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Old 05-10-2012, 08:01 AM   #15
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There is 240VAC available at a 50A pedistal service in a RV park. It is distributed to the circuit panel into the RV on 2-120V legs each carrying 50A.

Therefore there is actually 2 x 120 x 50 = 12,000 watts available distributed through the circuit breakers in the RV. The RV builder balances the panel the best they can, so in effect, all 12,000 watts are available.

In other words, microwave may be on L1; AC1 on L2; AC2 on L1; Converter charger on L2, etc.
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Old 05-11-2012, 01:29 PM   #16
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teddyu,
Sorry but the maximum amount available is 12,000 Watts.
L1 leg: 50amps x 120VAC = 6000 Watts
L2 leg: 50amps x 120VAC = 6000 Watts

So, 6000 (L1) plus 6000 (L2) = 12,000 watts total available.

The rub is, that you only have a max of 6000 usable watts on each leg. Even so, that is still 1400 watts (11 amps) more available on each single leg than what is provided on a rig designed to run on 30 amp service.

It is the responsibility of the original circuit designer to distribute (balance) the loads within the rig between each leg.

Dave
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Old 05-11-2012, 01:58 PM   #17
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Quote:
Originally Posted by teddyu View Post
I disagree in the 12,000 watts calculation for the RV. That calculated value is for a 240VAC system. Eventhough 240 VAC is available in the RV breaker panel, only 120 VAC is usable to the whole coach via two 120 VAC to Neutral circuits with their associate distribution breakers. The 240 VAC/12,000 watts is only available to a 240 VAC appliance. Just my two cents...
You're right in that no SINGLE appliance will have access to the ENTIRE 12000 watts unless it is a 240V appliance. However, Dave78Chief and Pusherman nailed it. There are a TOTAL of 12000 watts available in the coach on a 50A service. They're just not available at the same time to a single 120V appliance.
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Old 05-12-2012, 08:06 AM   #18
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Dave78chief,

The limiting component that should determine the maximum wattage useable in a 2-120VAC 50A system should be the shore power cable, based on the length and conductor size. The typical shore power cable is 3-6AWG + 1-8AWG conductors. The current carrying capicity of 6 gauge stranded copper wire is 37.5a. Since the limiting conductor is the neutral, used by both legs of the 120VAC supply, calcultion of wattage should use this limiting device. If one uses the breaker rating of 50A the wattage would be 6000W, not 12,000W. Because the nature of house/RV wiring and loading, 100% of the calculated capability is typically not used (check out ampicity). The ampicity rating for 3 conductor 4ga copper wire for a house is 100A; for 6AWG in an house/RV it is 55A. Therefore 1-120VAC x 55A = 6600W. Just my two cents...
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Old 05-12-2012, 05:45 PM   #19
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In a 240V application, each side of the breaker is coming from a leg which is 180 degrees out of phase with the other (as in standard home wiring). Therefore, in a 240V service wired with a double pole breaker, the neutral amperage cancels across the legs.

So in a totally balanced 240V 50A service at full draw, you would measure 50A on L1 on the Hot (Red, for example) side, 50A on L2 on the Hot (Black for example) side, and 0A on the neutral (white) side. If it's not equally balanced, then the neutral picks up the excess amperage, but it will never exceed 50A.

The ground should not be carrying any amperage. If it is, there's a problem with leakage.

This is why any properly wired 50A service for an RV is always connected to a double pole breaker, to guarantee the two hot sides are split to L1 and L2, which cancels the amperage draw on the neutral leg. That's why the neutral leg does not have to be a larger gauge wire than the 2 hot legs.
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Old 05-12-2012, 06:31 PM   #20
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teddyu,
Not sure if you realized it or not but you gave 2 different specs for the 6 gauge wire. To the best of my knowledge, this is governed by NEC Table 400-4 (Flexible multiconductor cable). Depending on insulator, that is around 63 amps for 6 gauge and 55 amps for 8 gauge. Now those are max rated values and the breaker is rated for max value. Should you run your system at max value for an extended period? No. Actually, the breaker itself should only be operated at 80% it's rated capacity for extended periods. Thats 40 amps for a 50 amp breaker (24 amps for a 30 amp rig). For effective long term operation and life, the upper 20% region should only be reserved for things like AC motor start. Is this sort of information effectively advertized within the RV community? Unfortunatly no. It fits in with rules of thumb like you should not draw you house battery below 50% capacity.

Dave
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