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I have what is rated by the electric company as a 200A entry into the house, but use a limited amount (largest draws are my HW tank, oven and Heat Pump). Recently, I was looking at upgrading some heating elements for my HVAC and I realized that I need to calculate an additional 60A for the elements. So I checked my entry to ensure that I had a 200A fuse there and was surprised to find only 130A.

However, I have the old style fuse cutoffs - with a dedicated fuse per phase. Each phase has a 130A fuse on it.

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Does that mean then that I actually have a 260A rating on my entry at the moment? (ie: up to 130A/phase?)

Further, I have a SquareD subpanel with a 120A master breaker on it. Does that imply 120A total draw to the entire panel, or 120A/phase?

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The overall service is 130A@240V -- each service leg is 130A@120V. The same rule applies to the subpanel. (I.e. you can hang twice as much 120V load off of it as you can 240V load, but not both at the same time -- a given amount of ampacity provides twice the power at 240V than it does at 120V, IOW.)

Also, you may wish to swap those fuses for 125A fuses if the extra 5A don't matter to you -- 125A is a standard North American fuse rating, while 130A isn't, so finding replacements for blown fuses is likely to be...much more annoying.

  • I'm not sure I understand why. Can't I draw 130A@120V per phase right now the way it is set up? Which would imply a 260A total draw no? Or is the entry always based on a 220V draw, in which case I agree... It would be 130A@220V. – Eric B. Nov 6 '16 at 23:42
  • @EricB. -- think of it as summing voltages across the two legs -- 130A@120V on one leg + 130A@120V on the other = 130A@240V, as if they were in series :) – ThreePhaseEel Nov 6 '16 at 23:48
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    Ok - so I guess at the end of the day, the household entry is always calculated as the Amperage based on the 220V draw. Which makes sense then. – Eric B. Nov 7 '16 at 1:22

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