Why some generators are wired to produce 240 volts and some are not.

July 14, 2020, 01:45:05 pm

Why some generators are wired to produce 240 volts and some are not.

Configuring PowerTech generators to NOT produce 240v:
ADVANTAGE
Both legs of generator windings share both hot leg loads so windings are always balanced, improving life of generator.
DISADVANTAGE
Neutral wire carries sum of amps on each hot leg, with potential BIG problems with transfer switch and 50-amp cabling within coach.
50-amp wires in the cable are size #6 rated to carry 50-amps, so if hot leg-1 is powering 40-amps & hot leg-2 30 amps, the white neutral will have to carry 70-amps, which may be ok for a while.
But if each hot leg is pulling 40-50 amps, the neutral would be asked to carry 80-100 amps, about double rating, burning wire AND transfer switch relay contacts, maybe quickly. This has historically been a destroyer of transfer switches.

Configuring generator to produce Split-Phase 240v:
DISADVANTAGE
Drawing high amps on one leg and low or zero amps on other leg, can stress generator windings. In the real world, generating stations go to major steps to keep their loads balanced.
ADVANTAGE
The white neutral wire only carries the DIFFERENCE in amps between the black & red hot wires, so if both roof airs are on and each leg is drawing 20 amps, the white neutral carries ZERO amps. If 20 & 30 amps, neutral carries 10 amps, etc.
Maximum amps the white neutral is called on to carry is the maximum amps of EACH black & red hot leg.

Some generators can be later switched from 120v to 240v & vis versa, but not all.

To help protect generator wires, Powertech neutral white wire that connects the short distance between generator and generator compartment wiring junction box is usually larger than #6.

Think lower rated generators are usually wired NO 240v, and larger generators are wired for 240v.

When installing 240-volt cooktops, dryers, etc, we have to consider not being able to use them in those many campgrounds that only offer 30 amp shorepower, and don't allow generators to run.

Also some campgrounds 'cheat' by wiring both hot legs of a 50-amp outlet to same transformer leg, which supplies 100-amps of 120v, but no 240v. This 'cheat' will cause same wire / transfer switch problems as above 'DISADVANTAGE' discussion.

Re: Why some generators are wired to produce 240 volts and some are not.

Reply #1 – July 14, 2020, 02:38:50 pm

Here's what the neutral looks like after carrying those heavy loads.
jor

Re: Why some generators are wired to produce 240 volts and some are not.

Reply #2 – July 14, 2020, 02:58:01 pm

Barry,

Thanks for laying this out so succinctly. I am sure I have heard all this before but a reminder is always welcome.

Richard

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