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Old 06-30-2012, 08:08 AM   #1
Bill Traver
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Breaker & Fuse Math

I have an older house ... it has 100 AMP service coming in from the street. The wiring is mixed (old and some new). Right now, the panel has 2 50 AMP fuses then 4 fuses to tie in circuits. They are all 15 AMP.

I am thinking of switching to Fuses. If I have 100 AMP service coming in, can I switch to 2- 20 AMP breakers (pull 12 ga wire) and 4 - 15 AMP breakers (total is 100, but not sure if I need to stay lower than that). OR Can/Should I just go with 6 - 15 AMP?

This stems from having issues time to time with older AC units plugged in (SEE SEPERATE THREAD for question on this).



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Old 06-30-2012, 08:20 AM   #2
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The total of the branch circuit breakers does not have to stay lower than 100 or whatever.

The individual branch circuit breakers can stay the same (15 amp for each with 14 gauge wiring, 20 amp for circuits with all 12 gauge wiring).

Only if the existing panel is rated (see its label) for 100 amps then can substitute two 100 amp fuses for the two 50 amp fuses in the main disconnect block. (Or 75 amp fuses for a 75 amp rated panel.) Alternatively install a new 100 amp panel and put a 50 amp double breaker in it to feed the old panel as a subpanel.

You can add more breakers to the existing panel (or add a subpanel) with additional 15 and 20 amp branch circuits (or 10 gauge wiring for 30 amp circuits, etc.) even if the main fuses are still 50 amp, subject to any load calculation required as part of your electrical construction permit. Because all branch circuits are not going to be fully loaded at the same time, the sum of the branch circuit breaker ratings can be much greater than the main breaker or fuse rating.

But in those less common situations where one meter feed goes to two panels with first disconnecting means inside those panels and neither is a subpanel of the other, the sum of their top breaker/fuse ratings may not exceed the service amperes rating. (A 100 amp 120/240 volt service provides 100 amps on each hot leg.)


The good conscientious technician or serviceperson will carry extra oils and lubricants in case the new pump did not come with oil or the oil was accidentally spilled, so the service call can be completed without an extra visit.

Last edited by AllanJ; 06-30-2012 at 01:04 PM.
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