Well pump with 15A fuses on a 20A circuit?

Question

I noticed today that my well pump, which is 240V (USA) and is on a 20A dedicated breaker has dual 15A fuses:

In 8 years in this house it has never blown a fuse nor the breaker. So I assume 15A is adequate. (That said, since the 2 fuses are different perhaps at some point one or both has gone out).

The circuit runs:

[Main panel] — [pump fuse + switch box] — [pressure switch] — [pump]

This brings up some related thoughts:

• Is there a good reason that the breaker and fuses would be for different current limits?

• What's the point of having the fuses anyway? They seem redundant to the breaker.

• Ditto in fact for this entire pump switch box.

• Should I change the breaker to 15 A? If there ever is an overcurrent issue it would be more convenient if the breaker tripped rather than a fuse went out.

The fuses are in the same room as the breaker panel which is the main panel in the house.

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I'm not necessarily going to change anything, really just trying to understand.

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(I'm not sure what the white wire is for, I've never actually seen the pump and don't know if it requires a neutral. Or maybe the pressure switch is 120V using one leg?)

Answer 1

Fuses and breakers may have different trip/blow curves.

Disconnect switches are for reasons other than merely fusing. Among other things so the person servicing a machine has line-of-sight to the person about to throw the switch on.

You can change the breaker if you like, but there is actually an exception in code for certain motor loads, to allow overbreakering of the motor to avoid false trips on startup. So I wouldn't be surprised if the 15A breaker had nuisance trips. This exception activates if certain mathematical formulas are satisfied; it is not carte-blanche to up-breakering any motor that trips.

Given that both are fused this may be a 240V motor. The white wire is the other hot. Under contemporary Code it is supposed to be marked with tape, but traditionally this was optional if the usage was obvious. The fact that it is not obvious to you is precisely why they changed the Code.

Answer 2

There may be a reason for this. Fuses and breakers have slightly different behavior when overloaded. In particular, the time it takes for the circuit to open can be much more tightly controlled with a fuse than with a standard breaker. For a given fuse current rating you can get slow-blow, fast-blow, etc.

The reason this may be important on a well pump specifically is that motors generally have high inrush current -- when you first switch it on, it may draw up to 10x its rated current for a small fraction of a second. This is fine -- the wiring won't have enough time to heat up to be a fire danger -- but it may be enough to trip a 15A breaker. So perhaps they used 15A fuses, which can handle this startup current better, and then upped the breaker to 20A.