Why does my Vanguard 8000 generator pop the 40 amp breaker on the generator when I plug in the 30 amp cord into the generator .

The more breaker space, the better provided panel mounting space is available.

Main two pole breaker in the panel examples:
40 amperes ( 40 x 240 is 9600W),
35 amperes ( 35 x 240 is 8400W),
30 amperes ( 30 x 240 is 7200W).

You can fit your 8000W power supply preferably into a 30A/120/240V panel, or a 35A/120/240V.

Try to keep the loads balanced. There are two 120volts lines, each, line voltage to neutral, and 240 volts line to line. Example, two 120 volt loads, 1500watts each, connect one of the loads to one lone and the other load to the other line so that each of the 120 volts lines has a load of 1500 watts avoiding both 1500 watts loads (3000W) from being connected to just one 120 volts line while the other 120volt line has no load. This is "balancing the load".

Product of volts and amps is watts. Transposable,
Watts divided by volts is amps.

You have a main breaker in the panel, most often 30 amps. (Possibly 50, depending on your hookup. If your cord you plug in outside has 3 prongs it's 30 and 4 prongs it's 50) then you have a 20 amp breaker for your air conditioning and a 15 amp for your microwave or outlets. As far as fuses go, usually you have a 30 or 40 for the main converter fuse(s) and then mostly all 15's in all the other slots.

Your question was worded poorly and it's difficult to understand what you are asking. As a guess, if you are looking for a 30-amp adaptor cord set, perhpas this will work:
Conntek 20601 25 Foot 10 4 30 Amp 125 250 Volt 4 Prong L14 30 Transfer...

If it's a 3-prong adaptor:
Camco 55333 30 AMP 3 Prong Generator Adapter

Or a 15 to 20 adaptor:
Coleman Cable 09020 12 3 9 Inch Generator Cord Adapter 5 15P to L5 20R

1) Match wire and breaker:
http://waterheatertimer.org/Color-codewire.html
http://waterheatertimer.org/See-inside-main-breaker-box.html

2) 7000 watts divided by 240 volts = 29.1 amps.
30 amp breaker x 80% safe maximum = 24 amps.
This means you should use 8 gauge wire and 40 amp double pole breaker for 240volt

3) 7000 watts divided by 120 volt = 58.3 amps.
60 amp breaker x 80% safe maximum = 48 amps.
So use 70 amp breaker and #4 wire.

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

1) You got a FIRE truck coming.

2) Wire is too small.
Problem is NOT circuit breaker.

3) Both 240Volt appliances on same breaker is a hazard, especially if space available in 200 amp service panel for more breakers.

4) Usually each 240V appliance has separate breaker.

5) Circuit breaker trips because of High heat on the wire.
If wire gets hot, it will start a fire inside the wall.

Electric code requires wire-size match breaker-size.

6) If you put larger amp breaker on same wire that is getting hot, then wire will get hotter.
Result: FIRE truck middle of night with folks jumping out window in glass-cut bare run across yard.

7) Each appliance has volts and watt rating on label.
EVERY appliance has label.

8) For example stove might be rated 8000 watts and 240Volt.
8000 watts divided by 240 volts = 33.3 amps
Stove would require 40 amp breaker and 8 gauge wire.

9) How to figure it out:
http://waterheatertimer.org/Color-codewire.html
http://waterheatertimer.org/How-to-install-a-subpanel.html

10) Do same calculation for new furnace.
12000 watt furnace divided by 240 volts = 50 amps.
Use 60 amp breaker and 6 gauge wire.

11) Summary: fixya wants you to be safe, and buy stuff showing in the ads.
Use a bigger breaker ONLY when bigger wire is also installed.
Call furnace installer, the installer should know to check safe wiring before installing.

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

check for a broken wire some where

You will have more available power (and use a different larger breaker already on your generator) if you use the 220 plug instead of the 115v plugs. Have an electrician make you an breakout box separating the 220v plug into two 115v outlets. Or your can replace the push button breakers with better quality 25 amp. Problem is that the 115v receptacles are probably 15 amp. Other solution is to try a 220v pump instead of 115v (more efficient draws less amps). Good luck

Extension cord will cause it to trip. Make sure it's big enough and as short as can be. worn breaker will trip easier.

You must find the reason the main breaker is popping ...

I'd start by unplugging everything and remove all breakers. Turn the main on ... does it pop? If it pops here look between the breaker panel and the end of your cord. Turn the power off - reinstall the first breaker - turn power back on. What happens ... keep doing this until the main or individual breaker pops - investigate that circuit to find your short.

If you get all breakers back in with out popping anything ... start plugging stuff back in. Something is shorting somewhere.

Let me know what you find please.

thanks

a

I take it you have RV plugged into generator. Hopefully direct with no extension cords. Are you using a 30 to 15 amp adapter to plug into generator? What is the rating of the breaker on the generator that keeps tripping, suspect it's only 15 amp?