
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I would add to check the size of the wire from Shore Power inlet to the breaker, that is actually probably inside the walls. The fact that they have a warning label tells me it is probably sized for 15 amps.greg
Excellent point I didn’t think of.
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Old 02-26-2021, 11:35 AM   #9
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Hi ORV - to help us Non-electric wanna bees - can you tell us what electric tester you've been using - and where purchases ? Thanks -

Stay safe -
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