I've learned a lot about DC electrical systems since buying my boat a few years ago but I'm still not clear on something. My system is 12 volt and I'm installing a pump that draws 20 amps. The wire from my distribution panel to the pump is 10 ft. long. This means that the length of the circuit is 20 ft. (10' there and 10' back). I get out my little wire sizing chart and determine that, if a 10% voltage drop is acceptable to me, using 14 AWG wire would get me about an 8.7% voltage drop. Since I have a large spool of 12 AWG wire, and since maybe someday this pump will go bad and I'll end up with one that draws a little more, I'm going to use the 12 AWG wire, which should give me a voltage drop of about 5.5%. My question is, with all this information, how do I determine what size breaker (or fuse) to use?
Electrical wiring question
- Thread starter Morrie
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Just look at the pump.
No formula needed. You know the answer already. Just use a breaker that's a little larger than the maximum draw of the pump. Twenty five percent larger is close enough. Remember, a breakers job is to prevent fire, not protect the appliance from over-current.
No formula needed. You know the answer already. Just use a breaker that's a little larger than the maximum draw of the pump. Twenty five percent larger is close enough. Remember, a breakers job is to prevent fire, not protect the appliance from over-current.
actually...
As much as I hate to disagree with Fred, I have to put my two cents worth in. What Fred said is correct in that "the breakers job is to prevent fire and not protect the appliance from over current". So, the proper breaker for 12 AWG wire is 20 amps. For 14 AWG it would be 15 amps. These are known values based on the safe current carring capacity of various size wires along with a safety factor tossed in. It works the same way for house wiring. Assuming you go with 12 AWG and a 20 amp breaker, the line you run will be protected against an over current condition and hence overheating and possible fire. If you also want to protect the pump from an over current condition, you would insert an inline fuse on the hot side of the pump's power feed. The fuse rating would be the maximum current the pump will draw, which should be on the name plate of the pump.
As much as I hate to disagree with Fred, I have to put my two cents worth in. What Fred said is correct in that "the breakers job is to prevent fire and not protect the appliance from over current". So, the proper breaker for 12 AWG wire is 20 amps. For 14 AWG it would be 15 amps. These are known values based on the safe current carring capacity of various size wires along with a safety factor tossed in. It works the same way for house wiring. Assuming you go with 12 AWG and a 20 amp breaker, the line you run will be protected against an over current condition and hence overheating and possible fire. If you also want to protect the pump from an over current condition, you would insert an inline fuse on the hot side of the pump's power feed. The fuse rating would be the maximum current the pump will draw, which should be on the name plate of the pump.
Paul ?
Where would I find these known values? I have some 16 AWG wire for lights and some 10 AWG for the referigerator. Are the values the same for AC current? I think all my AC is 10 AWG or 12 AWG. I sure don't know what I'd do without this forum to ask questions. The part about the breaker to protect against fire and a fuse to protect the equipment makes sense, but I never knew that before. Thanks
Where would I find these known values? I have some 16 AWG wire for lights and some 10 AWG for the referigerator. Are the values the same for AC current? I think all my AC is 10 AWG or 12 AWG. I sure don't know what I'd do without this forum to ask questions. The part about the breaker to protect against fire and a fuse to protect the equipment makes sense, but I never knew that before. Thanks
As far as I know,
Yes, I believe the standard is the same regardless of AC or DC. 10 AWG is good for 30 amps, 16 AWG is good for 10 amps. 30 amps seems like a lot of draw for something inside a boat, but I suppose it's possible. You can find the values in a copy of the National Electric Code, or ask at a Home Depot, or I'll bet you can google the information online. To me its just common knowledge because I've dealt with it nearly all my life. Keep in mind that if you are going to have a dedicated line for your pump (one power line with only the pump connected to it) you really only need to do what Fred suggested. Wire it with a breaker that is at least the value listed on the pump. It dosent matter if you under utilize the capacity of the wire. You only run into problems if you try to over power it.
Yes, I believe the standard is the same regardless of AC or DC. 10 AWG is good for 30 amps, 16 AWG is good for 10 amps. 30 amps seems like a lot of draw for something inside a boat, but I suppose it's possible. You can find the values in a copy of the National Electric Code, or ask at a Home Depot, or I'll bet you can google the information online. To me its just common knowledge because I've dealt with it nearly all my life. Keep in mind that if you are going to have a dedicated line for your pump (one power line with only the pump connected to it) you really only need to do what Fred suggested. Wire it with a breaker that is at least the value listed on the pump. It dosent matter if you under utilize the capacity of the wire. You only run into problems if you try to over power it.
The standard is the same for AC and DC
An amp is an amp just like a gallon is a gallon. You can just fry the wire faster with a 120 Volt circut than with a 12 volt one using the same size wire.
An amp is an amp just like a gallon is a gallon. You can just fry the wire faster with a 120 Volt circut than with a 12 volt one using the same size wire.
Wire and breakers
You should always use a fuse or breaker of the size recommended by the manufacturer. The size wire has absolutely no effect on the current draw. If the wire is too small, you will get a substancial voltage drop. And the amps will go up. When ever in doubt on wire size, just go over a size. Bigger won't hurt anything, smaller will cause all kind of problems. But still use a breaker or fuse of the recommended size.
You should always use a fuse or breaker of the size recommended by the manufacturer. The size wire has absolutely no effect on the current draw. If the wire is too small, you will get a substancial voltage drop. And the amps will go up. When ever in doubt on wire size, just go over a size. Bigger won't hurt anything, smaller will cause all kind of problems. But still use a breaker or fuse of the recommended size.
Get double duty
Just use the CB to protect the device and the wire will be protected because the device should be drawing less current than the wire can safely handle right. You only need in-line fuses when you have to protect a "sensative" device. By sensative I mean the manufacturer recommends quick blow fuse or something of the kind. They ALWAYS provide these if they are required. So CB rated at the sum of the loads on the circut, fuses to protect sensative items in the circut and the wire sized for voltage drop gets it.
Just use the CB to protect the device and the wire will be protected because the device should be drawing less current than the wire can safely handle right. You only need in-line fuses when you have to protect a "sensative" device. By sensative I mean the manufacturer recommends quick blow fuse or something of the kind. They ALWAYS provide these if they are required. So CB rated at the sum of the loads on the circut, fuses to protect sensative items in the circut and the wire sized for voltage drop gets it.
CB = 150% of the draw of the pump and...
100% inline fuse within 7" of the pump to add thermal protection from a locked rotor. Use waterproff butt connectors in the bilge and consider adding a high water bilge alarm. No frigging wire nuts or speaker wire! Also slow blow fuses are meant for electrical applicances that initially need more power at start up than once they are operating. Your application does not call for slow blow fuses.
100% inline fuse within 7" of the pump to add thermal protection from a locked rotor. Use waterproff butt connectors in the bilge and consider adding a high water bilge alarm. No frigging wire nuts or speaker wire! Also slow blow fuses are meant for electrical applicances that initially need more power at start up than once they are operating. Your application does not call for slow blow fuses.
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