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08012013, 10:02 AM #1
Determining the amperage at a main disconnect fuse block
I inspected an apartment in a building with 200 apartment units, The main disconnect for the sub panel of the apartment is located at the electrical meter in the basement of the building. There are two 60 amp cartridge type fuses in a fuse block as the disconnect. I stated that it was a 60 amp service and the building supervisor stated no it is a 120 amp service because you combine the two legs of the service. I am confusing my self, I always thought you rated the disconnect by the weakest link, which would be the 60 amp fuse. Which is correct, is it a 60 amp service or 120 amp service ? Thanks in advance.
Last edited by Scott Bray; 08012013 at 10:23 AM. Reason: Spelling

08012013, 01:00 PM #2
Re: Determining the amperage mat a main disconnect fuse block
That would be a 60 amp panel, not 120 amps. You do not add the leg ratings together when describing the service or feeder.
All answers based on unamended National Electrical codes.

08012013, 01:04 PM #3

08032013, 10:52 AM #4
Re: Determining the amperage mat a main disconnect fuse block
Think of it as a "2 pole" breaker that has fuses instead..... A 2 pole fuse block. As with breakers, the ratings are not additive. So, 60 amp is correct IF you confirmed that the service feeders to the fuse block can support 60 amps (# 4 Al or # 6 Cu).
Bruce Low
Bottom Line Home Inspection
Northeast Wisconsin
Go Pack Go

08032013, 11:19 AM #5
Re: Determining the amperage mat a main disconnect fuse block
"Get correct views of life, and learn to see the world in its true light. It will enable you to live pleasantly, to do good, and, when summoned away, to leave without regret. " Robert E. Lee

08032013, 11:36 AM #6

08032013, 12:16 PM #7
Re: Determining the amperage mat a main disconnect fuse block
"Get correct views of life, and learn to see the world in its true light. It will enable you to live pleasantly, to do good, and, when summoned away, to leave without regret. " Robert E. Lee

08032013, 12:29 PM #8
Re: Determining the amperage mat a main disconnect fuse block
Think about it, you could try to pull 75 amps on one leg of the 2 pole breaker while the other leg is at 0. Depending on the trip curve, the breaker will trip. You could also have each of the 2 hots pulling 60 amps and the breaker does not trip. 60 + 60=120.
All answers based on unamended National Electrical codes.

08032013, 12:48 PM #9

08032013, 12:54 PM #10
Re: Determining the amperage mat a main disconnect fuse block
I don't have a copy Vern, can you post the page?
All answers based on unamended National Electrical codes.

08032013, 01:05 PM #11

08032013, 01:08 PM #12
Re: Determining the amperage mat a main disconnect fuse block
Lets use instantaneous fuses at 60 amps for fun... show me where you could put an amp meter and actually measure more then 60 amps. the math is more like +60 +(60) = 0. 60 amps is still all you can draw with the fuses at 120 volts.. the 0 would be on the neutral with 60 respectively on each phase or leg..never more..
Breaker trip curves make this an unrealistic problem.
"Get correct views of life, and learn to see the world in its true light. It will enable you to live pleasantly, to do good, and, when summoned away, to leave without regret. " Robert E. Lee

08032013, 01:10 PM #13
Re: Determining the amperage mat a main disconnect fuse block
I've tried several times and IN does not tell me why it failed, just that it failed.
What the page shows is that the neutral leg has the difference current of the two 240 legs, thus you can only have the max current of either of the circuit breakers not the sum of the two.
The beatings will continue until morale has improved. mgt.

08032013, 01:18 PM #14
Re: Determining the amperage mat a main disconnect fuse block
The load on the neutral doesn't matter in this example. Leg A carries 60 amps, leg B carries 60 amps equaling 120 amps of power. The 120 power is not returning on the other leg. It is on the neutral.
All answers based on unamended National Electrical codes.

08032013, 01:18 PM #15
Re: Determining the amperage mat a main disconnect fuse block
I think I have done it Yay
The beatings will continue until morale has improved. mgt.

08032013, 01:24 PM #16
Re: Determining the amperage mat a main disconnect fuse block
Ok, I understand that. Leg A carries 40 amps, Leg B carries 20 amps. The neutral load will be the difference of between A and B or 20 amps. This is why a MWBC requires the hots to be on opposite legs.
If both hots were on the same leg of the panel, the neutral now carries the additive of the two hots or 60 amps. This would create a fire hazard on the now undersized conductor.
Each leg can carry up to the limit of the breaker. It does not matter how much the other leg carries.
All answers based on unamended National Electrical codes.

08032013, 01:28 PM #17
Re: Determining the amperage mat a main disconnect fuse block
The beatings will continue until morale has improved. mgt.

08032013, 01:30 PM #18
Re: Determining the amperage mat a main disconnect fuse block
A load on Leg A does not affect anything on Leg B. The only thing that changes is the load on the neutral.
All answers based on unamended National Electrical codes.

08032013, 01:46 PM #19
Re: Determining the amperage mat a main disconnect fuse block
Think of this, a 20 amp breaker is installed in a panel. What capacity does the circuit have? 20 amps.
Now add another 20 amp breaker for a second circuit. Again the capacity is 20 amps. It has not affected the original circuit capacity.
The only thing is that you can now pull 40 amps of power from the panel.
All answers based on unamended National Electrical codes.

08032013, 02:01 PM #20
Re: Determining the amperage mat a main disconnect fuse block
Jim, Do you want to talk about amps or power. Cause if you are talking about power you have just changed the subject.
No where, No how will you ever be able to measure more then 60 amps with an amp meter. Even if you stand on your head and hold your tongue right....
   Updated   
Never heard of 40 Amps of power. What is it??
"Get correct views of life, and learn to see the world in its true light. It will enable you to live pleasantly, to do good, and, when summoned away, to leave without regret. " Robert E. Lee

08032013, 02:01 PM #21
Re: Determining the amperage mat a main disconnect fuse block
The beatings will continue until morale has improved. mgt.

08032013, 02:07 PM #22
Re: Determining the amperage mat a main disconnect fuse block
"Get correct views of life, and learn to see the world in its true light. It will enable you to live pleasantly, to do good, and, when summoned away, to leave without regret. " Robert E. Lee

08032013, 02:15 PM #23
Re: Determining the amperage mat a main disconnect fuse block
http://www.mikeholt.com/instructor2/...809sample.pdf
Maybe this will help, hopefully.
"Get correct views of life, and learn to see the world in its true light. It will enable you to live pleasantly, to do good, and, when summoned away, to leave without regret. " Robert E. Lee

08032013, 02:19 PM #24
Re: Determining the amperage mat a main disconnect fuse block
Each leg can carry 60 amps. You have two legs available at 60 amps. 60 x 2 = 120.
All answers based on unamended National Electrical codes.

08032013, 02:39 PM #25

08032013, 02:47 PM #26
Re: Determining the amperage mat a main disconnect fuse block
Never heard of amps of power? Power = amps x voltage. Can also be expressed in watts. The only difference is that amps ate dependent on voltage while watts are constant, regardless of voltage.
20 amps x 120v = 2400 watts. 20 amps x 240v = 4800 watts.
2400 watts/120 = 20 amps
2400 watts/240 = 10 amps.
   Updated   
The Op stated the fuse block had 2 60 amp fuses in it. Each leg can carry 60 amps. Two legs at 60 amps is 120 amps.
This is not a transformer discussion.
All answers based on unamended National Electrical codes.

08032013, 02:52 PM #27

08032013, 03:03 PM #28
Re: Determining the amperage mat a main disconnect fuse block
All answers based on unamended National Electrical codes.

08032013, 03:27 PM #29

08032013, 03:33 PM #30
Re: Determining the amperage mat a main disconnect fuse block
With 60 amps on each leg there would be 0 Amps on the neutral. The 120 power is not returned on the opposite leg, but the neutral. A 240 volt circuit would return the power on the other leg.

08032013, 04:00 PM #31
Re: Determining the amperage mat a main disconnect fuse block
Draw it out. And remember; " Kirchhoff's current law (KCL) The current entering any junction is equal to the current leaving that junction.
The beatings will continue until morale has improved. mgt.

08032013, 04:46 PM #32

08032013, 05:10 PM #33
Re: Determining the amperage mat a main disconnect fuse block
The beatings will continue until morale has improved. mgt.

08032013, 09:35 PM #34
Re: Determining the amperage mat a main disconnect fuse block
This should help. Note that all of the current goes through one of the legs.
The beatings will continue until morale has improved. mgt.

08042013, 05:59 AM #35
Re: Determining the amperage mat a main disconnect fuse block
it doesn't really matter what the fuse or breaker size is, what matters is the size of the main wire and what IT is rated for.We all have seen 100 amp breakers on wires rated for 60 AMPS. So what size was the main wire coming into the panel? You guys always get so caught up in who's got the biggest code book that you overlook the question or best advice and start running around in circles. The first question to Scott , In my opinion, should have been, What size are the main wires?

08042013, 06:33 AM #36
Re: Determining the amperage mat a main disconnect fuse block
Wayne, as described in the OP, there would be a descrepency as to the correct sizing of the wires. One person is saying the service is twice as large as it actually is. If you were to believe that person the wiring is too small when in actuality it could be correct. The wiring does not need to be sized for 120 amps, but you do have 120 amps between the two legs available for usage.
All answers based on unamended National Electrical codes.

08042013, 07:10 AM #37

08042013, 12:43 PM #38
Re: Determining the amperage mat a main disconnect fuse block
Yes, but you know we can never assume! That's why I asked the question. and, if the OP was answered in the first few posts, why are there 40 odd posts after the fact hemming and hawing about which end of a candlestick will light. I just think that sometimes a simple answer or question is enough. we don't need to turn a post into a $hit fight,

08042013, 02:51 PM #39
Re: Determining the amperage mat a main disconnect fuse block
Again, each leg can carry 60 amps. There are 2 legs, 60 + 60 =120.
Assume that you could get a single pole 60. Install it on leg A and load it up. You can flow 60 amps, ignoring trip curves. Now install another single pole 60 on leg B. Again you can pull 60 amps, even if you turned off the first breaker. With both breakers on you have 120 amps available.
All answers based on unamended National Electrical codes.

08042013, 03:36 PM #40
Re: Determining the amperage mat a main disconnect fuse block
The beatings will continue until morale has improved. mgt.

08042013, 03:50 PM #41
Re: Determining the amperage mat a main disconnect fuse block
It is not possible to exceed the amperage the fuse is rated for (unless its a slow blow fuse and the amperage is exceeded for a brief period of time such as starting a motor), but that's not the type of fuse were talking about. The only reason I mention slow blow is because I assume feed back on it. Keep in mind were talking AC current and the two legs are 180 deg out of phase. Leg A is positive 120 volts at the same time leg B is 120 volts. To have 120 volts you need one leg A OR B to neutral or ground. To have 240 volts you need legs A AND B with no neutral. Leg A OR B to neutral is (120 + 0 =120 volts). Legs A AND B is (Positive 120 + minus 120 = 240 volts). Remember were talking about vectors not sums. A positive 120 volts when added to negative 120 volts does not equal 0 as one would expect. What is being measured is the difference between the two peaks of a sign wave. The sign wave swings from positive 120 volts to negative 120 volts and the difference between the two peaks is 240 volts. The two peaks being 180 degrees out of phase means there is a voltage  not an amperage  differential between leg A and B. Regardless which leg is positive or negative at any given time each leg will only allow a load up to the fuse size which in this case is 60 amps. This is a 60 amp service. Jim, your logic isn't totally correct. You are saying that each leg has 60 amps available so this equates to 120 total amps. You are correct in saying each leg can supply 60 amps for a total of 120 amps, And, if each leg had a load of 60 amps at 120 volts, you would consume 120 amps and the most the neutral would carry is 60 amps because the legs are phased 180 degrees apart. So far so good. Here's where the formula fails; If you insert one double pole 60 amp breaker and put a 60 amp load on it, you've maxed out the available amount of current on both legs and any more breakers whether it be single or double pole will blow a fuse. The amperage available, determined by the amperage of the fuses, is based on 240 volts, not 120 volts. If you exceed 60 amps at 120 volts on either leg, you will blow a fuse, If you exceed 60 amps at 240 volts, you will blow fuses. this is why its a 60 amp panel.
Last edited by Mike Borchardt; 08042013 at 08:28 PM.

08042013, 09:26 PM #42

08042013, 09:38 PM #43

08042013, 09:50 PM #44
Re: Determining the amperage mat a main disconnect fuse block
In regards to the picture, I'm assuming due to the size of the light bulbs, this is an unbalanced circuit. Show a balanced circuit and the current path would be quite different. Jim is correct in stating a single pole 60 amp breaker on each leg could give you 120 amps total consumption. Think of it this way; If I'm running a 60 amp 120 volt motor connected to a circuit breaker off of leg A, and at the same time I'm running a 60 amp 120 volt motor connected to a circuit breaker off of leg B, even though each leg is maxed at 60 amps, I'm still consuming 120 amps.

08052013, 04:24 AM #45
Re: Determining the amperage mat a main disconnect fuse block
You are 100% wrong! If you would look at the circuit diagram you would see that ALL of the total current is passing through one of the legs of the transformer and any additional current on either side of the neutral will be added to that current, thus the fuse or breaker in that leg will trip.
And by the way you don't have a phase difference across the coil, it is a single phase. What you are describing is a multiphase where one leg is lagging the other by some number of degrees.
The beatings will continue until morale has improved. mgt.

08052013, 08:09 AM #46
Re: Determining the amperage mat a main disconnect fuse block
This is not about a transformer. This is about how much power can be supplied through a 2 pole 60 amp fused disconnect as asked about in the OP.
All answers based on unamended National Electrical codes.

08052013, 09:02 AM #47

08052013, 09:18 AM #48
Re: Determining the amperage mat a main disconnect fuse block
All answers based on unamended National Electrical codes.

08052013, 09:35 AM #49
Re: Determining the amperage mat a main disconnect fuse block
Read what is written in post #4, under "3Wire Edison Circuits". The issue is, and has been from since the statement that you gave, saying 120 amps could be drawn through a 60 amp service if it was all 120v loads. I put arrows showing the current paths on the fig. 42, which I stole from Code Check (hope I don't hear from there lawyer). The whole page 14 is to help understand where the currents are going and the role of the neutral. All of the current must pass through one of the legs and when the current exceeds the OPD, it trips or blows. So if you have 60 amps all on one side of the buss (transformer) and you add additional amps on the other side of the buss, they add and trip the CB.
The beatings will continue until morale has improved. mgt.

08052013, 11:53 AM #50
Re: Determining the amperage mat a main disconnect fuse block
If you look at the current flow in this diagram you will see that all the current does not flow through one leg. [img]http://www.ibiblio.org/kuphaldt/elec...s/AC/02168.png[/img]
If your statement about all the current through one leg were true, turning off or losing one fuse would cause everything not to work. That would not be the case as the 120 loads from the other leg would still function. Think about when one leg is lost at the meter or main breaker.
All answers based on unamended National Electrical codes.

08052013, 12:00 PM #51
Re: Determining the amperage mat a main disconnect fuse block
The beatings will continue until morale has improved. mgt.

08052013, 12:48 PM #52
Re: Determining the amperage mat a main disconnect fuse block
Just posting this one more time to show that I am not the only one that is saying you have 120 amp available between the two hot legs. Vern, I don't know why you cannot focus on this as two 120 volt 60 amp circuits. This has nothing to do with the transformer upstream from the disconnect.
All answers based on unamended National Electrical codes.

08052013, 01:44 PM #53
Re: Determining the amperage mat a main disconnect fuse block
Jim, it does not have to be a transformer, even though it is. It could be a series of resistors, a series of batteries (if you look at it as a snap shot in time) or any other means of creating a difference in potential +,,+,, at the three conductors, the result would be the same. All of the current would pass through one of the legs regardless and some of the total current will pass through the neutral as well in an unbalance load. I did not design this or discover this, it is just the rules of electron flow. Talk to Tesla or Kirchhoff or Edison if you want to change it. I have done all I can to show you where and why the current flows. Code Check has done all it can to ex plane the current flowing in the 3wire circuit. You keep insisting 120 amps can flow through the 3wire circuit with 60 amp fuses but have not produced a single bit of evidence this can happen. Your best bet is to not share this belief with any electrician to avoid embarrassment.
The beatings will continue until morale has improved. mgt.

08052013, 02:20 PM #54
Re: Determining the amperage mat a main disconnect fuse block
The beatings will continue until morale has improved. mgt.

08052013, 03:24 PM #55
Re: Determining the amperage mat a main disconnect fuse block
You have ignored my examples, posts from Robert M and also the recent additions from Mike B as evidence of the capacity of the service. You have not explained how the circuit could continue to work with the one breaker turned off even though it will.
I have talked to other electricians and they agree with myself and Robert. The capacity is 60 amps per leg. Two legs at 60 gives you 120 amps at 120 volts or 60 amps at 240 volts. Mike gives a good example of how this can work that echoed the example I gave. Leg A is powering a 120 volt 60 amp load. Add another 60 amp 120 volt load to Leg B. You are now running 120 amps of load through the panel, 60 on each leg. Turn either breaker off and the load will drop to 60 amps on one leg and 0 on the other leg. The load left on continues to function properly.
All answers based on unamended National Electrical codes.

08052013, 03:29 PM #56
Re: Determining the amperage mat a main disconnect fuse block
What examples? Anyone can say something is so, proving it is another story. I continue to wait for your or Roberts diagram showing where the 120 amps comes from and goes to. (I would not use your electricians).
In the Mike example (if that is the diagram you are talking about), the current on leg A is the same current that came from or goes to leg B not additive. You need to read things a little closer!
"Turn either breaker off and the load will drop to 60 amps on one leg and 0 on the other leg. The load left on continues to function properly." The amps will drop to the load on the not tripped side of the neutral and is shifted to the neutral that was carrying 0 amps.
Last edited by Vern Heiler; 08052013 at 03:41 PM.
The beatings will continue until morale has improved. mgt.

08052013, 04:09 PM #57
Re: Determining the amperage mat a main disconnect fuse block
The sine wave or sinusoid is a mathematical curve that describes a smooth repetitive oscillation. It is named after the function sine, of which it is the graph. It occurs often in pure and applied mathematics, as well as physics, engineering, signal processing and many other fields. Its most basic form as a function of time (t) is:
where:
 A, the amplitude, is the peak deviation of the function from zero.
 f, the ordinary frequency, is the number of oscillations (cycles) that occur each second of time.
 ω = 2πf, the angular frequency, is the rate of change of the function argument in units of radians per second
 φ, the phase, specifies (in radians) where in its cycle the oscillation is at t = 0.
 When φ is nonzero, the entire waveform appears to be shifted in time by the amount φ/ω seconds. A negative value represents a delay, and a positive value represents an advance. And by multi phase your probably referring to 3 phase which represents three wave forms. Your confused with the meaning of phase. Phase does not represent the time difference between two wave forms. It represents a point in time within a wave form. One wave is voltage raising above 0 volts to dropping through 0 volts to a negative voltage and rising to 0 volts. In a wave form representing any given AC voltage, the peak positive voltage with respect to the peak negative voltage is represented in degrees of the total wave. The two peaks are 180 degrees apart. Therefor the peak positive voltage and the peak negative voltage do not happen at the same time and in fact happen 180 degrees out of phase.
Last edited by Mike Borchardt; 08052013 at 07:50 PM.

08052013, 04:41 PM #58
Re: Determining the amperage mat a main disconnect fuse block
And if you were to look at any point on the secondary with an o'scope, you would see the sine wave. A representation of time on the X and amplitude on the Y axis. There is a negative high and a positive high developed on the single wire secondary at the same point in time. The voltages are positive and negative in respect to the neutral which is a chosen reference point. If you could see current as a larger wire being more current and a smaller wire being less current, you would see the wire (coil in this example) grow and shrink for its entire length at the same time. It would be like watching a window parting sill move up and down.
The beatings will continue until morale has improved. mgt.

08052013, 04:46 PM #59
Re: Determining the amperage mat a main disconnect fuse block
The same 2 pole 20 amp CB with a neutral (MWBC) can supply 40 amp (20*2) @ 120 volts each.
We probably did say the same thing. Yes you can supply a total of 40 amps but the amperage would be two circuits of 120 volt at 20 amps each. Maybe its the way you worded it that confused me.
   Updated   
The same 2 pole 20 amp CB with a neutral (MWBC) can supply 40 amp (20*2) @ 120 volts each.
We probably did say the same thing. Yes you can supply a total of 40 amps but the amperage would be two circuits of 120 volt at 20 amps each. Maybe its the way you worded it that confused me.

08052013, 05:12 PM #60
Re: Determining the amperage mat a main disconnect fuse block
The beatings will continue until morale has improved. mgt.

08052013, 06:21 PM #61

08052013, 06:41 PM #62
Re: Determining the amperage mat a main disconnect fuse block
OMG, If there was a negative high and a positive high developed on the single wire at the same point in time, the negative high and the positive high would be self canceling. This is not possible within a single wave form. It sounds to me your o'scope is not in sync with the wave form. I don't doubt what you're seeing, I doubt the settings on the scope.

08052013, 06:59 PM #63

08052013, 07:02 PM #64
Re: Determining the amperage mat a main disconnect fuse block
It really seems that people are talking past each other. I wonder if the following isn't a better way to look at it. As far as asking electricians, I don't know how many of them haven't insisted to me, when I was supplying pool pumps, that it is cheaper to run things on 240 than 120! When I did the watt calculation for them, they scratch their heads, and look dubious.
The service is 60 Amps @ 240 Volts which would allow a maximum of 14,400 watts  A * V = W. If you pulled 60 amps at 120 you get 7,200 watts, two such circuits would yield the same maximum Watts 14400. Watts is the total measurement of electricity used Power after all Watts as in Kilowatts is what you get billed for.
I think it is customary to list the rating of the service at 240 Volts, so we say 60 Amps. One could say it is 120 Amps @ 120 volts, but we all know that is not the custom. If we insist that it is 60 Amps @ 120 Volts, that can't be right, because we know that we can pull 14400 watts with two 60 Amp 120 volt circuits. The gentleman who said the wire size should be checked is of course correct, because either the wire size or the breaker could be the limiting factor, and of course the breaker should never exceed the wire capacity

08052013, 07:11 PM #65
Re: Determining the amperage mat a main disconnect fuse block
And as Roland said, " Do you want to talk about amps or power. Cause if you are talking about power you have just changed the subject."
My challenge stands; someone, anyone, draw a circuit diagram showing where the current goes that is different than the one I posted! Because with mine, there can not be 120 amps on a 60 amp service with 60 amp breakers!
The beatings will continue until morale has improved. mgt.
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